The Care and Keeping of Marine Hermit Crabs

Donya Quick

The Care and Keeping of Marine Hermit Crabs

The Care and Keeping of Marine Hermit Crabs Donya Quick [Working Draft v1.10, 05-May-2011]

Copyright © Donya Quick, 2011. All rights reserved. No part of this publication may be reproduced or distributed in any form or by any means, or stored in a data base or retrieval system, without permission of the author.

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TABLE OF CONTENTS Introduction......................................................................................................................... 3 Myths Surrounding Marine Hermit Crabs .......................................................................... 4 Frequently Asked Questions by New Hobbyists ................................................................ 6 Care ..................................................................................................................................... 9 Marine Aquarium Basics .............................................................................................. 9 Types of Aquariums Suitable for Hermit Crabs ......................................................... 20 Macroalgae.................................................................................................................. 24 Livestock Acclimation ................................................................................................ 26 Diet.................................................................................................................................... 28 Suggested Foods ......................................................................................................... 29 Molting.............................................................................................................................. 30 Common Community Tank Problems .............................................................................. 31 When Hermits Aren’t to Blame .................................................................................. 31 Compatibility with Snails and Other Sessile Invertebrates......................................... 33 Compatibility with other Hermit Crabs ...................................................................... 34 When Hermit Crabs Really Are Unruly Animals Bent on Destruction...................... 36 Signs of Impending Doom and Recommended Solutions .......................................... 37 Species Identification........................................................................................................ 39 Anatomy and Terminology ......................................................................................... 39 Species Descriptions ................................................................................................... 40 Notes on The Small Clibanarius Species .................................................................... 63 Claw Morphology and Adaptation.................................................................................... 64 General Claw Types.................................................................................................... 64 Claws in the Aquarium ............................................................................................... 67 Behavior............................................................................................................................ 69 Behavioral Vocabulary ............................................................................................... 69 Dominance Hierarchy ................................................................................................. 71 Introduction of New Individuals................................................................................. 71 Courtship Behavior ..................................................................................................... 73 Reproduction..................................................................................................................... 74 Injuries and Health Concerns............................................................................................ 75 Missing Limbs ............................................................................................................ 75 Soft Tissue Injuries ..................................................................................................... 75 Physical Abnormalities ............................................................................................... 76 Molting Complications ............................................................................................... 76 Oxygen Deprivation.................................................................................................... 78 Exposure to Toxic Substances .................................................................................... 79 Desiccation.................................................................................................................. 79 Swollen Abdomen Syndrome ..................................................................................... 80 References and Recommended Reading........................................................................... 82

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Introduction The bulk of the hermit crab-specific information in this document is based on my own experiences, research, and observations of hermit crabs both in the wild and in aquaria. When I discovered the remarkable lack of reliable species information, I took the approach of picking species and observing them primarily in captivity, since I have not had as many opportunities to examine species in the wild as I would have liked. In time I may be able to remedy that. Some of the information in the basic care sections can also be seen elsewhere on the web and in other books, since it is common knowledge in the marine world – the sort of knowledge anyone can acquire given enough time to proceed by trial and error. Still, the more commonly available information on marine hermit crabs rarely goes beyond stating the minimum requirements for the survival of hermit crabs in community tank and tends not to address the needs of individual species or what makes a good species tank. I have attempted to adapt the care information to hermit crabs specifically while including species-specific requirements where applicable. I have also focused primarily on concerns that are most pertinent to moderately sized and smaller tanks. In a wall-sized tank, many of the compatibility issues discussed here become less relevant, since there is enough space for small animals to successfully avoid each other. The care information here should not be uniformly generalized to all tanks, particularly those containing corals, which have far stricter environmental needs than hermit crabs. Hopefully the observations I have incorporated into this document will shed some light on the behavior of hermit crabs in captivity in ways not represented elsewhere, and provide a view of them as interesting animals in their own right rather than simple additions to a tank’s janitor squad. I also encourage anyone who is new to the hobby to seek out multiple sources of information and not treat a single source as a stand-alone reference. Knowledge changes over time, and it is an aquarist’s responsibility to keep up with those changes. [Work in progress – additional sections not included in this version]

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Myths Surrounding Marine Hermit Crabs The following are some well-distributed myths regarding marine hermit crabs and their care in captivity. Many experienced marine hobbyists will find at least some of these myths absurd – but I have heard or read each of them on numerous occasions in the last several years. The ideas are accepted by just enough people to seem plausible to someone who lacks the necessary information to debunk them, resulting in continued propagation over time. Some of the statements reflect ignorance of advances in the understanding of small-scale marine systems, and others are due to confusion regarding species requirements and the biology of hermit crabs in general. Myth #1: marine hermit crabs cannot survive in captivity and should never be kept. I’m still not sure how it came to be that certain communities of people hold this view, particularly while some of them understand that terrestrial hermit crabs can be kept without issue. I suppose it’s possible that this myth had an element of truth to it back when marine aquaria were a new frontier being explored. In modern times, however, the only significant remaining hurdle is the replacement of wild-caught hermit crabs in the trade with fully captive-bred strains. It is worth noting that this hurdle also remains for many fish in the aquarium trade as well. Myth #2, version 1: marine hermits never need access to land. Version 2: marine hermits always need access to land. Clearly both can’t be right at the same time. Both are also false when applied uniformly to all hermit crabs classed as living in marine environments. Pay attention to where a given species comes from. If it comes from a reef that’s quite far away from land, odds are it’s not going to need to be out of the water constantly. If it lives on a beach and spends a significant amount of time out of the water preferentially, you may need to be more creative with the tank setup and accommodate some land access if you don’t want your hermit crabs to get too curious about climbing power cords and other tank equipment. This is addressed in more detail in the sections on hermit crab care and species descriptions. Myth #3: you shouldn’t handle your hermit crabs, because it’s bad for them. It makes sense to advise someone not to handle hermit crabs that are large enough to be a hazard to fingers, although that would fall under “you shouldn’t handle your hermit crabs because it’s bad for you.” It also makes sense to discourage small children from handle hermits because of the risk of the animal being injured if dropped. A good fall can stun or kill a hermit crab (a tactic some seabirds use in the wild). Handling is an important part of inspecting hermit crabs to identify the species and to ensure that they’re healthy, especially in tanks where the hermits may not be easily visible on a regular basis. Although you don’t need to get cuddly with them, there is certainly no harm done in picking up a hermit crab by the shell and holding out of the water for a couple of minutes to give it an inspection. It is up to the handler whether to 4

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allow the hermit crab to walk around on his/her hand if it is willing – bearing in mind that doing so can result in some unwanted nips if the handler isn’t cautious. Some hermit crabs, even small ones, think human skin is rather tasty and also have the ability to snip small pieces off. That fact may open up the possibility for a novel approach to the removal of unwanted calluses, but it also means that larger species should be handled with care (or sturdy gloves that don’t smell like anything edible). Myth #4: hermit crabs are social and need friends to be happy. This idea is probably related to terrestrial hermit crabs, of which some species exist that live in groups peacefully. In fact, some terrestrial (and marine) hermit species can be kept in quite dense populations. These species exhibit some characteristics of social animals, although the word “social” is often taken too anthropomorphically by pet owners. Regardless, tolerance of other individuals is not uniform across all hermit crabs. Some species definitely do not enjoy the presence of others in the small space of an aquarium, and crowding will result in a hermit crab war with a high casualty rate. Calcinus seurati is a good example of one such species. This is addressed in more detail in the section on species descriptions. Myth #5: you should never help or disturb a molting hermit crab. Well, this isn’t so much a myth as something I’ve found to be a gross overgeneralization. Most of the time, it is true that meddling with a molting crustacean will do more harm than good. It is also absolutely true that anyone lacking in steady hands or a thorough understanding of the anatomy of the molting animal will do more harm than good unless extraordinarily lucky. However, when applied to all cases involving all people and all hermit crabs, it’s a bit like the myth that you will somehow harm a baby bird for life by assisting it if it gets stuck when hatching. Is it possible to harm a molting crustacean? Most definitely, particularly if one is careless. Is it possible to stress a molting animal into doing something that causes it to injure itself? Absolutely. Is it possible to safely save a crustacean from a bad molt that would otherwise significantly harm or even kill it? Indeed. Any disturbance during molting will add some stress. The question is whether the stress from disturbance will be less than stress from none, and animals that are regularly handled will be less likely to have a violent reaction to intervention than those that have experienced little human contact. The success rate isn’t very high for saving a bad molt in the most serious cases, since a really bad molt usually has a deeper underlying cause within the animal (old age, physical weakness due to other causes, etc.), but the success rate for all cases isn’t zero. When the hobbyist is experienced and intervention will clearly make the difference between a healthy animal and either a dead or cripplingly deformed animal, there is nothing wrong with lending a helping, extremely careful hand. Still, this kind of action is never territory for a novice. Molting complications and assisted molting are covered more in the section on health concerns. Myth #6: small hermit crabs are herbivorous and are great at cleaning up algae. Hermit crabs are omnivores. They will eat algae sometimes, but will not eat all types and are not purely herbivorous. The rock-cleaning behavior that is commonly

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pointed to as algae removal largely consists of sampling what gets pulled off of the rock and discarding the majority of it. If something tasty comes along, it will certainly be eaten, but the rest will be ignored and sent up into the water column in a plume of debris. Although at a glance the algae may appear to be gone, it has probably just been shredded and will likely re-establish itself elsewhere in the tank if the nutrients allowing it to grow aren’t eliminated. Some of the algae might be eaten by the hermits if there’s not much else to nibble on, but tankmates may also start to look tasty if there is no other food available. This is addressed in more detail in the section on diet for hermit crabs.

Frequently Asked Questions by New Hobbyists How does a hermit crab grow? Hermit crabs grow by shedding their old skin and forming a new one. See the section on molting for more information on this process. As hermit crabs grow, they require new shells to move into so that their soft bodies can stay protected, as they do not grow their own shells. Strangely this process of molting and needing new shells seems to cause confusion for lot of confusion among non-hobbyists and new hobbyists over how both hermit crabs and snails get bigger shells. Many times I have heard and read questions about where snails get their shells and whether they need new ones to grow into like hermit crabs. Both animals do have the similarity of wearing the same shells, but snails grow the shells. Hermit crabs just make use of it after the snail is dead. My hermit crab is missing limbs! Will it get them back? Yes, assuming that the environmental conditions are suitable to allow it to molt successfully. Crustaceans re-grow lost limbs at each molt, although the new limbs may not be the same size as the others until a few molts have happened. I found the front half of my hermit crab drifting in the tank! Is it dead? Probably not – most likely the hermit crab just molted and you are seeing the old skin. It’s common to see only the hardened exoskeleton, since the covering for the soft body is thin and disintegrates easily. However, you can check whether it was just a molt by following the steps in the next answer. How do I know if a limb or body is just the old skin from a molt? When the old exoskeleton is shed, it is hollow. If you see the entire front half of a hermit crab, you can check whether it is from a molt by picking it up and lifting the carapace. If it’s completely hollow inside and joints are still held together by thin tissue, the hermit molted. Sometimes corpses from long-dead hermit crabs may become hollow after scavengers have eaten them away, but these usually fall apart and are foul-smelling. Occasionally limbs may get stuck during molting and will be seen floating around the tank afterwards. These will not be hollow when examined, but do not necessarily indicate a dead animal. If you see the entire soft body that is usually hidden in the shell, then it is the corpse of a dead hermit.

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My hermit crab disappeared! Where did it go? If the shell is missing too, it probably disappeared because it molted and is hiding for safety. When crustaceans molt, they remain soft for some time afterwards. During this time, they are vulnerable to predators and may hide as a result. If all shells are accounted for and no hermit crab can be found several days later, it is possible that something else in the tank ate it. This is again common during molting, since soft, newly-molted crustaceans are quite vulnerable to attack from otherwise harmless animals. Finally, if the tank is open-topped, you should also check the surrounding area for an escapee. It’s not impossible for hermits to escape by climbing up power cords and airline tubing. If they fall out, they can dry out rapidly. Don’t assume that a dried out crab is dead – put it back in a cup of tank water and see if it reanimates. See the section on desiccation (under injuries and health concerns) for additional information. I don’t like the shell my hermit crab is living in. Can I make it change shells? There is a difference between “can” and “should” in this case. There are ways to force hermit crabs out of their shells, but they put the animal at high risk of injury. If your interest is in keeping the animal alive and healthy rather than extracting it for preservation as a dead specimen, then leave the hermit crab alone to make its own shell choices. You can try offering other, prettier shells, but if the hermit prefers the old, yucky shell, you’ll just have to live with its decision.

I brought a hermit crab home from the beach in a bucket of water. Can I keep it? If you don’t have an established marine aquarium ready for it and have no experience in keeping aquariums, the answer is always NO. Never collect an animal unless you already have the necessary equipment in place to care for it. If you do not have an appropriate setup to care for the animal and it has not been exposed to non-native species (for example, if it has only been in a bucket of water from the same site), take it back immediately and replace it where it was found if it is legal to do so. However, you should NEVER re-release an animal if either of the following two conditions apply: 1. You are too far away from the point of collection to return the animals. NEVER release it onto a beach in another area. 2. The animal has been placed into contact with non-native fauna. This includes other animals, water, plants, rocks, and substrate that have been collected from the other areas or purchased from a store. In either of these cases, re-release is risky since it can introduce non-native species (even at the microscopic level). If either condition applies or it is illegal to rerelease animals in the area of collection, it is far better to take the animal to the nearest pet store with a marine department. Most marine fish stores will happily take animals in these situations.

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On the other hand, if you do have a marine aquarium already set up, there are two main considerations. First, if you don’t know what species the animal is, you may be inviting disaster on your tank if it isn’t set up to be a species tank. Second, as already mentioned, you can’t always just put the animal back where you found it and may need to either place it in its own tank or give it to a pet store. If you have prepared for both of these scenarios and the animal was legally collected (some regions require permits for collecting hermit crabs or may forbid invertebrate collection completely), then you probably have little to worry about.

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Care Compared to other marine animals, hermit crabs are not terribly difficult of animals to care for, but only for those with some experience in aquarium keeping. To the complete beginner, no marine animal is on the same level of difficulty as a hardy freshwater fish. Although some, specialized freshwater systems can be quite complex, the majority of marine systems are more complicated than their freshwater equivalents in terms of equipment requirements, chemistry considerations, and required knowledge for success.

Marine Aquarium Basics Although it is possible to start a marine aquarium with no prior experience in aquarium husbandry, it is usually discouraged by knowledgeable aquarists unless the hobbyist in question is willing to invest a large amount of time doing background research on marine aquariums and proceeds slowly with setting up the tank. Even so, it is usually much simpler with a smaller risk of catastrophic events to spend a year or two dabbling in the freshwater world before considering a marine aquarium, since marine aquariums are, with few exceptions, more demanding than freshwater aquariums in terms of required knowledge and maintenance. For that reason, this section will be approached with the assumption that the reader already has a basic grasp of freshwater aquarium principals such as tank cycling and water chemistry regarding pH, kH, ammonia, nitrite, and nitrate. These topics will each be touched on briefly, but a reader encountering these terms for the first time should seek out additional information beyond this document.

Chemistry The primary chemistry factors in a freshwater aquarium are pH, kH, ammonia, nitrite, and nitrate. These are also important for marine aquariums, with the addition of some new parameters to monitor, namely salinity (measured via specific gravity). Some marine animals, such as corals, make having test kits for calcium, magnesium, strontium, and other trace elements worthwhile, but for less-sophisticated marine tanks it’s usually not a consideration. For a hermit-crab only tank, test kits for elements like calcium and magnesium are only really useful as diagnostic tools (and are rarely needed even then). The following are some good guidelines for water parameter ranges that will be acceptable for hermit crab species tanks:

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Parameter pH kH Ammonia Nitrite Nitrate Salinity* Specific gravity*


Range, as readable on a test kit 8.0-8.4 8-12dkH or 143-215ppm 0ppm (level undetectable) 0ppm (level undetectable) <10ppm 31-35ppt depending on the habitat (coastal, reef, etc.) 1.021-1.028 depending on the desired environment (coastal, reef, etc.)

*Hydrometers and refractometers do not measure salinity directly, but rather specific gravity and refractive index respectively. Refractive index is used to determine specific gravity, and both can be used to determine the approximate salinity of the water.

These ranges are narrower for other animals commonly kept in reef tanks, such as the salinity and specific gravity. While a specific gravity of 1.021 at 78-80ºF is too low for a reef tank, it is not uncommon to find coastal areas with a specific gravity as low as 1.019. Similarly, pH may be as low as 7.5-7.8 in some tide pools and small bodies of saltwater not directly connected to the ocean, although pH should be maintained above this in an aquarium for safety. pH, kH, and Calcium This is a measure of the acidity of the water in terms of the relative concentration + of H and OH– ions. A pH of 7.0 is considered neutral, with the normal measurement range being from 0.0 – 14.0 (although it is possible to have pH values above and below that range). Water that has been properly processed by a reverse osmosis systems should have a pH of 7.0 or at least very close to that. The pH in most marine systems exist within the range of 8.0 to 8.4, although in tidal areas it may differ. Due to the large amount of decaying organics in tide pools, the pH may drop closer to 7.5 in stagnant areas when the tide is low. However, drops down to that level should be avoided in the aquarium, since not many marine animals are content to remain in such an environment for an extended period of time. In the wild, hermit crabs may not spend very long in these sorts of stagnant pools. kH is a measure of carbonate hardness, or the amount of carbonate ions, CO3–2, present in the water. The carbonate hardness of water is usually measured either in degrees (DkH) or in the more standard parts per million (ppm). The conversion between the two measures is 1.0DkH ≈ 17.9ppm. Low kH values are characteristic of soft water, which has little buffering capacity against acids. Maintaining a kH within the range normal for marine systems is important for two reasons: many marine invertebrates require sufficiently high carbonate concentrations to produce shells, and spare carbonate ions help to buffer against pH drops. Calcium, Ca+2, is an important ion for shell-building marine invertebrates. If added in the right form and kept balanced with Magnesium levels, it also helps to buffer the pH of an aquarium. Calcium can come from calcium carbonate, CaCO3, which forms calcite and aragonite. However, aragonite in particular relatively insoluble and will only dissolve to release calcium and carbonate ions when the concentrations of those ions fall 10

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to fairly low levels. Calcium hydroxide, Ca(OH)2, is what “kalk” supplements use. Although highly reactive and therefore requiring some care when added to the tank (calcium hydroxide can temporarily shoot the pH in a small volume of water over 9.0 pretty easily), calcium hydroxide is a very effective way to raise the calcium content of a marine tank. Most marine tanks with only hermit crabs will not require the addition of supplemental calcium, but those heavily stocked with stony corals or large snails may require more. I will not address the topic further here, since a hermit crab-only tank will obtain sufficient calcium from regular water changes with a good salt brand, but the hobbyist interested in a reef tank so research the subject in greater depth and be sure to obtain the appropriate test kits. Ammonia, nitrite, and nitrate To keep it short, the first two of these chemicals are very bad if they show up at detectable levels in an established aquarium. Also keeping it short, the only parameters that mean water is “ok” in the marine world are undetectable levels of ammonia and nitrite and nitrate less than 10ppm. If a test kit shows any reading of ammonia or nitrite, the water is most certainly not “ok,” particularly where invertebrates are concerned. Animals produce ammonia through respiration and other waste production, and the ammonia is converted by bacteria to become nitrite. Nitrite is then converted to nitrate by other bacteria, and finally, in a fully cycled marine tank, nitrate is converted to nitrogen gas. Once a tank is fully cycled, ammonia and nitrite should remain fully undetectable by standard test kits, meaning that the reading should show up as zero, even though trace amounts will exist at any given time. Nitrate should not exceed 10ppm in invertebrate-containing tanks, and levels over 20ppm will be lethal to more fragile invertebrates. Hardy crustaceans can sometimes tolerate higher levels before showing signs of distress, but levels of 20ppm must be strictly avoided in all marine systems even when trying to get other parameters under control after a mini-cycle or other bad event. Levels ≥10ppm in an invertebrate-containing system should be an immediate cue that water changes are in order. If the level stays high after water changes or bounces back very fast, the tank may be overstocked, overfed, or have decaying material hidden somewhere. Salinity and Specific Gravity In the aquarium trade, the terms “salinity” and “specific gravity” often get used incorrectly and interchangeably. Hydrometers and refractometers are both used to measure specific gravity, not salinity. The primary difference between the two pieces of equipment is that hydrometers are more susceptible to changes in temperature. Salinity refers to the salt content of the water as ppm or ppt of sodium chloride, NaCl. Sea water averages 35ppt of NaCl. Specific gravity is the density of a liquid relative to pure H2O (the specific gravity of pure water is 1.000), and can be affected by many things. The primary factors are the ions present in the water and the water’s temperature. The specific gravity of seawater in a tropical aquarium is usually in the range of 1.021-1.028. However, it’s possible to produce the same specific gravity in a water sample just by saturating it with carbonates. Specific gravity can also be changed quite a lot by temperature. For reasons such as this, it is important to calibrate any piece of equipment used to measure specific gravity prior to using it on an aquarium and to try to measure at

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approximately the same temperature each time. It is also important to ensure that the kH of the water is within acceptable ranges, to eliminate it as a potential factor that could throw off the specific gravity reading. How often a tank’s specific gravity must be monitored and maintained depends on the size of the tank. Open-top tanks and tanks that are 20 gallons in volume or smaller will need to be “topped up” with freshwater on a daily basis based on how much water has evaporated. Larger tanks, particularly those that have tightly-fitting lids, may need less-frequent adjustment to maintain a stable salinity.

Equipment Below is a list of the important elements of a standard marine tank. I have focused on the elements of the equipment that are most pertinent to hermit crab species tanks where the level of necessity is concerned. Other authors may have their own views on the exact equipment necessary for marine systems, but the list of mandatory equipment below is what I use for my own hermit crab species tanks.

Mandatory Equipment The tank. This can be glass or acrylic, or even a plastic top-view tub for a more unusual and cost-efficient setup where transparency of the material is less important. A source of purified freshwater, RO (reverse osmosis) being best for beginners, although in some regions, chemically-treated tap water may be acceptable. RO water will always be best for beginners since it eliminates many potential worries. Tap water’s acceptability depends on factors such as carbonate and metal content. Water with high carbonate content will cause problems balancing KH, pH, and calcium concentrations with many salt mixes. Metals must be chemically treated using chelating agents. Tap water may also contain phosphates that fuel algal blooms. In short, tap water requires a lot of attention to detail and should only be used if the aquarist is willing to devote sufficient time to monitoring it and researching the water quality standards in his/her area. A source of new saltwater or salt to mix. Some pet stores will sell ready-mixed saltwater, but it is usually more cost-efficient to buy dry salt and mix it. Containers for storing/mixing water. RO water needs to be stored in a sealed container to avoid eventual contamination from atmospheric exposure and saltwater may require several hours to thoroughly mix. Powerheads or other filters totaling 10-30x gallons per hour depending on the type of tank desired. An air pump is often sufficient on tanks <5 gallons in size. Although 20x turnover is standard for community and reef tanks, this is not a hard-and-fast rule for all marine setups. Hang on the back (HOTB) and canister filters can serve as a useful place

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for storing chemical filtration media. Contrary to some mythology circulating in the hobby, these filters are NOT nitrate factories when properly maintained. Proper maintenance of these filters involves checking and possibly cleaning at each water change. The frequency of thorough filter cleanings can be dramatically decreased by adding prefilters to the intakes of HOTB and canister filters and regularly cleaning and/or replacing them when flow decreases. All types of impeller-based pumps will add heat to the water, which may be problematic in some small tanks and warm environments. Aragonite-based substrate (sand or rubble). Sand is usually preferred since it is less likely to accumulate debris. Quartz sand should be avoided as it provides no pH buffer. Calibrated hydrometer or (preferably) a refractometer to measure specific gravity. Hydrometers require regular checking against solutions with a known specific gravity to ensure accuracy. Live rock, approx. 1lb per gallon of water for a community tank or one where messy meaty foods are frequently given. Sparsely-stocked and species tanks fed mainly on algae and prepared foods may be sufficiently filtered by 1lb per two gallons. Test kits for ammonia, nitrite, nitrate, pH, and KH. A digital pH meter can be substituted for a pH test kit, and although the initial expense is higher, it can be cheaper in the long-run. Thermometer placed in an easily-viewable part of the tank. Glass thermometers should be kept in a location unreachable by any large crustaceans. Sources of aeration/surface agitation if there is no powerhead or filter output actively breaking the water’s surface. Simply aiming a powerhead at the water’s surface to ripple it is usually insufficient to break up organic films, such as those caused by sudden macroalgae die-offs. Lighting. Community and species tanks require less intense lighting than reef tanks. For tanks without photosynthetic animals and algaes, the amount of lighting needed is purely aesthetic. Macroalgae will grow well under T5 fluorescents as well. In smaller tanks or shallow tanks, LEDs will also work well while avoiding heat buildup. Tanks with corals require more attention to the type of lighting and the spectrum involved. Since corals are not the focus of this document, these lighting requirements will not be covered in detail and interested readers are encouraged to seek out supplemental books and websites on the topic.

Useful Equipment (Depends on Desired Setup) Protein skimmer. These are useful for reef and community tanks, although usually unnecessary on hermit crab species tanks.

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Sump and/or refugium, for large community or reef tans. These are usually unnecessary for invertebrate species tanks. Brushes or magnet-based cleaning tools to keep the tank walls free of debris and encrusting organisms. If encrusting organisms become established on the glass, it can be very difficult to remove them, while a few, occasional passes over the glass to keep organisms from settling on it can prevent this. Heaters if the desired tank temperature is higher than the room temperature. Cooling fans to help avoid heat buildup from lighting or room temperature via evaporative cooling. Chillers for tanks requiring a stable temperature significantly below ambient room temperature. These are usually only a consideration for coldwater tanks and those in very hot climates. Test kits for calcium, phosphate, and copper. These are useful for diagnosing problems such as algal blooms and unexplained invertebrate deaths and are a very good idea to keep on hand if using tap water as source of freshwater.

The Marine Tank Cycle Put briefly, a tank’s “cycle” is the period of biological and chemical stabilization that occurs just after the aquarium has been set up and a source of ammonia is introduced. The bacteria and microalgaes responsible for biological filtration become established in a few stages, causing changes in the water chemistry over anywhere from a few days to a few weeks’ time after the tank is set up. How long this period of instability lasts depends on a number of factors discussed later in this section. An unfortunate myth that continues to plague the aquarium hobby is the notion that ammonia must come from an animal, usually a fish. This subjects the fish to dangerous levels of ammonia, nitrite, and nitrate unnecessarily when other sources of ammonia (a decaying piece of seafood, for example) will do the job just as well. Baring emergency situations where there is no place to put animals or when a tank crashes and must recycle, there is no reason to do anything but a fishless cycle. When a freshwater tank is first established, successive peaks in ammonia and nitrite are observed, followed by a growing level of nitrate that only decreases when a water change is performed – assuming that the tank isn’t heavily planted enough to use up all of the nitrate produced. The marine cycle is essentially the same, but it differs from the standard freshwater cycle by the simple fact that nitrate also undergoes a peak and then decreases towards zero. In most tanks, achieving truly undetectable nitrates is extremely difficult, and levels of 10ppm or less are considered acceptable for most marine animals. Getting nitrate down to undetectable levels can be accomplished by using macro algae (either in a sump or in the main tank), additional chemical filtration, protein skimmers, and/or very frequent water

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changes. If the nitrate level refuses to drop down to 10ppm in an invertebrate tank that is already established and cycled, assuming the test kit isn’t faulty, it generally means something has gone wrong in the tank setup and there is more waste being produced than the system can handle (overstocking, not enough live rock, etc.). While nitrate levels of up to 40ppm can be handled by many freshwater animals without health complications, nitrate levels rising above 20ppm can be deadly to marine invertebrates and should be avoided as already mentioned. Some marine animals can tolerate elevated nitrate levels if the condition arises slowly over a long period of time, but this too should be strictly avoided since it subjects the animals to extremely suboptimal conditions and unnecessary stress. The cycle time will be determined primarily by two things: tank size and therefore required volume of live rock for the bacteria and other microorganisms such as microalgaes, and whether the rock has been pre-colonized (“cured”) to some degree. The time during which this cycle takes place can be dramatically reduced in marine aquariums by purchasing cured live rock. There will always be some die-off on cured live rock causing a mini-cycle, but the cycle time in such a case can be reduced from several weeks to one, or even a few days in smaller tanks. Once nitrate drops under 10ppm, the tank is safe for stocking. Waiting too long to stock the tank will cause a gradual die-off in the bacteria colony unless the empty tank is “fed” to keep the bacteria alive.

Live Rock Live rock is a term used for rock that harbors a population of beneficial bacteria and other organisms that are at the center of a biological filtration system. This rock is usually aragonite (calcium carbonate) from old coral skeletons. In a tank with a reasonably deep aragonite sand bed, bacterial colonies will form there as well, creating “live sand.” Although the bacteria can grow on other media, the porous types of rock used to create live rock often provides the best type of environment for bacterial and algal growth aside from the sand bed, which is particularly useful in small tanks where a deep sand bed can be difficult to maintain and may not be able to support the tank’s livestock by itself. Live rock may be housed out-of-sight in a sump or canister filter, but is also important in display tanks since it provides surfaces for grazing animals to feed on. Never assume that just because you bought something called “cured live rock” that it makes your tank immediately safe for animals. Unless you have a cycled holding tank ready, you should also never buy your cleanup crew at the same time as the live rock for a tank. Moving live rock from a store tank to a tank at home will always disturb it, and therefore one should always wait at least a few days to a week after adding the rock to make sure that no unexpected spikes in ammonia, nitrite, or nitrate occur. Any good pet store that sells marine animals will also at least periodically have stocks of cured live rock available for sale. Stores that intend to sell large volumes of live rock will have some kind of a large vat full of rock chunks and a somewhat complicated system for moving water in and out to keep good circulation. These vats should smell like a pleasant trip to the seashore – not like a trip to a raw sewage processing plant that’s experiencing plumbing difficulties. However, this may not hold for individual pieces of rock. Since rock is always being sold and restocked, rocks are likely to be at different

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stages in their “curing” process. If you want to know whether a piece of rock is a good choice, take it out of the water and smell it. Seashore or soil smell means it’s a good rock and ready to go in your tank. No smell at all, particularly if the rock is mostly white, means it’s a new piece of rock that and probably hasn’t even started the curing process yet. If your nose is punished with a foul stench, the rock also isn’t ready yet and is still experiencing some die-off from when it was shipped to the store. Of course, not every aquarist buys all of his or her live rock from a local store. Sometimes websites and catalogues offer pretty good deals on large volumes of rock, which may be more appealing to those wanting to set up large tanks quickly. This does carry the risk though that a bad event could occur in shipping (e.g. the rock is overheated, causing a lot of bacterial die-off) and rock could arrive at your doorstep that needs to spend as much as another month curing before it’s ready to support livestock in the tank. There are also those that want to make their own live rock by collecting other carbonates. Sometimes non-carbonates are also possibilities. Various types of igneous rocks, namely tuffs, can have excellent porosity and therefore surfaces for colonization. However, regardless of the type of rock desired, the seashore is not a place to find live rock. Many regions have strict laws making the collection of natural live rock illegal, although rock can easily be seeded with starter fauna from something as simple as a wet, well-weathered fragment of seashell (although collection laws must be kept in mind for shells as well). Bearing that in mind, if the rock is “dead” (e.g. not in contact with marine waters), aragonite, clean, and doesn’t cause noticeable pH drops when placed in a bucket of RO water for a few days, that rock is fair game to become live rock. If the pH drops, it could indicate hidden pockets of other minerals or organics and the rock should be cleaned thoroughly and tested again. If it fails the test repeatedly, pockets of other minerals such as sulfides are a more likely cause than organics, and the rock is not suitable for a marine aquarium. To avoid unnecessary risk, one should also have rudimentary knowledge of geology and mineral identification before looking for rocks to put in an aquarium. Although not to everyone’s taste where shape is concerned, limestone can make good live rock. Correctly-selected pieces can colonize well and be a very effective filter per pound of rock while providing a useful pH buffer. Other types of rock can have greater porosity, but silicates will provide little to no buffering capacity and will necessitate the use of a lot of aragonite elsewhere in the tank (such as the sand bed). With any such rocks, limestone or otherwise, it is possible to find pieces of limestone that carry small deposits other minerals as impurities and can negatively affect the chemistry of the water, so caution must be exercised. As described already, always soak new rocks in freshwater for several days and check for pH swings in the water before placing the rocks into a saltwater tank for curing. RO is ideal for this, but treated tap water can be used in its absence, provided that the pH is allowed to stabilize and is tested before the rock goes in. Once the rock is obtained and ready to cure, the next steps are simple. The rocks can be cured either in a dedicated tub or in-tank. Depending on the size, the tub/tank will need one or more powerheads for circulation or a strong air bubbler for smaller volumes of water. Then, a starter population of bacteria must be added. The easiest approach is to take an existing small chunk of live rock or a handful of live sand from an established tank and drop it into the curing tub. Alternatively, sludge from a canister or other type of

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filter can be used (useful in a curing tub, less advisable when curing in-tank). After that, the bacteria need food. Add some meaty food such as a popcorn shrimp and allow it to decay, providing the initial source of ammonia to kick off additional bacterial colonization. Additional meat should be added periodically as the old meat decays to ensure a continuous source of ammonia. Although it may seem rather morbid, animals that have died in marine aquaria from old age or injury can also be used to provide the decay, particularly if a live rock curing tub is kept going for some number of years while other tanks are running. This is also a useful way to clean shells from snails that have died of old age or injury. Of course, animals that have died from parasites or disease should never be used in this way, since it risks spreading the problem to any tank receiving the cured rock. To help maintain long-standing live rock tubs, dirty tank water from water changes on existing tanks and filter sludge can be added to the live rock tub. As the curing process progresses, white rocks should begin to turn brown, green, black, or other colors with time as they become colonized. If you supply light, you will probably see most exposed surfaces turn brighter colors due to microalgae. Check the cycle of the tub periodically to see when the nitrates have fallen, and then you’re ready to use your new rock. Make sure you don’t miss the nitrate drop by too many days; particularly if the source of ammonia has disappeared, you will then start to see die-off as the bacteria struggle to have enough ammonia to consume. Depending on how much rock you’re curing, this process can take anywhere from ~3 weeks for around 5lbs or rock to a few months if you want to cure an entire tank’s worth from scratch. Finally, there are those that just aren’t happy with the rocks they find at the store or in the backyard. Because all that is required of a suitable rock is that it be calcium carbonate, there are some innovative, cement-based solutions to creating live rock, although I will not discuss them here.

Tanks Designed Specifically for Hermit Crabs Most hermit crabs will fare just fine in a standard marine tank. This is the case for most of the small hermit crab species found in the aquarium trade. Larger hermit crabs and coastal species, however, may require some modifications to the standard marine tank design if the hobbyist wishes to accurately reproduce their original environment.

Access to Land On the whole, marine hermits don’t differ greatly from other marine animals in their environment requirements, but the details do vary between species. Your best course of action prior to putting any hermit crab in your tank is to determine what part of the world it came from and whether its habitat is likely to have had regular land access. Not all species are content to stay in the water 100% of the time, particularly if recently

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removed from the wild. If these hermits are put into an environment with no access to land, they may attempt to escape frequently. Should you wish to or need to provide land access for hermits, the best way to do so is with a low waterline and large rocks. Hermits are great climbers, so as long as the rocks have plenty of crevices for them to hang onto, the hermits should be able to climb without problems. Make sure that any piles of rocks are well-anchored though if you don’t want a landslide. The tank rim should be well above any rocks, such that power cords are exposed several inches above the waterline. If there is little distance between the edge of the tank and the waterline, it risks providing hermit crabs with a handy exit via the power cords or simply the tank rim itself. Some tidal species are good climbers and can be quite agile out of the water, making them difficult to track down if the initial escape isn’t observed. It’s also important to note that not every species that is forced to move across land in the wild will do so in the aquarium, provided the water quality remains high. Clibanarius vittatus is one example, and is eventually quite content to remain submerged even though they are forced onto land occasionally to move between tide pools in the wild. If easy land access is provided, some individuals may use it (particularly those that are recently caught), but most probably won’t as long as the water quality remains high. Along the gulf coast, particularly during the hotter parts of the year, C. vittatus that don’t follow the retreating tide get marooned in tide pools that rise rapidly in temperature and easily become depleted of oxygen. Exiting the water is more out of necessity than a love of land, since movement on land requires a lot more physical exertion from large individuals. Sometimes this species can be seen moving out of the water just for a quick breather before heading back in to the same hot water. Such conditions should be avoided in the home aquarium, of course, creating little need for the species to leave the water. Usually after some time in captivity, this species’ desire for land access diminishes to nil, although it can take several months before some individuals stop looking for a rock above the waterline.

Temperature It is a common misconception that all marine tanks should be run at 80ºF (27ºC) or just above that. These temperatures may lead to optimal growth for commonly kept coral and fish species, but often has bad consequences for a number of the more common snail and crustacean species, which may live at much lower temperatures in the wild. Consistent with the belief that temperature must always be high, other myths circulate that many snail and hermit crab species can only live for a year or less in captivity. This is often true at elevated temperatures, but easily shown false at lower temperatures. For example, snails that may only last for a year at ≥80ºF (27ºC) will have an extended lifespan at lower temperatures in the 72-76ºF (22-24ºC) range. When considering what temperature at which to keep a hermit crab tank, it is best to keep the temperature both within the temperature range experienced by the species in the wild and following a yearly cycle if one is experienced by the species. Annual temperature cycles trigger changes in metabolic activity in invertebrates and often are correlated with breeding seasons and periods of dormancy. If an animal is denied this

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cycle, particularly if kept at the higher range of its natural temperature range, its lifespan may be shortened and breeding cycles may occur abnormally frequently. Coastal species in the United States are among the species that may not tolerate constant elevated temperatures well in the aquarium. The Clibanarius and Pagurus species that occur on the Gulf and Atlantic coasts experience quite cold temperatures during the winter, living in shallow water where the ambient air temperature may near or drop well below freezing. For these species, peaks of 76-84ºF (24-29ºC) or warmer during the summer and lows of 50-60ºF (10-16ºC) would not be unusual, particularly in tide pools. Keeping these animals in an unheated, room-temperature tank will come closer to simulating the natural environment than will placing such species in a tank with an artificially raised, constant tropical temperature. Similarly, allowing the room temperature, and therefore tank temperature, to drop over the winter for these species will have no ill effects. Just as it is important to observe the naturally-experienced annual temperature ranges in coastal species, care should be taken with more strictly tropical species to avoid chaotic temperature fluctuations. Fluctuations of more than 4°F in a 24-hour period should be avoided for all species to prevent unnecessary stress on the animals’ metabolisms. Animals that experience relatively constant water conditions in the wild are likely to be stressed more by relatively rapid changes in metabolic activity due to temperature fluctuations. Gradual changes over the course of a year may not be problematic, but the aquarist’s aim should be to replicate the animal’s preferred environment as much as possible.

Equipment Precautions Probably the only place where any of the advice given so far on putting together a hermit crab-friendly tank would differ from most readily available information is in regard to the safety of certain devices such as powerheads and heaters. It particularly applies to very large and very small hermit crabs. Unlike the average fish, hermits have a remarkable ability to manipulate their environment. If something is within reach, the odds of a hermit attempting to interact with it in some way are fairly high. Large hermits can remove safety covers from powerhead intakes, and even remove the backing from smaller pumps that covers the impellers. For these species, a simple sliding lock is not enough to ensure that a powerhead won’t be dismantled, putting the hermits at risk of injury if they get access to the impeller chamber. Choose your pumps and powerheads wisely to either make sure that the impeller chamber is even difficult for you to get to or is inaccessible to tank creatures in some way (e.g. the case of a canister filter). For heaters, the primary risk is associated with water entering the glass tube on heaters that utilize one. Although not a problem with small hermits, it is better to place heaters in sumps for tanks with large hermits. Smaller hermits are not at risk from prying equipment open, but unprotected powerhead intakes are still a hazard. If the suction is too powerful and are accessible in some way, small hermits can become trapped on the intakes. This problem can be easily avoided by fitting a 3-4” (7.5-10cm) tube with very small holes or slits cut into it and a covered bottom onto the intake. The small ½-1” (1-2.5cm) covers that are built in to

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many powerheads may not be sufficient to protect smaller hermits while giving them something to grab onto to move if the suction is a bit strong. For the best protection, in addition to the piece of tubing, a sponge should be wrapped around the tube to make it more difficult for hermits to become stuck.

Types of Aquariums Suitable for Hermit Crabs There are a wide range of possibilities when setting up any marine tank. Hermit crabs aren’t usually picky, and a species tank for hermit crabs has more options than most reef tanks. The following section contains a few suggested tank designs.

Standard Marine Tank

Tank size: “nano” sizes of 10-20 gallons are ideal for most hermits. Larger, messier species such as Dardanus megistos may require bigger tanks if many individuals are kept, and it may be useful to include a sump for additional filtration to avoid filling the tank with live rock. A sump is especially useful if the tank also houses fish. These tanks may be open or closed-topped. Considerations: This is a standard marine tank design. If using multiple stacked rocks rather than one large rock, the rocks should be glued together to avoid toppling. Never stack the rocks against the tank wall if the tank is glass or acrylic. 20

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Hermit crab-only tanks don’t require much lighting. However, if the lighting is powerful and prone to heating up, as is the case in small reef tanks, it is a good idea to mount the fixture(s) in such a way that air movement around the fixture is possible. Mounting the fixture higher will also reduce the amount of salt it accumulates. If the tank is open-topped, leaving some space between the fixture(s) and the water allows the option of placing a small fan on the side of the tank and aimed at the water to further prevent heat build-up.

Tank Adapted for Coastal Species

Tank size: “nano” sizes of 10-20 gallons are ideal for maintenance purposes. For larger tanks, a custom tank with more horizontal space and proportionally lower tank walls may be preferable, since less vertical space would be wasted. Plastic tubs can serve this purpose just fine, although many people find them unsightly. However, they work well for replicating environments in a top-view setting with minimal cost. Considerations: If using multiple stacked rocks rather than one large rock, the rocks should be glued together prior to being put into the tank to avoid toppling. Never stack the rocks against the tank wall if the tank is glass or acrylic. Also, although it may be appealing initially, trying to create a tank that has a simple sand or gravel slope will not work well. Hermits will dig in the substrate and generally disturb it, an activity that will rapidly flatten the sand bed by creating small landslides or simply by gradual substrate agitation. The amount of space above land may not need to be much. Some species spend a lot of time above the waterline in the wild simply because the tidal pools they travel between can grow quite stagnant. Provided with better water, they may prefer to stay submerged the majority of the time.

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“Pico” Tank

Tank size: “pico” tanks <5 gallons work best with this setup. The shape need not be round, but fish bowls are often made of sturdy, continuous glass and are cheap for their size. Small acrylic tanks intended for freshwater betta fish can also make good tanks. If an in-tank pump is not used, the tank must be small enough that sufficient water circulation can come from the air stone alone. Tanks that are bigger than 2 gallons may need a small powerhead, but temperature must be monitored closely if one is used due to most powerheads’ capacity for heat generation. Desk lamps are acceptable for these tanks provided they will not contain corals. Considerations: The lamp needs to be low enough wattage that it will not overheat the tank. Alternative lighting methods exist, such as LED lights and small, clip-on aquarium lights. LED lighting systems can avoid many of the heat-related problems other lighting systems present for small tanks. Hermits must be sparsely stocked in such a tank, and generally must be the only animals unless small, compatible snails exist. Clibanarius digueti can be stocked with small snails in tanks this size, but other, larger species are best kept by themselves. If intended for a Calcinus species, only one individual hermit should be in the tank. The small Clibanarius can be stocked as a pair or small group depending on how much space is present.

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Tanks with Deep Sand Beds

Tank size: any, although a “pico” tank (1-5 gallons) is shown above. When considering small tanks, many fish bowls are oddly shaped or tapered in some way such that they would not provide much substrate surface unless filled quite high with sand. When the sand level is raised until it provides a reasonable area for hermit crabs to run around, this leaves little room for rock. The sand bed in small tanks like this becomes the primary source of filtration for the tank. While these tanks can be very stable if set up correctly, deep sand beds are easier to maintain and therefore a more common practice in larger tanks. Considerations: this is a not a good setup for a complete beginner to attempt if the tank is small. Deep sand beds can be tricky to establish and keep healthy, especially in small tanks where common sand-sifting animals are impossible to support. The sand bed will also require a longer setup time and will need to be seeded with many small invertebrates, particularly beneficial Polychaete worms. Live rock that hasn’t been in contact with an established sand bed may not contain enough of these animals, so a substrate sample from an established sand bed may be necessary in smaller tanks. A healthy sand bed will grow progressively darker farther down and will not show any signs of large gas bubbles building up. The buildup of large bubbles indicates that the sand is not being disturbed enough (worms and other burrowing animals will help avoid this).

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Macroalgae The reader will note that I include macroalgae in each example. While macroalgae is another subject of hot debate among aquarists, I consider it to be a very beneficial addition to non-reef aquaria. Particularly for hermit crab tanks, it provides both a nutrient sink as well as a potential food source. In reef aquaria, caution is warranted with macroalgae unless placed in the sump, in part because some algaes are aggressive enough to encroach on corals. Small fluctuations in pH that the algae can cause in small tanks on a daily basis may also be undesirable in a reef tank. Regardless of those situations, the primary complaint against the algae is the tendency of some species to release spores or die suddenly in response to an unstable environment or physical damage. This is often cited as the cause of ensuing tank crashes, although I have heard of very few such cases where there wasn’t another underlying issue contributing to the problem.

Managing Die-offs/Spores Macroalgae seems to have its own set of myths and superstitions surrounding its maintenance, particularly concerning avoidance of the dreaded spore-release or unexplained sudden die-off. When macroalgae releases spores, portions of the algae will change shape and release streams of small green particles. A wispy, sometimes white husk is left once this process is complete. Macroalgae can also die from sudden fluid loss or other damage, and the algae may become soft and sludge-like over the course of a few days if not removed from the tank. Some propose that light must be on a strict 12h on 12h off schedule to prevent spore release and/or death, while others assert for the same species that 24h lighting is the only way to ensure that neither event occurs. In reality, neither of these approaches is actually a failsafe, since either dreaded event can be triggered by something as simple as physical damage to the algae by a large snail. However, as far as I have observed, neither spore release nor die-off is as dreaded as most aquarists think, particularly if the tank is equipped with additional means of nutrient removal such as protein skimmers. Some sensitive corals may also not tolerate any mild parameter shifts that cold occur in a small tank with a large macroalgae die-off. Over the years, my tanks (including those with soft corals) have experienced such events numerous times due to large gastropods take the occasional nibble on the macroalgae that they would normally ignore. It has been my experience that something as simple as an air bubbler can make the difference between gasping fish and an otherwise content tank that simply happens to have turned green overnight and requires an earlierthan-normal regular water change or two. Sufficient surface agitation can prevent the formation of films that could inhibit gas exchange. Oxygen deprivation is commonly a problem with dying macro algae when the tank has no backup source of surface agitation or aeration. The fastest effect that a mass die-off of macroalgae usually has on a tank is a decrease in dissolved oxygen. This can be for two reasons: breakdown of the released organics (although that usually takes longer,

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since freshly released particles won’t decay within minutes), or formation of a film on the surface of the water. A film or small layer of foam forming on the surface is very bad indeed, since it inhibits gas exchange with the surrounding air. When this happens, fish will start gasping and invertebrates will head to the surface of the water or exit the water completely. Invertebrates that don’t make it to the surface may lie on the bottom of the tank sluggishly. Other invertebrates may seize this opportunity to have a snack if they are less-effected by the drop in oxygen. An extra air supply to the tank will easily cure this problem and break up the film or prevent it from forming. Sometimes a bubbler with small enough bubbles will have a skimming effect on the film and cause the gradual accumulation of the excess particulates on the walls of the tank above the waterline. While the tank will be an unsightly green color, the animals in the average community tank will not come to harm over the course of a day or two if they have sufficient oxygen and there will be plenty of time for the hobbyist to perform water changes. If left completely unchecked, the large amount of excess organics released into the water can eventually cause ammonia levels to rise, so it is still important to try to dilute the pollution to more manageable levels rather than to simply ignore it.

Types of Macroalgae The most common genera of macroalgae sold in the aquarium trade belong to the Chaetomorpha and Caulerpa genera. As a nutrient sink, Chaetomorpha is preferred by most aquarists, since it is very hardy and unlikely to die or release spores in response to either physical damage or parameter shifts within reasonable ranges. Chaetomorpha is a very good addition to a sump or refugium for these reasons, since it is also very unlikely to spread to the main tank. It can also be added to clear canister filters that receive lighting. It makes an effective form of nutrient export, since it can simply be trimmed back as it grows without fear of causing the remaining portions to die. Caulerpa species are more controversial, since they are somewhat fragile and more likely to release spores or die if damaged/stressed. Large snails can bite through the stems, triggering spore release or fluid loss. Trimming the algae can also cause this type of response unless it is first tied off or pinched for several minutes at sites where it will be cut in order to constrict the vascular structures. Species of Caulerpa with broad, flat, leaflike structures are generally hardier than their feather-like and grape-like relatives. Feather Caulerpa species are some of the most fragile. Despite the fragility, Caulerpa species can make a nice addition to a tank and can be kept without problems, provided that they are properly maintained. Whether Chaetomorpha or Caulerpa is used, it is a good idea not to put both in the same immediate environment. If Chaetomorpha is allowed to grow around the stems of Caulerpa, the risk of Caulerpa releasing spores increases since it will be progressively choked out by the Chaetomorpha. If left unchecked, the Chaetomorpha can form a dense mat near the surface of the water that deprives any deeper algae of light. Removal of the Chaetomorpha becomes nontrivial if left to grow to this extent, since it is difficult to remove the Chaetomorpha without damaging the other algae. Chaetomorpha is best

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allowed to grow in an environment where the density it reaches will not adversely affect other organisms. Other soft macroalgaes occasionally appear in the aquarium trade, although they may be quickly eaten by both fish and invertebrates and may also prove difficult to grow due to their lighting and/or temperature requirements (common problems for many species easily collected from beaches). Chaetomorpha and Caulerpa are more commonly avoided by hermit crabs and snails than other genera of algae. There are also some calcareous macroalgaes available in the aquarium trade, although these may be at risk of being damaged in tanks containing large hermit crabs. Some coastal, temperate algaes can also be kept in hermit crab tanks, although the softer algaes often make better hermit crab food than anything else. Species of “sea lettuce” in particular are quickly eaten by many hermit crabs, even those that may ignore other types of algae.

Livestock Acclimation So you have your tank, live rock, substrate, equipment, etc. all set up and the aquarium is cycled. It’s ready for stocking. You go to the pet store and bring home a bunch of new pets in little plastic bags partially filled with water, eager to introduce them to their new home. This is the point at which many new hobbyists have their first catastrophe: improper acclimation. Improper acclimation is the cause of many invertebrate deaths, which many not directly follow the acclimation. Sometimes the damage done is slow to have effects, and the animals may die anywhere from minutes to a week later after exhibiting odd behavior. The most common symptoms of a bad acclimation are sluggishness followed by sporadic twitching over the course of a couple of hours after starting acclimation. These symptoms can even occur before the acclimation is finished if the initial change in parameters experienced by the animal(s) is too fast. Although not true of all marine hermit crabs, many marine animals live in environments where parameters like salinity, kH, and pH change very gradually, if at all. These animals may not react well if suddenly thrown into a new environment that differs greatly from what they were in previously. Coastal animals can endure greater shifts in environmental parameters, but if you want to minimize risk and aren’t 100% sure of what an animal can tolerate, use one of the following acclimation methods when introducing it into a tank. Acclimation for hermit crabs can be done successfully over approximately within a couple of hours depending on how different the bag water is from the tank water.

The Drip-Line Method This is the safest approach. Take some airline tubing, punch a small hole in the bottom of a soft plastic cup, and glue or wedge one end of the tubing into the hole. You’ve just set up a piece of equipment for siphoning water, with the cup being used to start the siphon. Cut the airline tubing midway and insert a valve for controlling water

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flow (a simple, cheap plastic air valve will work). Set up the siphon, and regulate the flow going from the tank into the bag at a rate of a 2-3 drops per second. Monitor the salinity and pH of the bag when the water inside has doubled in volume. If the pH differs by more than 0.1 or the salinity differs by ≥ 0.001, poor out half the bag water and repeat. Once the parameters are sufficiently the same between the bag and tank, float the bag if necessary to reach temperature equilibrium over the course of ~5-10 minutes. Hermit crabs are best acclimated over the course of 1 hour, but other more fragile invertebrates, such as echinoderms, may require up to 3 hours. Most fish can be acclimated faster using the floating method.

The Floating Method Floating acclimation in the marine world is not the same as tossing the bag into the aquarium for 10 minutes to reach temperature equilibrium and then transferring animals. Instead, it involves floating the bag (or preferably clipping it to the side of the aquarium) to keep temperature equilibrium and periodically changing out 1/4 to 1/3 of the bag water, replacing it with tank water. This is a rougher type of acclimation that, although handled well by fish, is not recommended for any invertebrate. It should also be strictly avoided when there are significant differences between the bag water and tank water, since a big water change with drastically different new water can be enough to shock more fragile animals. An alternative method for invertebrates that is more similar to the drip line method is to float the bag in the tank clipped to the tank rim, and add small amounts of tank water every few minutes. Half the volume of water in the bag should be removed after it doubles in volume, and the process repeated until the bag water matches the tank water. This method can be very preferable to a drip line when temperature is an issue. If the room is significantly cooler than the tank water, keeping the bag in the tank during acclimation will avoid additional stress on the new animals’ metabolisms.

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Diet As addressed in the myths section, the notion that any of the commonly available hermit crab species are fully herbivorous is incorrect. Many hermit species can play an important role in tank maintenance by picking over the rockwork constantly, but they are rarely a flawless cure for excessive amounts of algae. Although algae is on the menu for a good many species, that doesn’t mean that they’re strictly herbivores. Some species, particularly larger ones, will do an effective job of destroying initial algae outbreaks in the aquarium but don’t actually eat any of it, thereby allowing it to establish elsewhere. The result may simply be a more widely spread algae problem in the tank. This behavior would seem to be driven by an attempt to search for better food sources, and algae simply happens to get destroyed along the way. Some species can survive on a more herbivorous diet if no other foods are provided (although it is not a good idea to restrict any species to this – some sort of higher-protein food should at least periodically be made available, otherwise tankmates may be on the menu), but others can’t. Small Clibanarius species may be content to live on a diet of seaweed for long periods of time, but others require at least some meaty foods or prepared carnivore foods, otherwise they will start to view their tankmates as protein sources. The large species, Clibanarius vittatus, eats algae in the wild and scavenges opportunistically. In the aquarium, however, it will generally ignore algae unless no other food is provided, preferring meaty foods whenever they are offered. A few less common algae species in the trade, such as those in the Ulva genus, are exceptions that will be readily eaten. It should also be noted that hermit crabs are not necessarily adapted to tank life where food is provided at a specific time on the dot each day. In the wild, this doesn’t happen. Hermits have to take meals when they come and dead animals that are especially appetizing will also act as a magnet for other predators that might consider hermit crabs a side dish. Behaviors adapted to this sort of life style mean that hermit crabs, although sometimes requiring target feeding, should be fed carefully and watched closely to ensure that the food is actually eaten. Never give a hermit crab a large piece of meat that it can’t eat in one sitting unless the food is intended for multiple animals or plenty of other scavengers are present. If only given to one hermit crab, the hermit will either take a nibble and leave the rest to float away, or it will eat the meat as it runs away, disappear behind a rock someplace, and appear again a few minutes later looking for more. The hermit crab didn’t magically eat the entire meal during that time, but rather stashed the food. Hermits and other crustaceans, such as true crabs, can stash food in clever places when given the opportunity and an over-abundant food source. If no other food is provided, they can go back and eat the leftovers later. Sometimes they do, but not if they get another big chunk of meat before they become hungry enough to eat the old one. Why eat the old stinky food when there’s new food? Unless you have an impressive scavenger population in your tank, if hermits are allowed to accumulate food stashes, you’ll have a nice surprise waiting for you when you decide to clean the tank after noticing a funny

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smell coming from the water. Empty barnacle shells and small caves are favorite spots to place uneaten food. The best ways to avoid this stashing behavior is to simply remove the hermit’s ability to make food stashes. This can be done in the following ways: 1. Only feed prepared, pelleted foods. Hermits can’t cart large volumes of these around as easily and usually have to sit and eat pellets wherever they’re dropped. The pellets will break down rapidly and be easy food for other scavengers, such as Nassarius snails. 2. Intersperse seaweed with meaty foods. Hermits don’t tend to stash seaweed as frequently in my experience with them, and when the seaweed decays it doesn’t produce the same rank concoction that meaty foods can. If meaty foods are only given scarcely, even if they’re stashed they’ll eventually be dragged out and eaten again. 3. If you have to feed using large pieces of meat (squid, for example, can be difficult to chop finely because of how slippery it becomes as it thaws), anchor the food someplace. Don’t make it possible for the hermits to run off with the food. This may require some creativity on the part of the hobbyist when large species are involved.

Figure 1. A female Clibanarius digueti digs for food.

Suggested Foods Feeding hermit crabs can be expensive, or it can be done very cheaply. Here are some foods that work well for commonly kept marine hermit crabs. The best diet for any animal includes some variety, so a combination of several the following is ideal rather than just one or two items. •

Fresh foods o Fish, offered as a supplement. o Seaweeds and macroalgae, offered as a supplement or allowed to grow in the tank.

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•


Frozen foods (thaw before feeding) o Popcorn shrimp or prawns, which can form the bulk of the diet or be used as a supplement. o Krill, which can form the bulk of the diet or as a supplement. o Fish, offered as a supplement. o Seaweed (usually in cubes), offered as a supplement. Dry foods o Dried seaweed or Nori, offered as a supplement. o Krill/shrimp sinking pellets. These can be used as the main part of the diet for most hermit crabs. Dried krill will often float even after soaking and may need to be weighted down. o Other sinking marine carnivore pellets, which can be substituted for krill pellets.

Molting Hermit crabs must shed their old exoskeletons and produce new ones to grow and heal from injuries This process is called molting. Hermit crabs will molt many times when young to grow rapidly and then the molting rate slows. Depending on the species, adults will molt anywhere from one every couple of months to only once or twice per year. Injured hermit crabs may molt after a shorter than normal time period to regenerate lost or damaged limbs. Hermit crabs molt by backing out of their old skin just behind the carapace. Limbs and other eventually hard body parts will be soft and must be extracted through the old skin. The part of the skin covering the soft body that stays hidden in the shell peels off in pieces. Molting hermits will rotate the shell backwards, crawl partway out, and then back into the shell slowly again, leaving the old skin behind. Small animals can perform this process quickly in under a minute. Larger animals may require more time. Molting is a delicate time for all crustaceans. They remain soft for some time after they have molted and are vulnerable to attack from fish and other crustaceans. In mixedspecies environments, molting animals may be subject to attack from other hermit crabs. Same-species attacks may occur as well if the tank is too densely stocked. Avoiding these kinds of casualties is one of the motivations for stocking only a single species in small tanks and stocking sparsely. If a hermit crab is in a shell that is too small, it may have to leave its shell briefly to complete the molting process. This situation should be avoided by ensuring that plenty of larger shells exist. Hermit crabs experiencing such a molt are at great risk of injury and predation. When in possession of s sufficiently large shell, hermits will disappear entirely into the shell for safety after molting and may not reappear until their new exoskeletons have hardened.

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Common Community Tank Problems The view put forth by many in the trade is that a marine tank has some number of relatively uniform niches to fill in order to create a complete ecosystem. While this idea is a good way to present the ecology of a marine tank in brief, it is massively oversimplified and, as a result, responsible for many tank janitor-related problems. In the case of hermit crabs, these problems often get blamed on the demeanor of the animals rather than the aquarist’s stocking decisions, and thus hermit crabs have acquired a bad reputation as troublesome, unpredictable little animals that happily wreck their environment. In reality, they’re no more troublesome and unpredictable than the average omnivorous fish. What makes the difference is that many people will have more realistic expectations about the fish than the hermit crab. A hermit crab plucked at random from the aquarium trade is not a cookie-cutterstyle animal that can be used to fill a generic role in an aquarium. To assume otherwise is no different than assuming that a fish is a fish. There are many common species that grow to different sizes and have different behavior, just as is true for different fish species. The key to avoiding a cleanup crew disaster is as simple as employing careful observation and research instead of filling a tank with mystery critters. The marine aquarium community is often strongly divided on the decision to use hermit crabs as part of a tank’s cleanup crew. The most frequent complaints originate from problems involving snails, sessile invertebrates, and particularly other hermit crabs. Often the problems are simply attributed to the fact that hermit crabs were included in the tank, and hermits are written off as incompatible and useless. However, for cases involving commonly kept species, there is often an underlying problem that is correctable. Unless the species are very poorly selected relative to their environment, problems are unlikely to be caused by unruly hermits bent on destruction being introduced into an otherwise happy tank.

When Hermits Aren’t to Blame One of the most common types of mistakes people make when assessing community tank problems is the following type of statement: “if A was seen eating B, then A killed B.” Although that may be a tempting assumption to make, the conclusion that A killed B is only logical if one or more of the following are true: 1. A was observed in the act of killing B. 2. A is a known predator of B. This can be established by prior occurrences of case 1 or as documented in scientific or other reliable literature. 3. A had a probable way and reason to kill B. This includes cases 1 and 2, as well as cases where A is under sever stress or starvation conditions or when A is known to be territorial and was defending its territory. Still, A must obviously be physically able to cause harm to B.

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4. A could easily kill B by accident. This is mainly the case when A is large and strong while B is small and very fragile or if A is poisonous in some way that could harm B. These criteria may seem obvious to anyone with a bit of experience in the aquarium world. If none of the above situations hold, the notion that A killed B is an unfounded assumption – yet it is a frequent assertion made whenever someone’s prized fish or invertebrate turns up dead overnight and is seen being eaten by something else. It is an assumption made over and over again regarding hermit crabs, particularly where dead snails and other invertebrates are concerned, often ignoring any problems when trying to reconstruct how the crime was committed. It’s the sort of faulty logic that results in people panning hermit crabs as beneficial animals in an aquarium and asserting that all hermit crabs are destructive in nature. Case 1 is obvious and conclusive evidence of one animal killing another, but the observer should be certain that the animal being attacked is not already dead or so severely compromised by illness or prior injury that it could be perceived as food by a scavenger. An animal physically compromised by illness or injury may be preyed upon by animals that would not attack it otherwise. Nassarius snails are known scavengers, so when they go after a dying animal nobody bats an eye – but crabs are known predators as well as scavengers, so when they do the same as a Nassarius snail, they rarely get the same treatment. Case 2 cannot be established by something heard anecdotally, as there are simply too many people in the hobby who fill in gaps in observation and lack of concrete proof with unfounded supposition. Proof for this case instead requires some kind of formal documentation that proves case 1. An example of this would be the case for Petrochirus diogenes and the queen conch, Strombus gigas (Brownell and Stevely 1981, p5). Case 3 requires an assessment of whether assumptions are physically possible and likely given the circumstances. Is it realistic to assume that a juvenile “dwarf” species of hermit crab really killed a rather large, adult Turbo snail it was seen snacking on? Probably not, since it fails the test for case 3 by being a physically unrealistic assumption. On the other hand, the same hermit crab seen snacking on the body of a small snail removed from its shell is a different story – particularly if the snail’s shell is now inhabited by the hermit crab in question. That becomes evidence in support of case 3, showing possible motivation for the hermit crab killing the snail (desire for a new shell). In all likelihood, a hermit crab is not the culprit in a mysterious animal death if the dead animal meets any of the following criteria: A. Significantly larger/stronger/heavier than the hermit crab. B. Moves very fast when healthy (e.g. fast moving fish like tangs). C. Would release tank-nuking toxins or ink if attacked, injured, or severely stressed, yet everything else in the tank is fine. D. Heavily armored such that there is no obvious way the hermit crab could pull it apart to attack it. All four of these fail the test of probability and reasonability stated in case 3. Hermit crabs are opportunistic scavengers. If something dies and isn’t poisonous, they’ll eat it.

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This is actually a good thing, since it can cut down on the damage done to the water quality from a rotting carcass. Other animals often die as a result of poor tank conditions that hermit crabs can temporarily survive – many hermit species are remarkably hardy and can withstand ammonia and nitrite spikes that some fish and particularly gastropods cannot. Similarly, if an animal is near death for unrelated reasons, a hermit crab will not be above taking advantage of the situation if it becomes hungry. Even in this case, the hermit crab is still not the real source of the problem. Quite simply, if a hermit is seen eating a dead animal and the method of the animal being killed isn’t immediately obvious (e.g. a snail freshly dead snail pulled fully out of its shell, while dead snails only “fall out” of their shells after decay and impressive stench have set in), check the environment first and look for other potential aggressors before condemning the hermit.

Compatibility with Snails and Other Sessile Invertebrates Hermit crabs use snail shells as their protection; this should immediately suggest that combining hermit crabs with snails should be done with care. Indeed, many hermit crabs will attack snails for food and to get a new home. Smaller, weaker species will only do this as a last resort, particularly if plenty of other potential homes are available and food is ample. It is therefore important to pay careful attention to the diet, growth rate, and preferred shell shapes for hermit crab species. Many hermit crabs that are useful for removing nuisance algae from marine aquaria are not strictly herbivorous. The primary genera used as “janitors” in marine aquaria are small Clibanarius and Calcinus species, Paguristes cadenati, and the occasional small Dardanus. Information regarding the behavior of each of these species towards snails is often contradictory. For example, Clibanarius tricolor will be declared incompatible with snails by one aquarist and snail-safe by another aquarist. The difference lies in the choice of snail species and the size of the snails introduced into the aquarium, in addition to other factors such as food and shell availability. Snails classed as “grazers” and included in tank cleanup crews are strictly herbivorous, which immediately introduces a potential problem if hermit crabs are also introduced to also remove algae. Snails and hermit crabs are easily overstocked, leading to food source problems. Grazing snails can only eat the algae, while hermits don’t have the same limitation. When the algae in the tank disappears, the snails will be at risk of starvation and predation. The hermit crabs will also be struggling to find food unless there are sufficient leftovers present from fish foods, or unless the hermit crabs are target fed. If neither is the case, snails and sessile invertebrates smaller than the hermit crabs are the next most convenient food source. Hermit crabs will generally scavenge other meaty foods rather than put out the effort to attack snails (unless the hermit crabs are in need of more suitable shells), so if the food and housing supplies are sufficient, the safety of the snails is more assured. An easy way to avoid the problem of hermit crabs attacking snails for the purpose of shell-stealing is to simply not stock hermit crabs with snails that are within a size and shape range where the shells would be useful to the hermit crabs. An example of this

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would be stocking “dwarf” hermit species with adult Turbo snails. For a small hermit, taking on a well-armored snail many times its size would be a tremendous waste of energy and probably be a futile effort unless the snail is already compromised in some way. One combination that is a recipe for disaster is that of young individuals of Calcinus species (particularly Calcinus seurati) and marine Nerite snails. Those snails have a shell shape that many Calcinus hermits prefer, and most of the Calcinus species have a combination of claws that are effective can-openers. For the sake of the snails, don’t combine species where an obvious risk exists.

Compatibility with other Hermit Crabs Another common complaint among aquarists is the “hermit crab war” phenomenon. In my time working with hermit crabs, I’ve heard and read numerous such cases involving a group of small hermit crabs being introduced into a tank and a short time later, usually over the course of a few weeks, only one or two hermits remain due to constant fighting. These cases are generally attributable to three problems: competition over food and/or shells as was already discussed, mixing hermit species, and having an unbalanced number of males and females. The situation arises when the group of hermit crabs is in a non-ideal situation (overstocking, poorly chosen species combination, etc.), and then something in the environment changes just enough to push the conditions over the edge and trigger aggression. Some examples would be adding an empty shell (or sometimes even a snail) to a tank where all shells are already occupied by hermit crabs and introducing a new individual, particularly one of a different species, into a heavily stocked tank.

Mixing Species Imagine a world where two groups of people live on continents that are very far apart. The languages have little or no overlap, cultural customs are vastly different, and the two populations may have never seen each other. Each population is also very xenophobic when survival is an issue. All of a sudden, handfuls of people from both populations are transported onto a small island in the middle of nowhere with nothing to eat. Several days of fear and hunger later, a cake magically appears in the middle of the island. What do you think the odds are of a civilized attempt by the two parties to discuss the situation and divide the cake equally amongst everyone on the island? Perhaps the odds aren’t zero, but a success rate of 100% is unrealistic. While this silly example may sound contrived and irrelevant, some aspects of the situation aren’t too different from those that arise when randomly selected hermit crab species are thrown together (and hermits in these cases don’t even have the benefit of being the same species). A few of these and a handful of those get sprinkled into a new tank with little or no food and what a surprise – some of the time it doesn’t work out! Aquarists aren’t amazed when various fish species turn out to be territorial or aggressive, but the assumption is too often made that hermit crabs are just a passive service provider that can

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be added to a tank. If something just performs a task, why not have some red ones and some blue ones for variety? There are, in fact, some very good reasons why not. Mixing species of hermit crabs is a risky choice to make in any aquarium when the hermits are being added as tank janitors that have to compete for very limited food. Although hermit crabs have a number of similar communication behaviors that span across species, individual species can also exhibit very specific behaviors that simply don’t occur in others. Clibanarius vittatus, as well as its smaller Clibanarius relatives, have a well-defined set of leg and pincher movements used to show aggression. Aggressive individuals will perform a somewhat elaborate dance of posturing and legwaving. Dominant individuals will also perform a very deliberate “head-butting” behavior at times, where the aggressor carefully lines up the shell apertures before forcing them together with one quick bang. In contrast, an aggressive Pagurus longicarpus is more likely to run up and jump on top of another hermit, shake its shell violently for no more than a second or two, and then lose interest and run off again. Two different behavior sets for two different species. When a food source becomes scarce, it is also more adaptive for a species to attack a member of a different, competing species than it is to attack its own species. Hermit crabs may be pushed to attack each other during shedding if insufficient space is present in a tank, but will also fight over food and territory the rest of the time as well. Although ample food and space go a long way to eliminating same-species aggression, inter-species aggression occurs fairly frequently when neither food nor space was an obvious problem. To avoid these problems, there’s a simple solution: don’t keep more than one species of hermit crab in a marine tank, particularly small tanks. Some species combinations may work in one tank but not in another, depending on the exact environment. Since it is difficult to predict the outcome of any given species combination in a tank, it is therefore safer to not have to try to predict the result and simply not mix species unless you want to turn your tank into a population experiment.1 While I have not taken the time to collect concrete statistics on which mixedspecies environments lead to problems, the vast majority of problematic cases I have heard and read about from other aquarists have involved two species: Clibanarius tricolor and Paguristes cadenati. The result is commonly that the P. cadenati win after obtaining a larger size (in fact, I cannot recall any examples of C. tricolor being the victor). In “nano” and “pico” tanks, the attrition sometimes continues down to just a single, large P. cadenati. Clearly achieving harmony between these two species is tricky, so why do so many hobbyists attempt it? My only explanation currently is many hobbyists seem to treat small invertebrates as a collectable, like stamps. I have nothing wrong with invertebrate collecting, but please, for the sake of the animals, don’t put them all into the same small tank!

1

For the hobbyist who does want to set up a tank as a population experiment, spare tanks to accommodate immediate species separation are a necessity, and the experimental tank should not be so complex that the species interactions are difficult to observe. The experimental tank should also be under very close observation in case disaster strikes and the species need to be separated.

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Same-Species Aggression Aggression within a species can occur due to the following: too cramped an environment, not enough empty shells present for hermits to grow into (causing one or more individuals to be stressed when seeking new homes), insufficient food, incompatible species, and a high number of mature males relative to females in the same tank. The first four are the most common causes, with the last being a relatively rare issue that tends to be most noticeable in densely-populated species tanks. If there appears to be too much aggression, removal of individuals from the tank is the first step to alleviate those problems since insufficient food is by far the most common cause of scrappy behavior. If the aggression persists, careful attention should be paid to the behavior to see if any hermits are engaging in courtship behavior (described later in this document). Male hermit crabs will demonstrate aggression towards each other when attempting to court and mate with females. This aggression may be eliminated if competing males are separated. Large-scale fights among a single species may also occur when shells are scarce. If there are no free, clean shells2, dropping a single, empty shell into a tank is enough to create chaos. For this reason, it is advisable to always keep plenty of empty shells in the tank. The shells should have a range of sizes such that none of the hermit crabs will feel stressed if it sheds and requires a larger shell to move into.

When Hermit Crabs Really Are Unruly Animals Bent on Destruction Not all cases of death and destruction involving hermit crabs in a marine tank are due to simple ecological imbalances such as lack of food or competition between species. Occasionally there is a massive species incompatibility present. These cases usually involve large hermit species that can be preferentially predatory and often grow to inhabit shells more than two inches across (although there are some smaller species that are just as destructive for their size). Hermit crabs that are strongly predatory on other sessile invertebrates and grow to large sizes do not belong in the average community tank any more than large predatory fish do. Some of the larger and/or more aggressive hermit crab species may not even be compatible with other individuals within the same species due to food and space needs, unless the tank is sufficiently large. The existence of these species in the aquarium trade makes research and careful observation of any new hermit crab in a tank essential to avoiding disaster. If kept in a species tank, aggressive hermits can make for quite an interesting aquarium, but extra precautions may be required regarding equipment and tankmates. 2

Cleanliness of spare shells is extremely important! Shells inhabited by bristeworms or pest anemones may be cleaned out by a non-stressed animal out of curiosity, but a stressed animal will often pass over shells that are not immediately inhabitable. Ensure that spare shells are regularly purged of sand and other debris. If unwanted animals are seen taking over spare shells, the shells should be dried, washed thoroughly to remove any debris, and returned to the tank.

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Signs of Impending Doom and Recommended Solutions Significant aggression resulting in injuries or deaths can usually be caught long before any individuals get severely damaged. The following are some common symptoms of problems that may not be too bad when first observed, but are easily corrected and can quickly become worse if ignored. Problem: one or more hermits are seen running around the tank shell-less for more than a minute or two. Solution: something is causing these hermits to be bullied out of their shells, which puts them at risk of predation and injury to their soft bodies. Supply more shells or remove larger hermits that do not exhibit this behavior, since they are the most likely culprits. If it is unclear which hermit is the culprit, watch closely for shell-clacking and shell-shaking behavior to determine the aggressors. Any hermit seen stranded without a shell should be immediately presented with another shell for protection even if the fit isn’t perfect. Otherwise, the odds of that individual surviving long-term are low. Problem: multiple hermits suddenly turn up with missing claws and limbs. Solution: one hermit missing a few legs may just be a bad molt. Many turning up like that, however, is a classic sign of aggression caused by overstocking and/or mixing species. Hermits missing both claws and/or with multiple missing limbs in a community tank or a tank with multiple other hermit crabs should be isolated immediately so they can recover. Alternatively, for species tanks, hermits with no injuries should be removed temporarily. Consider stocking fewer hermits and, if multiple species were involved, only stocking one species. Also look closely for other non-hermit aggressors if the situation persists. True crabs are common culprits and will happily remove limbs from other crustaceans. Problem: one hermit crab repeatedly bullies or drags other hermits out of their shells. Solution: this hermit needs a new home for one reason or another. There aren’t enough suitable shells available so it’s left with one alternative: steal one from someone else. Solving the problem is as easy as adding a handful of new, suitable, CLEAN shells. Include shells that will give the largest hermit ample growing room while not being so large that it will hinder mobility. Problem: serious fights break out, but only at feeding time. Solution: either there isn’t enough food to go around or the food isn’t spread around enough. Try feeding in multiple locations at the same time. Some species, such as Pagurus longicarpus, may require as many pieces of food as there are hermits to avoid a short outbreak of chaos. Problem: hermits repeatedly try to escape the tank.

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Solution: there are two considerations here, species and environment. If all of the hermit crabs of one species are doing this, you need to make sure the species you picked is one that is content in the provided habitat. If only a couple of individuals are trying to run away, the tank may be overstocked and they may be trying to escape due to harassment; try cutting back on the number of hermits in the tank. Checking water parameters is also a good step, although hermits are often one of the last animals to react to poor water quality in a community tank.

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Species Identification One of the most important parts of keeping hermit crabs is knowing what species you have. This is important not only for reasons of compatibility with other hermits, but also what you have in your tank in general. Unfortunately, identification of hermit crabs can become quite difficult. The bright side to this is that relatively few species are frequent occurrences in the pet trade, and they tend to be different enough that they can be easily identified. Don’t be fooled by common names though – those are often inaccurate and used for multiple different species. Size in hermit crabs is usually referenced by the size of the snail shell inhabited. Unfortunately this is only a very rough estimator, since the length of the snail shell may not be a good indicator of the aperture size, which is often the limiting factor for hermit crabs. Except where otherwise noted, size estimates given here use Turbinid snail shells, such as Turbo genus shells, as a model. These shells are not terribly globose and do not have any extensions that would exaggerate the length in the way that is characteristic of some conch and whelk shells.

Anatomy and Terminology

Figure 2. Anatomy of a hermit crab, highlighting areas important for species and gender identification.

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Abdomen: the soft part of the body, sometimes called the tail, which remains hidden in the hermit crab’s shell. Antennae (feelers): sensory organs used to communicate with other hermit crabs and feel the surrounding environment. Antennules: smaller sensory organs used for smell/taste. Carapace: the back of the hard exoskeleton. Cheliped: the limbs with claws used for grasping. The upper part of the appendage is commonly referred to as the arm. Eyestalk: the movable stalk on which the eye sits. Pleopods: usually only visible on females, these limbs are used to hold eggs. Looking for the presence of these limbs when a hermit crab is extended far out of its shell or when the hermit crab changes shells is one way to sex hermit crabs. However, checking the location of the goniopores will yield greater gender certainty (see the section on sexing hermit crabs). Uropod: limbs on the end of the abdomen that are used to anchor the hermit crab into its shell. Walking legs: the four long limbs that do not have claws on the ends.

Species Descriptions Although this is not a complete list of hermits commonly seen in the pet trade, it includes a number of the most common species. Please keep in mind that the diagrams are to illustrate the general coloration of the species only. Individual variation within a species may mean that a hermit doesn’t exactly match the picture, but still matches the textual description. Hair is not shown on any of the diagrams to make the underlying exoskeleton color as clear as possible.

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Calcinus elegans Common names for blue form: electric blue hermit, blue hermit. Common names for orange form: Halloween hermit, orange-striped hermit.

Figure 3. Coloration of Calcinus elegans, showing both the blue and orange forms.

Temperament: usually peaceful. Adult size: shells usually not exceeding 2” (5cm), sometimes up to 3” (7.5cm). Reef safe: yes, for large nano or bigger tanks. Native range: [pending] References: [pending] Physical Description The walking legs have alternating bright blue and black stripes, with black spots on the tips of the legs. The arms are brownish with white spots, and the claws have white tips. The eyestalks are blue, and both the short and long feeler pairs are red. Tufts of magenta hair may be visible on the backs of the legs and on the body of some individuals, particularly after shedding. In addition to the blue form shown above, there is also a striking orange version where normally blue portions of the legs and body are orange instead. The eyestalks range from nearly black to lighter blue. The arms and claws retain the same coloration as in the blue form. The orange color form is often confused with Ciliopagurus strigatus in the aquarium trade due to overlap in common names.

Compatibility and Behavior Compared to other Calcinus species, this hermit is generally peaceful. Mixing C. elegans with smaller species is a bad idea, although it will generally be safe with snails provided that enough empty shells are present and that the snails are also large enough. It is also a fairly good hermit for ridding tanks of algae outbreaks. It should be provided with meaty foods occasionally, but is generally content to live on what it can scavenge from the tank and dried seaweed or protein-rich pelleted foods. It is my experience that this species will ignore even smaller snails when food and shells are abundant. However, the hobbyist should be aware that, for any hermit crab, too little protein-rich food (or too little food in general) will make snails look more appealing as a meal.

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Calcinus laevimanus Common names: zebra hermit, striped hermit, orange-clawed hermit, orange-legged hermit, left-handed hermit.

Figure 4. Coloration of Calcinus laevimanus.

Temperament: semi-aggressive. Adult size: shells commonly up to 1” (2.5cm), sometimes up to 2” (5cm). Reef safe: yes, for nano and larger tanks. Native range: [pending] References: [pending] Physical Description The walking legs are reddish brown with white and black striping on the tips and not above the joint. The right claw is black with white tips on a black arm. The left claw is enlarged and may or may not be solid black. The eyestalks are blue at the base, with an orange section before the eyes, which are blue. The antennae are orange, and the antennules are blue at the base with orange tufts at the end. Compatibility and Behavior Due to being somewhat aggressive, this species shouldn’t be kept with any snails that aren’t significantly larger, and probably is best not mixed with other species of hermit crabs. Being fairly omnivorous, it will do a good job scavenging in a tank for food but may also snack on smaller animals that can’t move away fast enough. Like C. seurati, this hermit crab is capable of being quite the escape artist. Particularly for tanks with young C. laevimanus, tight-fitting lids or smooth plastic disc-shaped guards around all airlines and power cords to prevent climbing are a must. Due to its potential maximum size, Calcinus laevimanus should be stocked more sparely than small Clibanarius species in most tanks.

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Calcinus seurati Common names: zebra hermit, striped hermit, dwarf zebra hermit, left-handed hermit.

Figure 5. Coloration of Calcinus seurati.

Temperament: aggressive. Adult size: shells commonly up to 1” (2.5cm), sometimes up to 2” (5cm). Reef safe: yes, for nano and larger tanks. Native range: [pending] References: [pending] Physical Description The walking legs have alternating white and black stripes, as do the upper arms. The claws may be white or gray, and the left claw is enlarged. In adults, the right claw develops a cup-shaped tip. The eyestalks are blue at the base, with orange stripes right before the eyes, which are also blue to black. The antennae are orange, and the antennules are blue with orange tufts. Compatibility and Behavior Calcinus seurati is often confused with its relative, C. laevimanus, because of the common names. C. seurati’s coloration makes it an attractive animal to keep, but it is highly aggressive towards tankmates and strongly territorial where other hermit crabs are concerned. This species should be sparsely stocked, and a single adult can easily claim an entire nano tank for itself, killing any hermits that aren’t big enough to fight back. Adult C. seurati will show aggression first towards other species, but will kill members of their own species as well if sufficient space is not provided. This species is probably not a wise choice for serving the roll of janitor in most marine tanks unless it is the only animal intended for the role. Another quirk of the species is its talent for escaping. If these hermits feel cramped for space or are unsatisfied with the tank conditions, they can do a surprisingly fast combination of an arm-over-arm crawl and inchworm-motion with their walking legs up airline tubing and power cords and quickly escape a tank. Juveniles are more capable of and likely to do this than adults. When content (and mature), the species will generally not try to escape, and constantly escaping hermits should signal to the aquarist that there is a problem at hand, usually either overstocking or poor tank conditions. 43

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Calcinus tibicen Common names: tibicen hermit, striped hermit.

Figure 6. Coloration of Calcinus tibicen.

Temperament: generally peaceful. Adult size: shells commonly up to 1” (2.5cm), sometimes up to 2” (5cm). Reef safe: yes, for nano and larger tanks. Native range: [pending] References: [pending] Physical Description Coloration is variable and some individuals are substantially darker than others, appearing nearly black over most of the body and legs, although the distinctive markings at the ends of the walking legs remain. Highlights around the joints on the arms are also variable. The walking legs and arms are usually dark reddish to purple-ish brown with yellow to white highlights near the tips. The last joint of the walking legs have a reddish brown band and black tips. Eyestalks and feelers are orange to light brown. Compatibility and Behavior This species is similar to Calcinus laevimanus in its behavior, potential threat to small/medium-sized snails, and size. Due to its potential maximum size, Calbinus tibicen should be stocked more sparely than small Clibanarius species in most tanks.

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Ciliopagurus strigatus Common names: Halloween hermit, orange-striped hermit, demon hermit.

Figure 7. Coloration of Ciliopagurus strigatus.

Figure 8. Two small Ciliopagurus strigatus, both carrying cone snail shells.

Temperament: generally peaceful within the species, but semi-aggressive towards other invertebrates. Adult size: elongate cone snail or conch shells usually over 1” (2.5cm) and sometimes up to 4” (10cm) in length (from left to right in the orientation shown in the picture above). Reef safe: no. Native range: [pending] References: [pending]

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Physical Description The limbs are covered in striking orange to yellow stripes. The darker bands range from a deeper brownish color to a brighter red/orange hue. The eyes, antennae, and antennules are red to dark orange. The carapace is white. The carapace is flat and wide. Compatibility and Behavior One of the most notable traits of this species, aside from its coloration, is its strongly flattened body compared to other species discussed in this document. The flattened shape enables the species to make use of shells that would ordinarily be impossible for other hermit species of the same size to fit their bodies into: shells from cone snails (preferred to all others), young conches, and sometimes cowries. As a result, the average Turbinid shell will be a bad fit due to having round apertures. These hermits require cone snail or similarly-shaped conch shells as housing options. They may take Turbinid shells if no other options are presented, but this will likely lead to unnecessary stress on the hermit since the shell’s fit will be poor. An illfitting cone snail shell may be preferred to a better-sized but slightly rounder shell, such as a conch shell. The choice of shells for this species suggests that conches are a risky choice as a tankmate. When in need of new shells to grow into, hermits are more likely to predate on the type of snail whose shells they prefer. Conches’ lack of tight-fitting operculums make them especially susceptible to attacks from crustaceans. Behaviorally, this hermit is still not well-documented in captivity. It is slowmoving with proportionally small claws, and it would appear to be non-aggressive towards others of the same species (although compatibility with other hermit species is unclear). Large individuals may eat soft corals and other sessile invertebrates. The species will happily eat hair algae and other soft algaes, but it will also pick the rock clean of anything else living on it. Ciliopagurus strigatus is probably best suited to either species tanks or non-reef tanks containing non-aggressive fish and no other hermit crab species. The maximum size for Ciliopagurus strigatus is usually reported to be in the 1-2” size range, similar to that of Paguristes puncticeps. While these quotes reflect the most common maximum size observed in captivity, substantially larger individuals do occur. It is currently unclear why larger individuals are uncommonly observed in captivity, although it may be related to failure to provide suitable shells, attrition due to mixing species, or failure to provide an appropriate diet.

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Clibanarius digueti Common names: red-tip hermit, red-leg hermit, dwarf hermit, mini-hermit, Mexican redleg/tip hermit.

Figure 9. Coloration of Clibanarius digueti.

Figure 10. A male Clibanarius digueti showing abundant blue spots.

Temperament: peaceful. Adult size: shells usually about 0.5” (1-1.5cm), very rarely up to ~1.5” (3.5-4cm). Reef safe: yes. Native range: Gulf of California, Gulf of Mexico. References: CalPhotos, Brusca 1977. Physical Description The legs and claws are a dull greenish brown to dark brown with red tips. Some individuals may have visible blue spotting on the legs and a blue stripe down the front of the tips, but on others it is too faint to see clearly. The amount of blue speckling may also change with successive molts in some individuals, and some may have such dense speckling that they appear more blue-green than brown to the naked eye. The eyestalks are greenish to reddish brown, antennae are red, and the antennules are brown and end in

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bright red tufts. This species is usually a deeper brown as an adult than Clibanarius erythropus, although the color range overlaps with that species and color can vary within an individual as it grows (as with the blue speckles). Compatibility and Behavior If a perfect hermit crab species exists to fill the roll of reef janitor without unnecessary aggression, this is probably it. Clibanarius digueti is a very small species, easily kept in groups, and if sufficient shells and food are present, no problems will arise in the group. They are easily picked on by other animals, however, since they generally stay smaller than other commonly sold species (which would perhaps make this the only species deserving of the “dwarf” label) and are not all that strong. Most adult Calcinus species would be able to kill an adult Clibanarius digueti without much difficulty, so mixing C. digueti with other species is not recommended unless it is with another small, usually peaceful Clibanarius species, such as C. tricolor. C. digueti is often sold when immature and about half of its normal maximum size. It is also my experience with this species that it does exceptionally well kept as a male-female pair, even in “pico” marine tanks as small as 1 gallon, where other species would feel cramped. Of course, determining when a pair contains a male and a female is a bit tricky, and often the best method for acquiring a pair is to get a bunch for a larger tank, watch for any courtship behavior, and then move the courting pair to the desired location. The move shouldn’t disrupt the courtship too much, and it will resume once the pair are settled.

Clibanarius erythropus Common names: red-tip hermit, red-leg hermit, dwarf hermit, mini-hermit.

Figure 11. Coloration of Clibanarius erythropus.

Temperament: peaceful. Adult size: shells usually about 0.5” (1-1.5cm), very rarely up to 1.5” (3.5-4cm). Reef safe: yes. Native range: [pending] References: [pending]

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Physical Description This species and Clibanarius digueti are very similar in appearance, particularly when small. The legs and claws are a dull greenish brown to dark brown with red tips. The red tips have blue stripes running lengthwise on the side that range from being very faint to quite pronounced depending on the individual. Some individuals may have faint blue spotting on the legs, while it is absent on others. The arms usually have blue spotting present and smaller, less-pronounced blue stripes on the tips of the claws. The eyestalks are usually brown, and the antennae are red. The antennules are brown with bright red tufts at the ends. Compatibility and Behavior Behaviorally this species is very similar to C. digueti. If stocking is cautiously sparse and food is abundant, may be combined with it and other small Clibanarius species such as C. tricolor and C. digueti. This species’ range also makes it suitable for some cooler tanks in the lower 70°s Fahrenheit, and perhaps room temperature tanks similar to Paguristes longicarpus.

Clibanarius striolatus Common names:

Figure 12. Coloration of Clibanarius striolatus.

Temperament: semi-aggressive Adult size: shells usually up to 2” (5cm), sometimes up to 4” (10cm). Reef safe: no. Native range: [pending] References: [pending] Physical Description The walking legs are dark green with black stripes running lengthwise. The joints have orange or yellow highlights. The arms are mottled green and black and the claws have orange tips. The eyestalks and antennae are green. The eyes are usually lighter colored, although the change in color may be difficult to see on smaller individuals. On smaller individuals where the colors may be difficult to determine, the presence of orange highlights near each joint distinguishes this species from Clibanarius vittatus.

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Compatibility and Behavior This species is sometimes confused for Clibanarius vittatus. It is relatively uncommon in the aquarium trade compared to the smaller Clibanarius species. Due to its potential maximum size, recommended tankmates would be similar to those for Clibanarius vittatus.

Clibanarius taeniatus Common names: yellow-striped hermit, skeleton hermit.

Figure 13. Coloration of Clibanarius taeniatus.

Temperament: usually peaceful. Adult size: shells usually up to 1” (2.5cm), rarely up to 1.5” (3.5-4cm). Reef safe: yes. Native range: [pending] References: [pending] Physical Description The walking legs are a dark, often black, with yellow stripes running lengthwise down the legs all the way to the tips. The arms are also very dark with yellow spots and some striping. The feelers are yellow. The eyestalks and carapace are black and yellow striped, which distinguishes it from Clibanarius vittatus. Compatibility and Behavior This species is more or less like Clibanarius tricolor, although it is less commonly kept. Its behavior in captivity is less well documented than for other Clibanarius species as a result. It is generally peaceful towards other similarly-sized hermits, although mixing with Calcinus species is not recommended for the same reasons discussed regarding Clibanarius digueti.

Clibanarius tricolor Common names: blue-leg hermit, tricolor hermit, blue hermit, dwarf hermit, mini-hermit.

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Figure 14. Coloration of Clibanarius tricolor.

Temperament: peaceful to semi-aggressive. Adult size: shells up to 1” (2.5cm), sometimes to 1.5” (3.5-4cm). Reef safe: yes. Native range: [pending] References: [pending] Physical Description Identifying features include the bright blue legs with black dots, yellow tips, and red/yellow stripes near the joints. Claws are yellow tipped with the upper part of the limbs being darker blue/black with lighter blue to white speckles. The eyestalks are blue, and the long pair of feelers have a red base with yellow ends. The shorter pair of feelers is usually bright orange. Compatibility and Behavior These are, without a doubt, beautiful hermit crabs to keep. It is fortunate as well that, as hermit crabs go, Clibanarius tricolor is a relatively peaceful species. Although not quite as compatible with all animals as C. digueti, unless there are small snails in the tank, these hermits will not go out of there way to harm other animals. They are also unusually compatible with other Clibanarius species, and can be kept successfully with C. digueti, provided that the tank is not overstocked with hermits. It should be noted though that most individuals sold in the aquarium trade, particularly those sold as “mini hermits,” are immature and may grow significantly within a few months of purchase.

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Clibanarius virescens Common names: yellow-tipped hermit, yellow-leg hermit.

Figure 15. Coloration of Clibanarius virescens.

Temperament: generally peaceful. Adult size: shells up to 1.5” (3.5-4cm). Reef safe: yes. Native range: [pending] References: [pending] Physical Description The walking legs are a dark blue to black with yellow tips and one yellow stripe near the last joint. The arms are the same dark blue or black color with yellow to white spots and claw tips. The eyestalks are dark, as are the antennules and the base of the antennae. The antennae may have yellow ends. Compatibility and Behavior This species is more or less like Clibanarius tricolor and Clibanarius taeniatus, also being one of the less commonly kept Clibanarius species. It is generally peaceful towards other similarly-sized hermits, although mixing with Calcinus species is not recommended for the same reasons discussed regarding Clibanarius digueti.

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Clibanarius vittatus Common names: skeleton hermit, striped hermit, gulf coast hermit, large reef hermit.

Figure 16. Coloration of Clibanarius vittatus.

Figure 17. A very dark, adult Clibanarius vittatus.

Temperament: semi-aggressive to aggressive. Adult size: shells up to 4” (10cm), sometimes larger. Reef safe: no. Native range: Gulf of Mexico, Florida, Caribbean References: [pending] Physical Description The walking legs may be red-brown to nearly black with yellow stripes running lengthwise down the legs. Each leg has 2-3 stripes, which may intersect. The claws are reddish with black tips and the arms are mottled brown and yellow. Yellow ringlet patterns may be present on/near the claws. All feelers and the eyestalks are brown, as is the body and carapace.

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Compatibility and Behavior Clibanarius vittatus is often sold incorrectly in pet stores as a “large reef hermit.” They are not reef animals, and are poorly suited to such an environment – or rather, such an environment in an aquarium would be poorly suited to them. They can probably be placed in just about any marine tank due to their hardiness, but will tear things apart where it is possible for them to do so. They can be found in great abundance on the sandy beaches of the gulf coast, where they feed on dead animals washed up on shore, seaweed, and encrusting bivalves. Don’t put these in a reef tank unless you want your corals damaged. The hermits may not eat the corals, but are not gentle with their environment. To be completely safe, these hermits require a tank built around their curious and easily destructive nature, which often means a species tank. These hermits, due to their large size, are also not suitable to add to tanks where there are smaller hermits that can’t escape quickly. Clibanarius vittatus’s claws would easily be able to inflict injury on other smaller species. Strangely though, if sufficient food is present, adult C. vittatus will make no attempt to go after even quite small marine snails. Larger snails may actually be at a greater risk, since their shells might make a good home. Despite this anomaly with adult C. vittatus and small snails, immature individuals will continue to go after snails when the shell size is within the range that would interest them as a new home.

Dardanus megistos Common names: giant hermit, strawberry hermit, speckled hermit, white-spotted hermit.

Figure 18. Coloration of Dardanus megistos.

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Figure 19. A male Dardanus megistos in the aquarium.

Figure 20. The same individual from the previous figure exhibiting an interesting digging behavior in the sand bed. The shell was carefully and very deliberately covered with sand using the walking legs.

Temperament: aggressive. Adult size: Turbinid-shape shells up to 5” (12.5cm), more elongate shells up to 8” (20cm). Reef safe: no. Native range: [pending] References: [pending]

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Physical Description The hermit is a mostly uniform bright to dark red with white spots. The white spots are generally rimmed with black. The carapace is a lighter yellow color with the same type of spots. The eyestalks are red as are the antennules and the base of the antennae. The antennae have white ends. Although not shown in the picture above, these hermits are also much harrier than the other species covered so far. The hairs covering the legs and body range from almost transparent to dark brown depending on the size and individual. Compatibility and Behavior There is a reasonable amount of conflicting information about this species circulating in the hobby. In fact, the species is probably a good example why it is important to make careful observation of the environment and history leading to behaviors before classing them as standard. When I have seen these animals in pet stores, they always appeared rather ferocious – always grabbing at small fish and storming around the tank tearing things apart. This roughly matched the reputation these hermits have the average aquarium: hairy legs of death and destruction. However, there are some important conditions to note that coincided with this behavior whenever I saw it: the sand beds were usually coarse and thin, the rock was picked clean, and the tanks were densely stocked with small, active fish like damsels. It is unlikely under these conditions that the hermits would find much to eat. Presented with a deep bed of sugar grain-sized sand and abundant food, there is something less barbaric about the species. The dense, stiff hairs on the walking legs transform the limb into a shovel. The same is true for both clawed arms. Given such a sand bed, these hermits will begin shoveling sand into their mouth to eat any organisms they can sift out. The benefits are that the sand bed will be well-churned so that it doesn’t accumulate gas bubbles, and the hermit crab spends its time digging in the sand rather than showing interest in other livestock in the tank. However, the sand bed will also be filtered of beneficial organisms. In a small tank, this could easily deplete populations of sand-dwelling worms. Regardless of the disparity between starvation behavior and well-fed behavior, Dardanus megistos is still probably best kept in a moderately-sized species tank or a rather large tank with reasonably large or aggressive fish as tankmates. Since fragile and encrusting organisms are on the menu and the species’ maximum size makes it unsuitable to be kept in smaller community tanks. The species also has a reputation as a snail-eater, although strangely enough I have yet to observe this from the species. D. megistos can be very rough with snails, forcibly prying them away from rock and tossing them about like a soccer ball if they are in the way, but I suspect that any habitual snail-eating may be in part due to failure to meet dietary requirements or provide sufficiently large shells to house them. Snails not within an obvious size-range to have their shells considered as housing will be safer, of course. Occasionally, very small Dardanus megistos show up in the aquarium trade, sometimes under misused common names, and may seem compatible in a standard reef environment. However, these hermits will grow, eventually becoming too destructive to be kept in reef tanks without damage to fragile corals and other invertebrates.

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Dardanus pedunculatus Common names: anemone hermit.

Figure 21. Coloration of Dardanus pedunculatus.

Temperament: semi-aggressive. Adult size: shells up to 4” (10cm). Reef safe: no. Native range: [pending] References: [pending] Physical Description The coloration of this species is somewhat variable. The lower portion of the walking legs may be orange to reddish, and the arms may be tan/pink to brown. The left claw is enlarged and differently shaped. The eyestalks have alternating red and white bands. Compatibility and Behavior This species is most well-known for its relationship to a species of anemone that it carries on its shell. When the hermit crab changes shells, it may remove the anemones from its old shell and place them on the new one. Dardanus pedunculatus’s size makes it unsuitable for the average aquarium, and its special relationship with anemones means that the aquarist must not only care for the hermit crab but also ensure that the tank provides adequate food to the anemones it carries.

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Paguristes cadenati Common names: crimson hermit, scarlet hermit, red hermit, strawberry hermit.

Figure 22. Coloration of Paguristes cadenati.

Temperament: semi-aggressive. Adult size: usually shells to 1.5” (3-4cm), sometimes shells up to 2” (5cm). Reef safe: yes, for nano and larger tanks. Native range: [pending] References: [pending] Physical Description This hermit is generally a uniform red color, ranging from bright red to deep red in some individuals. The eye stalks are yellow with green eyes, and the antennae are red at the base and may fade to yellow at the ends. The antennules have red and white bands with yellow tufts. Compatibility and Behavior Paguristes cadenati, often sold as the “scarlet reef hermit,” is one of the most abundant marine hermit crab species in the U.S. aquarium trade. They stay reasonably small and tend not to be a problem with many other common reef fixtures such as corals and sessile invertebrates that are larger than the hermits. However, they are commonly reported as either the instigators or victims of inter-species aggression involving species such as Clibanarius and Calcinus species, which are commonly stocked together with Paguristes cadenati. Perhaps for no other reason than it’s availability, P. cadenati is also one of the species that has contributed to the commonly held belief that all hermits will go after snails unnecessarily. Because many of the snail-predation cases when sufficient shells are present are likely due to insufficient food sources, P. cadenati’s reputation as a snail-killer may or may not be founded. Primarily for size reasons, it is inadvisable to mix P. cadenati with small snails or other hermit crab species such as the small Clibanarius, particularly in smaller tanks.

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Paguristes puncticeps Common names: white-spotted hermit, white-speckled hermit, speckled hermit.

Figure 23. Coloration of Paguristes puncticeps.

Temperament: aggressive. Adult size: conch and other elongate shells up to 6-8” (15-20cm) depending on shell shape. Reef safe: no. Native range: Gulf of Mexico, Caribbean References: Benedict 1901. Physical Description This species is usually a rosy-red hue with dense white speckles over the entire body. The speckles are in two primary sizes, with large speckles being surrounded by numerous smaller ones. The eyestalks are speckled as well, with light to dark blue eyes. Although not shown in the illustration of the coloration, this species is also covered in fine white hair that is most dense on the body and becomes thinner on the legs and claws. The last joint of the walking legs also has short, thick black hairs. Compatibility and Behavior Paguristes puncticeps rarely occurs in the aquarium trade. It is a species from the Gulf of Mexico, frequently seen from Florida to the Caribbean both coastally and in deeper water. When it does make its way into the pet trade, it is usually through smallscale collection by hermit crab enthusiasts or by accident. It is a coastal species much like Clibanarius vittatus, but larger and stronger. These hermits will move rocks and tear apart anything that isn’t sturdy enough to resist their strength, making corals an unwise stocking choice with these hermits. Although it would be unlikely that the corals would be eaten, they would be at a high risk of being badly damaged by the P. puncticeps’ foraging behavior. This species can be quite aggressive when pursuing food, and can move surprisingly fast over short distances in response to a bit of motion from something that might be edible. P. puncticeps does not belong in the average reef tank, perhaps being best-suited for a species tank similarly to Dardanus megistos. It may not be too

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destructive in a fish-only tank, provided that the tank is quite large and houses few if any other individuals of the same species and the fish are not small enough to become lunch for hungry hermits. Smaller, bottom-dwelling fish are a risky choice with P. puncticeps given its speed and ability to move quickly over short distances. P. puncticeps seems to prefer more elongate shells than most of the species discussed so far (with the exception of Ciliopagurus strigatus), as it has a more flattened body. This species requires large conch and whelk shells, or other shells with suitably elongate openings. Such shells are not always easy to find, but are sometimes sold in craft stores or by pet stores that sell large terrestrial hermit crab species.

Pagurus longicarpus Common names: long-armed hermit

Figure 24. Coloration of Pagurus longicarpus.

Temperament: aggressive. Adult size: shells up to 1.5” (3-4cm). Reef safe: no. Native range: Eastern coasts of North America References: [pending] Physical Description These hermits range from light to dark brown in color, and exact coloration differs by location. Speckles may or may not be apparent, although the carapace is usually mottled. The walking legs may or may not have clearly visible alternating bands of light and dark brown. A few darker bands are present on the last segment of the long arm and high on the walking legs. The claws are lighter at the tips, and the antennae are all dull colored. The most pronounced physical feature of the species is its enlarged right arm. Compatibility and Behavior Not only are these hermit crabs aggressive, they are also extraordinarily fast. Compared to the Calcinus and Clibanarius species, these hermits are very fleet footed and have interactions at such a fast pace that they may go unnoticed by the inobservant aquarist. The sheer speed at which these hermits operate makes them incompatible with slower species that would be unable to keep up and compete for food. Even when fed

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daily, a war can erupt if there aren’t enough pieces of food to ensure that each individual has something to eat at the same time. These aspects of the species can also make Pagurus longicarpus great fun to keep. Not many other species will see your hand approach and come running to the front of the tank and try to grab your finger through the glass. If kept in a small tank in a conspicuous location, this species will not ignore you. You may find that these hermits line up to watch you as much as you sit to watch them. Despite potential problems if species are mixed, P. longicarpus can live in very crowded conditions with its own species. Same-species aggression is most often limited to short conflicts involving shell swapping, territory, and food. These hermits have one very large arm, but relatively small pinchers. Instead of pulling at limbs or feeler pinching, as is common in Clibanarius and Calcinus species as a sign of aggression, these hermits preferentially “slap” each other with the longer arm and sometimes both arms. If one hermit feels that another has gotten too close, it will quickly flail an arm out to, quite literally, slap the encroaching individual in the face. This behavior is also done to snails and sometimes even inanimate objects. If the population is not under stress, it is quite rare for aggressive interactions to result in limb loss or other injuries. P. longicarpus is an inter-tidal species that has no trouble scampering across land. This means that, if it escapes, it can get quite a distance between itself and the tank and may be tricky to recapture if it finds a good hiding spot. Although they are content to remain in the water and will usually go back to water if placed on land, these hermits should not be provided with an easy escape rout. Unlike Calcinus species, Pagurus longicarpus seem less adept at climbing up power cords and airline tubing, so neither of those is likely to serve as an escape rout. The primary concerns would be stacks of rocks or other equipment that sits too close to the edge of the tank. Pagurus longicarpus can sometimes manage small, vertical jumps if startled, a behavior not present in the other species already discussed. This jumping tendency also means that great care should be taken when handling these hermits. Although the aquarist should have no fear of hand injury, a risk of hermit injury exists if they are able to catapult themselves off of a hand and fall any significant distance with only a hard surface to land on. Perhaps more importantly, this is not a strictly tropical species. Although uncommon in pet stores, the species seems to be slowly creeping into the online aquarium trade. These hermits are in no way suitable in a reef tank, and are not suitable in most tropical tanks either. P. longicarpus will be happy in an unheated, roomtemperature tank, provided the ambient temperature is in the range of 60-75°F (16-24°C). In the wild, they can survive much colder temperatures over the winter. Higher temperatures may be endured seasonally in the wild, but not year-round. Possible compatible invertebrates include mud snails (such as Ilyanassa obsoleta3) and other small whelks with the same native range. Provided with suitable spare shells and enough food, P. longicarpus will usually ignore these snails.

3

These snails are common carriers of the fluke responsible for the skin condition known as “swimmer’s itch.” Although not a serious problem, it is advisable to treat this species with Albendazole in edible form to ensure de-worming prior to introduction into any aquarium. Ilyanassa obsoleta will also not tolerate tropical temperatures (>75°F) for more than brief periods of time.

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Petrochirus diogenes Common names: giant hermit.

Figure 25. Coloration of Petrochirus diogenes.

Temperament: aggressive. Adult size: shells up to 12” (30.5cm), including those of the queen conch, Strombus gigas. Reef safe: no. Native range: [pending] References: [pending] Physical Description The coloration is fairly diverse on this species, which may be a dark, almost purple-black color to a brighter red. The arms have a distinct, scaled appearance, with lightening of the body color on each scale. These highlights may be almost white. The walking legs grow darker at the ends and are also covered in thick, thorn-like hairs. The antennae are striped with alternating brown and white bands, and the eyestalks are yellow to white with green eyes. Compatibility and Behavior This species grows to be far too large and aggressive to be kept in a standard community tank, being suited mainly to species or predator tanks. One individual will grow large enough to require a substantially large tank for itself. Although uncommon in the aquarium trade, these hermits occasionally appear as juveniles. Petrochirus dioenges should not be combined with other hermit crab species and does not belong in the average aquarium. This species will prey upon any other sessile invertebrates in the tank, so it may be difficult to maintain much life in the sand bed.

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Notes on The Small Clibanarius Species The Clibanarius genus has species covering a wide range of sizes. However, in the aquarium trade, species within the genus can often be placed into one of two categories: large or small. Species in the large group easily grow over 2”, while those in the small group rarely grow to occupy shells that large in captivity. It is easy to identify reasonably-sized specimens belonging to the large species group (e.g. C. vittatus and C. striolatus), but trying to identify a small brown hermit crab in a shell the size of one’s pinky fingernail without a additional magnification is rather difficult. Even slightly larger individuals falling into the small group can be tricky to identify due to the range of color variation present in some species. There are three or four small Clibanarius species that are both relatively common in the aquarium trade in North America and Europe and easily confused: C. digueti, C. erythropus, and either C. antillensis or C. ransoni (perhaps both). In general, the only clue one has to distinguish these three species is down to how much blue spotting is present and how far the blue stripe extends up the walking legs. It may not even be possible to see these features on a pinky-nail-sized animal, making identification difficult beyond “small brown Clibanarius.” Previous sections made the case that throwing a mix of hermit crab species into a tank is a recipe for disaster, but what is one to do with these mysterious “small brown Clibanarius” hermits? Even the best of intent to keep a single species can result in a mix of the three when obtaining juveniles from a store. Fortunately, the average aquarist with a moderately large tank need not worry about mixing these species. The small Clibanarius are often an exception to guidelines related to inter-species aggression, and can co-exist provided that there is no environmental stress (lack of shells, food, etc.). Clibanarius tricolor may be added into this group as well. Tanks at the larger end of the “nano” size-range (and of course all bigger tanks) offer enough space for these species to avoid each other and still co-exist. Still, when small Clibanarius species are mixed, behavior may be different than in a single species environment. Like-species individuals will often forage together or remain in the same general area rather than spreading out across the tank. My personal experience with Clibanarius digueti and Clibanarius tricolor is that they will co-exist peacefully in a “nano” tank, but will usually try to stay on opposite sides. Some interspecies aggression occurred during this experiment, but was limited to injury-free posturing and kicking fights over food pellets. While the small Clibanarius may present an exception to the species mixing guidelines in most aquariums, care should still be taken to keep a single species when stocking exceptionally small tanks, such as those in the “pico” size-range. The smaller the tank, the less room hermits have to place distance between each other and establish territories. When dealing with the small brown Clibanarius discussed here, small tanks should still be watched closely for aggression and individuals may need to be removed if fighting gets out of hand.

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Claw Morphology and Adaptation One of the more outwardly obvious features that differs between species, other than coloration and size, is shape and differentiation of the two grasping claws, or chelipeds. Aquarists often give quite a lot of attention to claw shape when considering the true crabs, namely when determining whether a particular hitchhiker crab will be an unstoppable force of destruction or a beneficial addition to a tank. Although attention to the claws is important, the conclusions I read and hear from such discussions are often strange and sometimes arbitrary. For hermit crabs, the claws are usually ignored except to note their size, despite the fact that the same general types of claws show up both in hermit crabs and true crabs. For true crabs, excessively broad generalities are common, sometimes without attention to other aspects of the animals’ behavior. When considering animals’ adaptations, it is often said that form follows function. For a hermit crab, claws are tools to interact with the world and obtain food. Although some might find the observation odd, the claws of hermit crabs and true crabs bear similarity to a number of common tools. This may shed some insight at a glance as to what the claws are most useful for – an important consideration when trying to determine what effect the claw-bearing animal might have on a given environment. General Claw Types There are a few basic types of claws that crop up repeatedly in both hermit crabs and true crabs. Below I have focused on some of the shapes that are most applicable to the hermit crabs seen in the aquarium trade. Claws for Cutting

Figure 26. Claws of Clibanarius vittatus compared to simple wire cutters. The leftmost claw is shown in crossection to highlight the fact that the only point of contact between the two halves is a sharp edge on the outside of the claw. Small sensory hairs line the inside.

The claw shape exhibited by the Clibanarius genus, as well as some species in other genera, such as Paguristes (including P. puncticeps and P. cadenati) and Ciliopagurus strigatus, close along sharp edges. Assuming no molting problems have 64

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occurred, the two halves of the claw normally meet perfectly along this surface. On these species, the sharp rim is very dark compared to the surrounding exoskeleton. Hermits with these claws are generally not very strong for their size compared to other crustaceans, but can deliver quite the nip due to the fact that the force gets concentrated on such a small area. The shape of these claws closely resembles that of small wire cutters used by jewelers. Not surprisingly, Clibanarius hermits are particularly adept at cutting up algae ranging from hair algae to thicker macro algae. They won’t always eat it if they’re searching for a meatier food, but they can certainly mow down and generally snip apart most marine algaes. It seems little coincidence to me that these hermits are also some of those that fare better on a diet heavier in algae.

Claws for Precision

Figure 27. Claws of the long-armed hermit, Pagurus longicarpus (left) and the blue crab, Callinectus sapidus (middle) compared to elongated pliers. Depending on the species, the claws may be more similar to forceps, tweezers, or other precision instruments.

In addition to concentrating a lot of force on very small, sharp tips, and the middle portion can get a good grip on slippery objects. As far as hermit crabs are concerned, this shape of claw is also worth some consideration, particularly where compatibility with small, unarmored animals. Both Callinectus sapidus and Pagurus longicarpus are able to move quickly to grab objects of interest, although Pagurus longicarpus’s long arm is a bit too unwieldy to make a good tool for predation against most fast-moving animals (although it can reach into some snail shells). This makes it much less of a worry than when considering true crabs with similarly-shaped claws. Indeed, a true crab with this kind of claw should raise a red flag to most hobbyists if found in a reef tank or community fish tank.

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Claws for Gripping

Figure 28. The claw of the European green crab, Carcenus maenas compared to a similar-looking tool.

This type of claw usually meets at various points along the length of the joint, although the tip may still be the main focal point of pressure if no object is being grasped. Hermit crabs such as Petrochirus diogenes exhibit this type of claw, as do many true crabs. This is also the type of claw that appears on the enlarged arm of some Calcinus hermit crab species. Although versatile in its usage, the type of claw has the ability to get a firm grip on an object. These types of claws are useful for opening up snails with thick operculums or cracking the exoskeletons of other invertebrates. A number of terrestrial hermit crabs also exhibit this kind of claw. These claws also make excellent hair algae removal tools – but the animals bearing them are rarely inclined to eat algae. The true crab, Carcenus maenas exhibits these claws (with very slight differentiation between the left and right claws). Given other opportunities, algae will generally be ignored and the claws will get used to crack or pry open other food sources. Similarly, Calcinus species have a claw like this on the enlarged arm, and it usually isn’t used for herbivory.

Claws for Leverage

Figure 29. The claw of a Mithraculus sp. true crab compared to similarly-shaped pliers.

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Figure 30. The claws of the zebra hermit, Calcinus seurati. The enlarged arm bears more of a grippingtype claw, while the smaller arm has a leverage-type claw with thick, bristle-like hairs along the inside. The smaller claw bears a strong similarity to the claws of various Mithraculus species.

These claws seem to be the focus of some debate in the marine community. One person will look at a Mithraculus claw and draw the conclusion that it’s a perfect hair algae trimmer, while another will look at it and conclude that it’s a perfect fish catcher. I suspect that one observation is based on seeing that the crab can eat hair algae and the other is based on the fact that it can catch a fish (or at least hold a dead/dying fish). The fact that some Calcinus species also exhibit the same claw type generally goes ignored when trying to determine the purpose of the Mithraculus claw. As Calcinus seurati can happily demonstrate in the aquarium, the oddly-shaped claw tips are well-adapted to removing small, encrusting tube worms that fit the curvature of the claw tips. Other encrusting animals like vermetid snails, barnacles, and some bivalves, may also be pried off with such a claw. Of particular interest are the cuspshapes at the ends of the claws, which provide a better grip on small, curved objects than the previously discussed claw types. Of course, claws that are well-suited for gaining leverage on small objects are surely useful for quite a lot more than that. However, while it is certainly true that both Mythraculus and Calcinus species both may eat algae to some degree (they are both omnivorous after all), I have my doubts as to whether the claws are the perfect hair algae removers that some claim. The claws seem better adapted to removing difficult-to-pulloff organisms, “rooting” varieties of macroalgae, and other more substantial algaes. From my own observation of the various claw types in action on hair algae, cutting-type claws of Clibanarius species appear to do a more effective job of rapid hair algae destruction and removal, even if the algae isn’t eaten most of the time. This is because the cutting-type claws can snip off and remove larger sections of wispy algae relative to the size of the claw than a similarly-sized prying-type claw that can only grasp firmly at the tip. Think of it as trimming a small patch of grass – would you rather use shears or tongs? For the purpose of hair algae removal, true grazing animals (such as sea hares or large Turbinid snails) are likely to do a much better job. Claws in the Aquarium Since claw morphology is a potential indicator of diet, it can be used as a way to make a best guess about which species to place in an aquarium to fill a certain ecological niche. Hermit crabs are usually introduced into marine aquaria for the purpose of rock 67

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cleaning – specifically to clean up uneaten fish food and prevent certain nuisance algae varieties from taking hold in an aquarium. Even if the algae isn’t eaten, it can be uprooted often enough that it doesn’t become noticeable. As already described, not all hermit crab species perform this roll equally well. Ignoring species compatibility and other stocking concerns, the hermit crab species usually considered to perform janitorial duties with high success rates are the small Clibanarius (C. tricolor, C. erythropus, and C. digueti) and Paguristes cadenati. Calcinus species are less favored as a primary janitor staff. Aggression is unlikely to be the deciding factor in this trend, since there are species of Calcinus that are, in general, less aggressive than P. cadenati can be towards other hermit crabs and snails. Similarly, size is an unlikely factor since P. cadenati can obtain roughly the same maximum size as some Calcinus species, such as C. seurati and C. laevimanus. What the small Clibanarius and P. cadenati have in common are undifferentiated, cutting-type claws. This claw type is more effective than precision or leverage-type claws at the removal of most fine nuisance algae varieties. These species are also less prone to predation on encrusting animals such as tube worms. It would seem that hermit crabs with cutting-type claws are more likely to be reef-safe than those possessing other claw shapes (although reef-safety is not ensured, particularly for easily-disturbed corals). Large, gripping claws also can do a very good job at cleaning rockwork and controlling algae (although not always hair algae). However, this type of claw also makes a good can opener and shell-crusher, making it a much bigger potential hazard to other invertebrates in the tank than is true for cutting-type claws of similar size. [Work in progress – additional sections not included in this version]

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Behavior At a glance, it would seem that most hermit crabs behave in scrappy and unorganized ways. Closer observation will give a different picture. Hermit crabs are clumsy, blundering invertebrates most of the time, but are capable of significantly more complex interactions with other hermit crabs. Hermit crabs are covered in environmental sensors, from the hairs on their legs to antennae and eyes. The set of antennae at the front of the animal are one of their primary ways of interacting with their environment. Objects of interest will be followed by the feelers, even if it’s on the other side of the aquarium glass. When two hermit crabs encounter each other head on, a furious conversation of “feeler bopping” may commence before the hermits decide how to respond to one another. Individuals that have never seen each other before will do this for longer than those that have met before. Hermits also have an incredible sense of smell. They can respond within seconds to food being placed in the tank. Their claws are capable of a fair amount of dexterity despite their bombastic style of movement. All of these attributes point to these animals being capable of more than just blundering around the tank in search of food.

Behavioral Vocabulary Hermit crabs communicate with each other through sight, smell, and touch. In addition to the antennae and antennules, the small (and sometimes not-so-small) sensory hairs coving the limbs allow a hermit crab to chemically evaluate its environment. Two hermit crabs will use their long antennae and sometimes antennules to smell and identify each other before engaging in other behaviors. This is typically the first type of interaction that takes place between two individuals that haven’t seen each other before. Beyond that simple interaction that spans the majority of, if not all species common in the pet trade, the vocabulary of behaviors differs from one species to another. Allowing for species-specific difference, there a few general categories of behavior that can be observed: 1. Touching with antennae/antennules – one hermit crab moves its antennae and/or antennules to touch an object of interest. It is quite common for hermit crabs accustomed to human hands to see a hand on the other side of the aquarium glass and move a feeler over to touch it, indicating that sight is a trigger for this behavior. 2. Changes in posture – hermit crabs may try to raise themselves up to appear bigger, or shrink down without retracting into the shell. This is used when establishing dominance and performing threats.

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3. Open-clawed displays – even fairly peaceful species will threaten each other with open claws, even though no injury occurs if one hermit grabs the other. Openclawed thread displays are frequently in response to the same types of stimuli that cause retreat into the shell in more timid individuals. 4. Kicking/slapping – one hermit crab flails a single limb at another individual. This is a common way for one hermit crab to signal to another that it has gotten too close and should move away. Most species use their walking legs, but some use their arms. Pagurus longicarpus will “slap” other hermits with one arm instead of kicking them, although it will also use kicking when the other hermit is too far to the one side or the other to be hit with either of the arms. 5. Shell knocking – one hermit crab bangs its shell repeatedly against another hermit crab’s shell. Depending on the species and the exact orientation of the shells, this can be a way of initiating a shell swap or asserting dominance, or it can be part of courtship behavior. a) Aperture-to-aperture – this is a form of dominance display in Clibanarius hermits, and very common when new individuals are added to an established population. This behavior may be done to other species as well as within the same species. b) Approach from behind – when one hermit crab approaches another from behind unseen, rapid clacking of the shells and shaking is a way of one hermit crab scaring another one out of its shell. This is one way that smaller hermit crabs can move into larger shells. The hermit crab being attacked will have no way to size up its aggressor, and may flee after sufficient harassment. The smaller individual can then hop into the larger shell and run off before the naked individual can retaliate. c) Approach from the front – this is most commonly part of courtship in various species. Small Clibanarius males will roll the females onto their backs and clack the shells together rapidly as one of the early stages of courtship. Sometimes it can be aggressive when a larger individual is trying to force a smaller one out of its shell, although the aggressor will usually grab the victim’s limbs at the same time and alternate pulling and shell knocking. 6. Limb-grasping – one hermit crab carefully, but firmly grasps another’s limb without causing damage. This is another common form of either dominance assertion or courtship behavior, depending on the species and context. The males of some species will grasp the female’s legs or pinchers and perform specific, repetitive movements as part of courting.

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Dominance Hierarchy So you just introduced a bunch of new hermit crabs into a tank, and now they appear to be fighting every time the bump into each other. Why? There are plenty of suitable spare shells, food is available, lots of space, and there seem to be no signs of courtship behavior. If a “hermit war” doesn’t break out in the tank resulting in numerous casualties (likely due to overcrowding or species incompatibilities), the unsettled state tends to correct itself within a few weeks and the tank is again peaceful. Like many animals, hermit crabs will establish a pecking order. They will fight for dominance, achieve it, periodically have their dominance challenged by other individuals, and sometimes fall down the hierarchy. It is especially common in some species when more than one male is present during the breeding season. Perhaps surprisingly, the pecking order isn’t always dictated by size. A good indicator of a dominant hermit is if one repeatedly turns out as the “winner” in food fights and is able walk off with the biggest piece of food. Initially it would seem logical that it’s a case of larger individuals being stronger and more capable of winning by brute force, but sometimes smaller hermits successfully bully larger individuals that would seem quite capable of defending themselves. Hermits will display dominance by intimidation and direct physical bullying. In the most common form, one hermit will approach another from behind and violently shake its shell for a while. This tactic is used to scare another hermit out of its shell when done for several minutes, but it appears to also be a method asserting dominance in some species, such as Clibanarius species and Pagurus longicarpus, when the attack is much shorter. Larger individuals will also pick up and move smaller ones, and sometimes hermits will team up in pairs (particularly male-female pairs in some Clibanarius species following courtship) to force another individual to retreat. Unlike the dramatically obvious type of aggression seen in hermit crabs that are fighting to the death, within a group of hermits that are living relatively peacefully, smaller interactions that take place that maintain the dominance hierarchy. A common method of asserting dominance in an established population involves a dominant hermit crab flipping another over and forcing it to remain in its shell by tapping the shell with one or more legs each time the flipped hermit attempts to right itself. Sometimes all it takes to keep the upside-down hermit in its placed is a raised leg in threat, and the dominant hermit may not actually need to strike the shell. In Clibanarius vittatus, one hermit may confront another head-on, grab its shell, and appear to “head butt” the other hermit one or more times by clacking the shell apertures together loudly. Both males and females will engage in this behavior.

Introduction of New Individuals When the social structure of a group of hermit crabs has reached a peaceful state, a few things can throw it out of equilibrium. The causes of unrest in an otherwise stable group of hermit crabs tends to be due to a single individual undergoing some sort of physical change (sudden growth spurt, an injury, etc.) that alters its status, or the introduction of a completely new individual. Hermit crabs are very aware of their 71

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surroundings and instinctively gravitate to anything new in their immediate vicinity that might be a shell or food item. In the wild, simply picking up a hermit crab and moving it to another location with a dense hermit crab population can be enough to prompt a sudden change in behavior, such as other hermits rushing over to investigate the newcomer. The most obvious reason for this behavior centers on shell possession. If shells are scarce and a new individual arrives, it may be worth trying to bully that hermit out of its shell. On the other hand, when shells are abundant, different motivations exist for the same behavior. The next most obvious reason would be selection of a mate if during the breeding season or establishing dominance over a rival. Regardless of the gender of the hermit introduced, it will likely elicit a response from most of the other hermits in the area. Dominant individuals in particular will seek out newcomers and sometimes lash out violently for a brief period of time, even when shell stealing clearly isn’t the goal. Considering these behaviors in the aquarium, it would seem best to introduce all of the hermit crabs to a tank at one time to avoid multiple “hermit crab war” occurrences. That’s not always possible of course, and when it isn’t, it is important to keep an eye on the situation after the new hermit has been introduced in case it gets mobbed. Here are some tricks to reduce the risk to new individuals in an already well-established group of the same species of hermit crab: 1. Re-arrange the tank a bit. Move empty shells around and disturb the environment just enough that the hermit crabs will become distracted with other things at the time the new individual is added. It also helps to disrupt territorial behavior – and is also a common practice for introducing new fish to established tanks. 2. Keep a lot of empty shells available, and add a couple new ones near the time of adding the new hermit crab(s). This will decrease the risk that a new hermit will be forced out of its shell. If it is forced out, it will also have plenty of opportunity to get a new home. 3. Feed the existing hermits not too long before adding the new hermit(s). This is especially important if a new hermit is much smaller than one or more existing ones, and/or if the tank contains multiple species of hermits. Remember that hermits aren’t above attacking, killing, and eating other hermit crab species and sometimes even their own species if they’re stressed enough for food.

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Courtship Behavior Each hermit crab species has a different set of behaviors that take place during the breeding season. In many cases, the interactions between males and females can be quite elaborate. Unfortunately, courtship is not very well documented for most species except to note the more common features that tend to be present in multiple species.

Figure 31. Illustrations of several hermit crab behaviors. The far left image shows the approach from behind that is commonly used aggressively and should not be confused with courtship. The hermit crab being attacked is presented with a pair of pinchers in the face and no other physical contact. This tactic can enable smaller individuals to bully larger ones out of shells with enough persistence. The middle image is also commonly seen in aggression, but when it is aggressive, the aggressor will keep its feelers out of the way and try to forcibly remove the other individual from its shell. The right image can be seen when a hermit crab investigates a spare shell (lasting seconds to minutes), or during courtship. During courtship, the male will roll the female over into the same position and tap on her shell. If the female doesn’t respond violently, the male may move mostly inside the female’s shell for some time. This can leave him vulnerable to attack. The male may or may not place his walking legs into the females shell depending on the relative sizes of the individuals and the amount of room in the female’s shell. Smaller species in shells with plenty of space are sometimes able to pull the shell apertures tightly together for long periods of time, leaving nothing exposed. This stage of courtship usually lasts off-and-on for much longer (hours to weeks) interspersed with other courtship behaviors.

Hermit crab courtship can take hours to weeks. For Clibanarius digueti courtship may last over a week, while in Pagurus longicarpus courtship may last less than an hour. Although the exact process differs between species, most courtship behavior has the same elements. The male will rotate the female onto her back, and perform some sort of complex shell tapping behavior. In small Clibanarius courtship behavior, it is common for the males to do a little “dance” of sorts, rapidly tapping their shells on the female’s shells and drumming on the female’s shell with their pinches. Pagurus longicarpus has an unusually large variety of courtship behaviors that are not always used in the same combination during courtship. This set of behaviors includes the male enticing the female to grab his claws in an effort to draw her partially out of the shell, the male “tickling” the underside of the female’s legs, and the male grabbing the female’s walking legs and alternating shell clacking with moving the legs back and forth. Larger species may be more subdued. If the female doesn’t respond aggressively and the shells are not spacious enough, the male will crawl mostly out of his shell and into the female’s shell, and may 73

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remain there for a few minutes to most of a day. Smaller species with roomy shells may pull the apertures of the two shells together for protection. If the female is unreceptive, she will lunge out of her shell with open claws and force the male to leave. If the male is significantly larger and is undeterred by the female’s display, the female may panic and leave her shell until the male moves away. If the female is receptive, the actual act of mating requires both hermit crabs to crawl quite a ways out of their shells such that the female’s gonopores are exposed. Mating is usually complete within minutes of the female being coaxed out of her shell. [Work in progress – additional sections not included in this version]

Reproduction Breeding hermit crabs in an aquarium is an extremely difficult task. There are problems that will be encountered at many levels. First, gender ratios have to be such that courtship behavior is successful. The females then have to be able to carry their eggs unharmed. Females carry the eggs in large clumps on the back appendages that are normally not visible. They will move to areas where the current is higher, crawl part-way out of their shells, and wave the egg-bearing limbs around in the water to increase circulation over the eggs. Males sometimes appear to guard the females during this time, which may last days to weeks depending on the species and environmental conditions. When the eggs finally hatch, hundreds of planktonic larvae will be released into the water. Once hatched, power-heads can be deadly for the new hermits and food must be tiny. The larvae must then survive for some time in the tank and transform into small hermit crabs. Then they require miniscule shells. At this point, they are easy prey for even some usually benign and beneficial animals such as amphipods, isopods, and marine worms. Needless to say, it’s a complicated process to have take place successfully in an aquarium, and therefore a very difficult process to facilitate all the way to successful development of the offspring into young hermit crabs. [Work in progress – additional sections not included in this version]

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Injuries and Health Concerns As aquatic animals go, hermit crabs are quite hardy. Their lives in the wild are filled with predators and aggression from other hermits, and they have adapted to handle a fair amount of rough treatment. There are a number of injuries they can get, but fortunately recovery is possible from the most common types.

Missing Limbs There are many reasons why a hermit crab may loose a leg. It may have been attacked by a predator, gotten stuck in the old skin during shedding, or been trapped somehow such that the hermit crab had to remove it to free itself. One lost limb is not cause for worry; the hermit crab will simply obtain another one at the next molt. Hermit crabs, like other crustaceans, are built for this type of injury and rebound from it relatively quickly with their remarkable regenerative abilities. Multiple lost limbs are a slightly different consideration from a single missing limb. Hermit crabs have four walking legs and two pinchers. Being minus one walking leg and a pincher might not hamper it too much, but two missing walking legs and particularly two pinchers will put the hermit in pretty bad shape. When both pinchers are missing, the hermit will have difficulty eating and will be largely unable to defend itself. Similarly with multiple missing walking limbs, hermits may be unable to escape situations that would lead to conflict. If there are other animals in the tank, particularly hermits, it will be at risk of being bullied and deprived of food. If the situation persists even after the hermit has molted (e.g. limbs are lost again even after the missing ones were regenerated), a serious effort should be made to identify the reason why so many limbs are being lost. Causes could range from environment-related issues such as the water to aggression from other hermit crabs and other predatory animals in the tank. If a hermit is missing either both pinchers or has less than three walking legs, it should be isolated immediately if other animals are in the tank, particularly hermit crabs or other aggressive invertebrates. Isolating hermits can be accomplished by placing them in a separate tank, a sump or refugium, or something as simple as a lidless jar wedged into the substrate. The injured hermit will have to be fed separately until its next molt, at which point is should recover any missing limbs. Once the new exoskeleton has hardened up, the hermit should be ready for reintroduction into the main tank with caution and careful observation.

Soft Tissue Injuries This is not generally possible for hermit crabs that are in a sturdy, safe shell. Hermit crabs can get their soft bodies injured when changing shells or if the only 75

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available shell has holes that allow other animals to pick at the soft tissue. Sometimes a bad molt can result in soft tissue injuries. There is not much that can be done for a hermit crab that has suffered damage to its soft body other than to isolate it and, if its shell is in poor shape, offer it an alternative home with better protection. If the injury is sever, the hermit crab likely won’t last more than a few hours since critical organs are housed in the softer potions of the body. If the injury is minor though, the hermit crab should recover with the next molt.

Physical Abnormalities Not all abnormal hermit crab health conditions are injuries. Sometimes the bizarre but benign happens. The most common things I’ve seen are malformed limbs and tumors. Malformed limbs can occur from the limbs getting stuck during a molt, or from other growth-related problems that cause abnormal development. These types of unusual appearances are not cause for alarm as long as the hermit crab is otherwise functional, since the problem will most likely be corrected with the next molt. Sometimes this problem is preventable if caught in time, as described in the section on molting complications. Tumors are a slightly different problem. They are quite rare in hermit crabs in captivity, but sometimes occur on the soft back part of the body that is almost never visible. Knowledge of tumor treatment in invertebrates may improve over time, but currently all that can be done is to simply to ensure that the hermit crab has a sufficiently large shell that offers enough protection such that the affected area isn’t exposed to be picked at. Tumors may be fatal during shedding since they can cause complications with the molting process.

Molting Complications Molting is a sensitive operation for all invertebrates with an exoskeleton. A healthy crustacean preparing to molt will form its new exoskeleton under the old exoskeleton. The new skin will thicken as the old exoskeleton is dissolved away. Depending on the size and species of the animal, the actual shedding event where the old skin is discarded may last anywhere from less than a minute to a few hours. There are several common events that can cause injury or death during a molt: 1. One or more limbs are lost when pulling rapidly out of the old skin. If too many limbs are lost, the individual may require special care until its next molt. This is particularly true of hermit crabs that have lost both arms. If the next molt is successful, all lost limbs will be re-grown, although they may be small and require additional molts to return to full size.

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2. One or more limbs are deformed due to failure to free them from the old skin before they harden. Some crustaceans may remove their own deformed limbs after the new exoskeleton hardens. 3. Injury or tearing of the new skin due to failure of the old skin to separate cleanly. In shrimp, sever forms of this are referred to as jack-knifing. Although hermit crabs are unlikely to suffer this injury, it is not impossible. Minor soft tissue injuries may be able to heal. 4. The animal becomes trapped before it is able to free any limbs. This is usually due to a combination of the old carapace not giving way and the underside of the old exoskeleton not breaking apart enough to allow any movement of the limbs. If the gills are trapped within the old skin, the animal can suffocate. Even if eventually able to free itself, the animal may die of exhaustion or be seized upon by anther hungry animal before it re-inflates its new limbs. 5. The old skin fails to thin prior to molting. Although rare, this causes the animal to be fully unable to molt in most cases, and it will simply die after a few panicked hours of twitching and erratic behavior. Limbs may fall off or be forcibly removed by the animal during this time. Examination of discarded limbs will show an abnormally thick exoskeleton. The difference between the new and old skin may or may not be visible without magnification. In cases 1, 3, and 5, there is little or nothing the hobbyist can do. Lost limbs are not a serious problem, and soft tissue injuries may not be fatal. There is no warning or way to diagnose the 5th case with enough time to have any affect on the outcome. It is a problem that takes some time to develop and may only manifest itself as unusual mottling or coloration of the exoskeleton. If case 5 is observed and the animal was in the tank for at least a month, the tank water should be tested to see if all parameters are within acceptable ranges, since this problem may be environmental. Case 2 is a place where I am in favor of breaking the “never touch a molting crab” rule. If caught early, intervention needed to free trapped limbs is minimal and probably less stress overall than allowing the animal to remain trapped until it hardens up enough to eat the old skin off of itself. In a community tank, freeing a trapped animal also decreases its chances of being attacked by scavengers. Intervention should not occur unless the hermit crab has been struggling for some time, since molting is not always fast and intervention should only occur when it becomes obvious that something abnormal is happening. If the new exoskeleton is already starting to harden, trapped limbs must be removed to avoid possibly crippling deformation. Although the limbs may not appear normal after removal, they may still be functional afterwards if they can re-inflate slightly. If all of the walking legs are trapped, the underside of the old exoskeleton may need to be broken if it has not split apart at all, and the trapped limb may need to be carefully removed from what it has become trapped inside. Removal of a trapped limb from the body cavity is easy. Removal from the old limb’s exoskeleton is trickier and is unwise to attempt except on reasonably large individuals that are unlikely to have their

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limbs crushed by accident from a human hand. Depending on how the limb has become stuck, the old exoskeleton may need to be broken, or the old exoskeleton may simply need to be wiggled a bit while the leg is held in place. When the old exoskeleton requires this kind of treatment to free the limb, the hermit has little hope of freeing the limb on its own – particularly when the new skin has already started to take the shape of the object trapping it. When case 4 occurs, if the animal is still struggling after several hours and has made no progress, it is highly likely that it will perish either due to exhaustion or predation. Usually the old carapace and underside of the old exoskeleton will not split in a way to allow the animal to back out of the old skin. The most help that can usually be administered is a brief intervention to split the old carapace to allow water flow to the gills. This will prevent suffocation, but survival of the animal is still unlikely if it is quite large and old for its species. Old individuals may simply give up out of exhaustion. If the animal is able to continue to back out of the old skin but becomes trapped again partway out, the underside of the old exoskeleton may need to be separated along the middle to allow the limbs to move. Once again, the measures described here are drastic ones, and should only be attempted on cases involving large animals that would almost certainly perish if left unaided. Interfering with a normal molt only risks causing harm to the molting hermit.

Oxygen Deprivation As has already been stated, hermit crabs as a group are remarkable survivors. However, that doesn’t mean they’re invincible. Oxygen deprivation is a common event that can happen during shipping, temperature spikes, and power outages. When a system with animals uses up oxygen faster than it is provided through surface gas exchange or supply from additional aeration, every animal is put at risk of the same condition. Fish will respond by gasping at the surface, or in worse cases trying to jump out of the tank. Snails will also head to the surface. Hermit crabs don’t usually have that luxury – unless they can climb out of the tank or hitch a ride on a snail, they’re stuck. Hermits that are starved of oxygen will usually sit half-out of their shells and appear less responsive as the condition progresses. Their limbs may be stiff, but they will often give a twitch in response to repeated prodding. Treatment must be fast when this occurs. There are two options: for coastal species that spend a good deal of time out of the water anyway, they should be removed from the oxygen-depleted water and allowed to sit in the air briefly. This may show a slight increase in activity. In the wild, these hermits would evacuate to land if the water became too stagnant. The next step, which is the only step for non-coastal species, should be to get the hermit into highly oxygenated water as quickly as possible. It is best to increase oxygenation in the tank or bag water so that any pH shift as a result of gas exchange will occur gradually. Most hermits will rebound within 10-15 minutes and resume normal activity as though nothing had happened. The treatment described above, if handled quickly, is really quite simple. The bigger concern is identifying the cause of the condition. When out on a collecting trip,

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oxygen-depletion can be caused by not changing out bag water frequently enough, allowing the water to overheat even when open (although coastal species are quite temperature-resistant, hot water holds less oxygen), or simply not opening up air-tight bags frequently enough for gas exchange. In a tank, the situation is often more complicated. Oxygen can be sucked out of the water in a deep tank that is heavily stocked just be not having sufficient circulation due to a power outage. The solution is to keep backup battery-powered devices that can provide at least a small amount of surface agitation in the even of a long power outage (such as a multi-day outage). A lot of tanks, though, simply run on a bare minimum amount of surface agitation. What happens if the surface of the water gets gummed up with an organic film that inhibits gas exchange? Some write it off as an inexplicable tank crash – but it’s easy to prevent by always keeping a backup method of agitation and aeration that does not rely on powerheads. A simple air bubbler can make the difference between a catastrophe and a non-event.

Exposure to Toxic Substances Like other marine invertebrates, hermit crabs are susceptible to poisoning from certain chemicals. In addition to the standard ammonia, nitrite, and nitrate list that affects all aquatic life, metals are another strong concern with any crustacean. Copper is one of the worst, and, in high concentrations, will kill crustaceans. When metal poisoning occurs, the symptoms and unusual behaviors usually show up far too late to do anything, and the animal may die once removed from the contaminated water. Copper is commonly encountered in many fish medications for parasites and acidic tap water that runs through copper pipes. Fish medications can contain more than just metals that are toxic to invertebrates. These chemicals can also be problematic in low concentrations long after the treatment has finished and water changes have been performed. Unfortunately, most cases of poisoning in invertebrates (including non-crustaceans, such as mollusks) causes behavior very similar to that of oxygen deprivation. That makes the case difficult to identify early on, and only differs by the fact that the animal usually dies after being moved into a safe environment. If crustaceans die repeatedly without explanation in a tank where the fish show no problematic symptoms, poisoning from residual chemicals from medications or metals may be to blame. A good rule of thumb is to never use medications in a display tank with invertebrates and to quarantine and treat the fish separately when possible. In areas where the quality of the tap water is unreliable, avoid use of tap water unless it is first put through reverse osmosis. Water obtained from reverse osmosis units should be free of any hazardous chemicals and is the best choice for top-offs and mixing new saltwater.

Desiccation

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Sometimes even a tight-fitting tank lid isn’t enough to stop a hermit crab from taking a leap. This is particularly true for species like Calcinus seurati and C. laevimanus, which are observant enough to see an open tank lid and make use of the opportunity to crawl up rocks and power cords to explore. Although they may not intend to completely leave the tank, once the top of the tank is reached, hermit crabs tend to fall off quite easily. Species that are mostly or fully aquatic are at risk from desiccation when this happens. That doesn’t mean they’ll give up the ghost easily, but it’s an unfortunately common mistake for aquarists to see a shriveled, dry animal that has fallen out of an aquarium and assume that it must be long dead. While a dried-out, shriveled fish that is too stiff to move may be long gone, the same assumption shouldn’t be made for hermit crabs. To be honest, I probably wouldn’t have believed that marine hermit crabs could make a full recovery after being turned into a dry, crispy husk of their former selves until I saw it first hand with a C. laevimanus. The one night I forgot to put the lid back down, the juvenile hermit took note of my forgetfulness and took the plunge. In the morning, it was out of its shell and showed no signs of life. The poor thing was dry as a bone, curled up into a ball with a shriveled body. Less than 5 minutes after being back in the water, a leg started twitching. A few more minutes and the body had returned to its normal shape. The next day, it was fully back to normal – although distinctly less interested in climbing power cords than before. Although it is remarkable that these animals can survive drying out to such a degree, it is critical that a desiccated hermit crab not be put back in a tank where it could be savaged by animals that might not otherwise be able to harm it. Even ordinarily harmless scavengers such as beneficial worms from the substrate and Nassarius snails can be a hazard. Desiccation results in temporary paralysis, so a desiccated hermit crab is at the mercy of its environment until it re-hydrates itself and is mobile again. Desiccated hermit crabs should ideally go into a cup of tank water with an air bubbler until they are able to move around easily. Hermit crabs that have recovered from moderate to sever desiccation may not eat for as long as 1-2 weeks after the event. Softer foods will be taken preferentially at first.

Swollen Abdomen Syndrome This is a poorly-understood condition that is observed in both terrestrial and marine hermit crabs: the abdomen of an otherwise apparently healthy individual will swell and cause difficulty entering/exiting shells and finding a shell of suitable size. The swelling may go down once in a sufficiently large shell, but affected individuals will often die within a few days. Hermit crabs with this condition will first appear to hang a bit too far out of their shell for no obvious reason. If prompted to retract, they are usually unable to do so. Whenever these symptoms occur, the first step should be to offer a larger shell. A hermit crab will not always leave its shell when the swelling starts, so it must be isolated and presented with other housing options if it is to switch safely.

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Unfortunately, beyond isolated affected individuals and providing larger shells, the only thing that can be done is to wait and ensure that the water quality remains high. As the cause of the condition currently isn’t understood (it may be a symptom common to more than one condition), there is little to be done to prevent or cure it.

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References and Recommended Reading [Work in progress – additional references will be added to later versions] Books Richard C. Brusca, 1977. A Handbook to the Common Intertidal Invertebrates of the Gulf of California. The University of Arizona Press. This is the first edition of a field guide that addresses many invertebrates from the Gulf of California. There is also a second edition, going under the shortened title “Common Intertidal Invertebrates of the Gulf of California.” Robert J. Goldstein, 1997. Marine Reef Aquarium Handbook. Barron’s Educational Series. This book is useful overview of marine aquariums in general for the beginner. Although there is little on hermit crabs, it contains more detailed descriptions of equipment and chemistry than occurs in this document. Pete Giwojna, 1978. Marine Hermit Crabs. T.F.H. Publications Inc. Although old and therefore outdated in some ways, this book contains some useful observations and insights into hermit crab behavior. It contains numerous useful species photo references and describes behaviors absent from other literature on hermit crabs in captivity. Ronald L. Shimek, 2005. A PocketExpert Guide to Marine Invertebrates: 500+ Essentialto-Know Aquarium Species. Microcosm. This is a valuable resource for the hobbyist interested in marine invertebrates, since it contains both textual descriptions and color photos of many of the most commonly seen invertebrates. It includes a number of hermit crab species, as well as having good sections on gastropods and echinoderms.

Websites Author’s website: “CalPhotos,” University of California, Berkeley, 2010. U.C. Berkeley hosts an excellent search tool for finding correctly-labeled photos of different species of animals. Although it has a bias towards species from California and the hermit crab photo additions are relatively new, it should prove to be a wonderful resource for species identification as more photos are added over time.

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“Marine Species Identification Portal.” ETI BioInformatics, 2010. This website contains useful photo resources for identification of many marine invertebrate species.

Papers Willard N. Brownell and John M. Stevely, 1981. The Biology, Fisheries, and Management of the Queen Conch, Strombus gigas. Marine Fisheries Review. James E. Benedict, 1901. The Anomuran Collections Made by the Fish Hawk Expedition to Porto Rico. NOAA Fishery Bulletin. B.A. Hazlett and L.C. Baron, 1988. Influence of shells on mating behavior in the hermit crab Calcinus tibicen. Behavioral Ecology and Sociobiology. Mark E. Laidre, 2007. Vulnerability and reliable signaling in conflicts between hermit crabs. Behavioral Ecology. < http://beheco.oxfordjournals.org/content/18/4/736.full>

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The Care and Keeping of Marine Hermit Crabs

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