There are many things I could discuss here, but I thought it might be most useful to focus on some of the species I have found most interesting over time. Hopefully the information on each species will help explain why I’m an advocate of both the “pick a species and stick with it” and “stock lightly and carefully” guidelines for keeping marine hermit crabs in an aquarium.
The Problem with Hermits as a Generic Janitor
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 the behavioral expectations are more realistic for the fish than the hermit crab.
A hermit crab plucked at random from the aquarium trade is not a cookie-cutter-style 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 (yellow tang in a nano, anyone?). There are many common species that grow to different sizes and have different behavior, just as is true for different fish species. Many species will work, many won’t. The key to avoiding a cleanup crew disaster is as simple as employing careful observation and research instead of just filling a tank with mystery critters and hoping for the best.
To exemplify some of these differences and show the more interesting aspects of hermit crabs as more than just a janitor, I will discuss my observations of experiences with some of my favorite hermits: the small Clibanarius species, Calcinus seurati, Clibanarius vittatus, and Pagurus longicarpus. The latter two are species are relatively rare in the aquarium trade (especially outside of North America), but they are native to the Gulf Coast and the eastern shores of North America, and I have therefore had the opportunity to observe them in the wild in addition to in the aquarium.
Before launching in to discussing discussion of species, I should point out that common names alone cannot yield accurate species identification. The vagueness of common names for these animals is a big problem, second only to the lack of sufficiently detailed physical descriptions of hermit crab species in the literature (or perhaps I should say the lack of descriptions that don’t require killing the animal and dissecting it for identification). At present, correct identification of many live hermit crabs is non-trivial. Of course, that doesn’t mean it can be ignored. It simply means that, for hermit crabs, a responsible aquarist’s job is a bit harder and may involve some careful trial and error to compensate for a lack of information.
For those who are only familiar with the common names, I will provide a brief “key” for the scientific names used here and the associated common names I’ve seen – but do observe the amount of overlap of the names.
- Clibanarius tricolor: dwarf hermit, mini-hermit, blue-leg hermit, blue-tip hermit, blue hermit.
- Clibanarius digueti: dwarf hermit, mini-hermit, Mexican red-leg hermit, red-leg hermit, red-tip hermit, red hermit.
- Clibanarius erythropus: red-leg hermit, red-tip hermit, red hermit.
- Calcinus seurati: dwarf zebra hermit, zebra hermit, dwarf hermit, left-handed hermit.
- Clibanarius vittatus: large reef hermit, reef hermit, skeleton hermit, striped hermit.
- Pagurus longicarpus: long-armed hermit.
The Small Clibanarius Species
There are many small Clibanarius species, but three in particular are both common in the aquarium trade and also very similar in maximum size and behavior. These are Clibanarius tricolor, C. erythropus, and C. digueti. All three species range over tropical areas, with C. erythropus also spanning some cooler regions in Europe. C. tricolor is by far the easiest to identify, usually going under the name “blue leg hermit.” However, the other two fall under the “small brown Clibanarius” label – a group of species for which reliable physical descriptions are hard to come by. Sometimes they’re called “red-tipped hermits” or “red-leg hermits,” but these names are equally nondescript.
For C. erythropus and C. digueti in particular, I must admit that even after several years of trying to hone my species identification abilities, I still make mistakes with these two when they are juveniles (at which point they are often sold as “micro hermits” – another misleading term). Collection location can separate the two sometimes, but that information is rarely known at a store. Both are a drab brown with red tips on the walking legs, and the coloration primarily differs in the amount of blue speckling present and the prominence of a narrow blue stripe on the red leg tips. These features can be almost invisible on small specimens.
Because of the similarity between species, small Clibanarius species are more interchangeable than other genera and species. The three species described are peaceful if not overstocked, and sometimes even show some preference for being kept in groups. In a mixed-species environment, same-species individuals will forage together, and individuals kept as breeding pairs in particular may show signs of distress if the other hermit either dies or is separated. Similarly, introduction of a new individual into an established group can attract small amounts of short-term aggressive attention from existing hermits.
Small Clibanarius kept in male/female pairs or relatively equal gender ratios are likely to breed in the aquarium, although it can go completely unnoticed or be mistaken for aggression. Courtship behavior consists of some combination of vigorous shell clacking and little dances done by the male over a period of days to weeks. Unfortunately, although eggs may result and be carried by the female until hatching, survival of the larvae in the average tank is highly unlikely.
Clibanarius tricolor is sometimes cited for snail-killing, but the snails that become victims small Turbinids, Nerites, and similar species that are poorly protected and in the right size-range to be used as a home by the hermits in question. If snail species are more carefully chosen (avoidance of mystery critters applies to snails as well!), this issue will be fully avoided. I imagine that if small Clibanarius were the only species used as janitors and snails were more carefully chosen to stay above the hermit’s maximum shell size, the number of compatibility-related complaints in the hobby would be much smaller.
Calcinus seurati
This hermit crab goes under the common name “zebra hermit” or “dwarf zebra hermit.” The second name is rather unfortunate, since the “dwarf” term seems to stem from the fact that most individuals seen in the aquarium trade are young and often don’t last long enough to reach the species’ maximum size. The maximum size is large enough and growth rate fast enough that my first seuratis actually had me wondering if I was in possession of another species. Although “zebra hermit” makes sense, “dwarf zebra hermit” is very misleading.
An adult C. suerati can grow to have a large claw about the size of a human thumbnail. Given that most are sold when the entire hermit and shell are about the size of a thumbnail, that’s a lot of room for growing. When small, they can be kept in a relatively cramped environment without trouble. However, as they grow, their tolerance for other species – and even each other – greatly diminishes.
Although there are other Calcinus species that are quite peaceful by comparison (such as C. elegans), C. seurati is one of the species that contributes to the reputation of hermits as both violent creatures and snail-killers. The species is quite adept at popping the operculums off of small snails if it wants a snack, and if a fight breaks out over some resource (shells, food, etc.), somebody will probably lose a limb. Despite the seemingly evil nature of these hermits, their aggression can actually make them very interesting to keep, since it means they can also be quite interactive. The aquarist must simply be aware that one of these hermits can easily claim an entire nano for itself and plan accordingly. It has been my experience that a single Calcinus seurati can easily do the same job as several small Clibanarius, such that the decreased population density will not have a negative impact on the state of the tank.
Clibanarius vittatus
Clibanarius vittatus is often sold incorrectly in pet stores as a “large reef hermit.” Although “large” is correct and they get significantly larger than most hermits sold in the aquarium trade, “reef” is not very accurate. 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. They are not strictly reef animals, and are generally poorly suited to such an environment in an aquarium – or rather, such an environment in an aquarium would be poorly suited to them. They could probably be placed in just about any marine tank due to their hardiness, but would tear things apart with their normal foraging behavior. Newly introduced hardware can attract just as much interest as a newly introduced empty shell, and these hermits can dismantle anything that isn’t securely attached. To be completely safe, these hermits require a tank built around their curiosity and strength. A species tank is ideal to avoid compatibility issues with smaller/weaker hermit crabs that would be at risk of injury.
If sufficient food is present, adult C. vittatus will surprisingly make no attempt to attack even quite small marine snails, such as Nassarius species. Larger snails may actually be at a greater risk, since their shells might make a good home. In the wild, shells are extremely scarce, so these hermits must be very opportunistic. 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.
C. vittatus shows a similar pattern of social behavior to the small Clibanarius species discussed previously. Adding new individuals into an established group will provoke aggressive responses from larger and/or more dominant individuals. Courtship behavior is also similar, although episodes of vigorous shell clacking are replaced with the male tapping his pinchers on the female’s shell.
Pagurus longicarpus
I chose to include this species not because it is commonly kept, but because I think it warrants more attention. I find its behavior fascinating, and the species is also representative of a group of often-ignored hermits: those that live in temperate to coldwater conditions. One advantage to these hermits is that they can be kept in an unheated tank year-round.
These hermits are extraordinarily fast. Compared to the Calcinus and Clibanarius species discussed so far, these hermits are very fleet footed and have interactions at such a fast pace that they may go unnoticed by the inobservant aquarist. They also do not show aggression in the ways typical of the species discussed above. I have yet to see one individual actually damage another in captivity, although they are very scrappy.
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 off limbs and feelers, these hermits will “slap” each other with the longer arm. If one hermit feels that another has gotten too close, it will quickly flail its longer arm out to, quite literally, slap the encroaching individual in the face. This behavior is also done to snails, human hands, and sometimes even inanimate objects.
Breeding behavior in P. longicarpus is much like that of the small Clibanarius, but there is a sharp difference in the type of little dance done by the males. The male rotates the female onto her back, and then he entices the female to grab his pinchers. The male then does a very jerky set of movements, wiggling his arms from side to side with the female’s arms and later tickling the underside of the female’s walking legs. Shell clacking sometimes precedes the dance.
These aspects of the species 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.
Closing Remarks
There are many other interesting species I have seen in the aquarium trade that warrant detailed documentation, some even more common than the ones I have discussed. Because of the distinct lack of reliable information for many hermit crab species, I encourage other aquarists to make careful observations about each species as it is encountered. In order to bridge a gap in knowledge, somebody at some point will have to turn his/her tank into an experiment to find the answer. It’s simply important to do so knowingly rather than accidentally.
but that last photo is awesome
. You should actually thank the crab. I think its the same one from the nudey shots. He's quite the exhibitionist!
