Hermit crabs are some of the best-known of the crustacean clan because of their habit of adopting empty snail shells as a home. These crabs have been of special interest to animal behaviorists because of their fascinating habits — in fact, scores of scientific papers have been written on shell utilization of the paguroids.
However, hermit crabs are not only of interest to ethologists. They have also received the special attention of aquarists since the first saltwater aquarium was set up. Today, any aquarium store “worth its weight in salt” will have numerous hermit crabs, often representing several different species. The algae-eating, scavenging and sand-sifting habits of some of the more diminutive forms have made them a popular choice for reef aquarists. In this article, we will look at the biology of these fascinating crustaceans, as well as their care requirements.
Hermit Crab Habits
These crustaceans belong the order Decapoda, the infraorder Anomuran and the superfamily Paguroidea. This group is composed of around 800 species of hermit and coconut crabs. The paguroids are found from the tropics to polar regions. There are members of the superfamily that live in trees (e.g., coconut crab, Birgus latro) and many species that spend much of their time right at the water’s edge. There are also species that live in the abyssal depths.
As mentioned, the shell-carrying behavior of these crustaceans has received the most attention. Their calcareous mobile home provides protection from predators, mechanical damage (e.g. being tossed about by heavy wave action) and desiccation (especially in those species that live in the intertidal zone). Not all of these crabs live in gastropod shells. Members of the primitive family Pylochelidae are found living in pieces of wood, rocks, sponges and tusk shells. Some of the miner hermit crabs (genus Cancellus) use rocks as a sanctuary, occupying tunnels created by burrowing mollusks. In some cases, the crabs will live in a stone as big as a fist, which it drags around after dark.
The many studies on shell selection in hermit crabs have yielded interesting and sometimes contradictory findings. One of the most important environmental factors influencing the types of gastropod shells utilized is availability. Empty shells can be in short supply, at least in some marine habitats. If there are more housing options (i.e., more shells), a hermit crab can be more selective about the type of shell it lives in.
It should not be surprising that larger hermit crabs can use larger, heavier shells. There are advantages and disadvantages to using a more robust shell. One advantage is that a crab in a stronger shell will be less vulnerable to predators; but a heavy shell can be more difficult to maneuver and will require more energy to transport. A large, cumbersome abode may also negatively impact mating success for male crabs. They may have a difficult time manipulating the shell, their mate and engaging in pre-copulatory behavior.
The roominess (internal volume) of the shell is important. The cavity in which the crab retreats must be big enough for it to withdraw its body and for appendages completely to avoid being nipped off. To increase the protective qualities of the shell even further, when some hermit species pull into the shell, the claw serves as a lid to cover the shell aperture.
When a hermit crab outgrows its current abode, it will attempt to locate a new shell to occupy. While the visual system may be important for shell location, other senses help to find and evaluate the suitability of an empty shell. For example, hermit crabs can smell the calcium emanating from an empty shell. Once it finds an empty shell, it will touch the outside of the shell, insert its appendages into the openings (possibly checking it out for roominess) and roll the shell about. If the potential new home passes the “feel test,” the crab will make the move. If it switches shells, but finds its new home to be too small or too heavy, it will return to its old shell.
If shells are in short supply, some hermits might attempt to usurp shells from confamilials or even rip their original owners (i.e., snails) from their homes. When hermit crabs fight over shells, they engage in a behavior called “rapping.” This is where an attacking crab rapidly strikes its shell against the shell of the defender. This will continue until the defending crab exits its shell or the attacking crab moves on. There are some hermits that are better at stealing shells than others. For example, the left-handed hermit crab (Calcinus laevimanus) is a more aggressive, better shell-fighter than the zebra hermit (Clibanarius zebra); the former species also has a higher intrinsic drive to change shells than C. zebra. One characteristic that makes C. laevimanus a better shell-fighter is that it “raps” harder than its sympatric kin.
It should not be surprising that when competition exists for shells, better “rappers” occupy more desirable shells than more timid species. When members of the same species are fighting over a shell, larger individuals are more likely to evict smaller defenders, though smaller crabs may defeat larger animals if the latter is still “soft” from a recent molt.
Some hermit crabs (e.g., Dardanus pedunculatus) adorn their mobile homes with sea anemones (e.g., Calliactis spp.).
Those species that carry sea anemones on their shells will transfer these animals when they move to a new home. They do this by tapping on the pedal base with their chelae, which causes the sea anemone to loosen its grip. Gradually, the hermit is able to work the sea anemone off with one or both chelae, after which it slaps the anthozoans on the new shell (the process of moving its anemone decor may take several hours). If a sea anemone is damaged in the process of transfer, the hermit crab will consume its symbiont. It has been suggested that the stinging cells of the sea anemones may provide the hermit crab with another layer of defense. At least some anemone-hosting hermits have been observed feeding their cnidarian partners.
Do hermit crabs, with their calcareous armor, really have to worry about predators? Yes, they do. This includes other crustaceans, including the box crabs (Calappa), which use oversized, toothed claws to break apart snail and hermit crab homes. There are also fish that have dentition that enables them to do the same thing. In a study on food habits of Caribbean fish, 34 species had eaten hermit crabs. This included morays, snappers, porgies, drums, large wrasses, triggerfishes, puffers, spiny boxfishes and porcupinefishes. In most cases, these fish do not wrest the crabs from their shells but crush the whole shell, ingesting the shell fragments along with the crab. These fish are a threat to small hermit crabs in the aquarium, as well. However, a large hermit with a heavy shell is probably going to be able to withstand the attacks of all but the largest moray, trigger, puffer or porcupinefish. Larger hermits may be less active during the day if kept with larger, troublesome fish that may pick at their appendages (e.g., antennae, eyes).
Hermit Crab Husbandry
Hermit crabs have been popular in the aquarium trade for many years. When I set up my first aquarium back in the early 1970s, larger hermit crab species (most in the genus Dardanus) were readily available and recommended as scavengers for the marine aquarium. I only found out about the downside of paguroid ownership when I found my medium-sized hermit with a struggling false percula clownfish trapped in its chelae. At that point, I swore off hermit crabs for more than a decade. It wasn’t until the smaller algae-eating varieties started showing up in the aquarium trade that they made their way back into some of my aquariums.
There are a number of smaller hermit crabs that are best employed in both a scavenging and algae-eating role. When attempting to acquire one of these animals, make sure you don’t end up with a juvenile of one of the more destructive species. Make sure you can identify the crab as one of the following (or a similar smaller species) before adding it to your tank. The most efficacious herbivores that are also some of the most benign belong to the genera Calcinus and Clibanarius. The species most often encountered include the orangeclaw hermit crab (Calcinus tibicen), dwarf zebra hermit (C. seurati), electric blue hermit (C. elegans) and the blue-leg hermit (Clibanarius tricolor). Another less common but equally desirable species is the red-legged hermit crab (Clibanarius digueti). This species not only is a great filamentous algae-eater, it will also consume blue-green algae with reckless abandon.
All of these are small hermits (less than 1 inch in length) that feed primarily on detritus and algae. Also, the food-foraging activities of these hermits can help keep the upper layer of the sand bed stirred. Some species/individual hermit crabs tend to show a preference for either hard or soft substrates. For example, C. tibicen tends to work over the rock work, while C. tricolor spends more time feeding off the sand surface.
While many of the members of the genus Paguristes get too large and are too dangerous to be kept with small fish and sessile invertebrates, there is one species that I would recommend for a reef aquarium: the red reef hermit crab (Paguristes cadenati). This is an interesting little crab because it rarely moves around much. Even at night, when feeding is at its peak, it does not cover much territory. The red reef hermit crab may feed on the sand or rock work. The polka-dot hermit (Phimochirus opercularis) is another pygmy species that will voraciously eat algae and will substrate-sift. It tends to be more aggressive than P. cadenati, stealing food from other hermits, and it may fight with members of its own kind.
As a crustacean grows, it molts its exoskeleton. In captivity, it is not uncommon for hermit crabs to molt once a month if they are well-fed, with the process taking as long as 10 days to complete. Some hermit crabs engage in what is called “prenuptial molting;” that is, they molt prior to mating. When they molt, it takes a while for the exoskeleton to harden, and it is at this point that they are most vulnerable.
One thing you must do as a conscientious paguroid owner is provide them with an assortment of gastropod shells that they can move into once they outgrow their current shells. If you do not provide new housing, they are more likely to attempt to oust snails from their shells and take them over or fight with each other for accommodations. Empty snail shells are often available from aquarium suppliers.
One of the biggest hermit husbandry obstacles is overstocking. Many reef aquarists, in an attempt to control or prevent the growth of undesirable plant material, chuck numerous small hermits into their aquarium. This often results in squabbles over domiciles, as well as food deprivation and subsequent starvation. If you are adding small, herbivorous hermits to a tank to control algae, a good rule is one crab per 5 gallons (of course, there are variables that will impact this rule, such as how much you feed the tank and how much natural fodder is present).
It is also imperative that some food gets to the substrate where these crabs can get it. Every third or fourth feeding, it is a good idea to turn off all the water pumps for approximately 10 to 20 minutes so that some food settles to the substrate and benthic scavengers can fill their bellies. You can also feed them the sinking pellet foods or algae wafers (available for freshwater algae-eating fish). These foods drop to the substrate where they can easily find and eat them. If your hermits are very active during the day, it usually is a sign they are hunting for food, which typically indicates they are not getting enough to eat. In nature, hermits usually spend the daylight hours in their shells, where they are less of a target to crustacean predators.
Remember that larger paguroids cannot be trusted with smaller fish or with sessile invertebrates. Even so, they can still make interesting pets. If you want to keep a larger hermit (e.g., Dardanus spp.) in a community situation, make sure the fish are much larger than the crab, so they are able to get away from a marauding crustacean. If you see wounds or torn fins on the body of a fish that cannot be attributed to fish-on-fish aggression, you may want to remove the large hermit. I suggest that you should keep the destructive and predatory hairy hermit crab (Aniculus maximus), which is a spectacular animal, by itself in a species tank.
That ends our looks at these fascinating crustaceans. While they have a place in certain aquarium venues, care must be taken to acquire the right species for your aquarium community. It is also imperative that you don’t place too many in your tank and that you make sure they get enough to eat. Happy crab-watching!
Feeding Hermit Crabs
Smaller herbivorous hermits are usually not a threat toward invertebrate or fish tankmates. However, if they are being starved, even they may go astray on occasion. This usually consists of their subduing small fish at night or grazing on sessile invertebrates (e.g., colonial polyps, stony corals). That said, more often than not, if you see them among polyps, they are harvesting the algae and/or detritus that grows between them. It is possible that starving hermit crabs may eat snails and/or other hermit crabs; they typically target other paguroids that have just molted.
Scott W. Michael is the author of Reef Sharks and Rays of the World, Reef Fishes: A Guide to Their Identification, Behavior and Captive Care; and more. His photos have appeared in publications around the world.