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Hermit crabs are found all over the world in shallow waters and are one of the more familiar coastal animals. Although they have an exoskeleton, as all crustaceans do, this protective covering only surrounds the head, legs and claws. To protect the soft rear part of the body they must find a suitable shell to crawl into.

This adopted shell is usually the remains of a snail that has died. When the snail was alive the shell grew with the animal. However, as the hermit crab grows it must periodically find a larger shell to move into. Because they change shells over time there is a common misconception that the crab doesn’t shed the exoskeleton as other crabs do.

Shedding is the most vulnerable time of an adult crab’s life. Because the new precursor shell takes several hours to begin to harden, the molted crab is helpless against predators. Hermits can avoid this dangerous time by shedding inside the snail shell, ejecting the old exoskeleton, and then hunkering down until the new one forms.

A riskier time for this animal is when it needs to change into a larger shell, which leaves the soft part of the body briefly exposed. For this reason the crab will make the switch as fast as possible.

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Hermit crab removed from its shell.

When a potential new home is found the crab will inspect it very carefully with its claws to be sure nothing is hiding inside. Then once the two openings are lined up it will slide out of the old shell and quickly slip into the new one. If it doesn’t like the fit it will return to the old shell and continue its search.

This shell-changing behavior is fascinating to watch in captivity. However, it doesn’t happen very often since each crab seems to have an uncanny memory of which shell it has already tried. You can trick them into switching pretty easily, though. Once he’s tried out a shell and opted out of moving in, you can remove the shell from the water, dry it out completely and then drop it back in. The crab will try the same shell over and over again as long as this process is repeated.

Some species can get fairly large, about the size of a softball. But the maximum size is determined by the species of snails that live in their habitat. Here in New England we have two species. The long-clawed hermit crab stays in very shallow water and is quite small as the intertidal snails like periwinkles and oyster drills do not grow more than an inch or so. Flat-clawed hermits live in deeper water and can grow to a much larger size by inhabiting the discarded shells of moon snails and whelks.

Both crabs are omnivores, scavenging dead animals and grazing on microalgae that grows on rocks and often on the shells of neighboring crabs. In spite of the name, hermits are not loners and tend to congregate in groups based on favorable feeding areas.

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Hermit crab colony.

Although hermits are mainly marine animals, landlocked readers may be familiar with them because of the popular "tree crabs" sold in most pet shops. These are tropical species, and although they do indeed live on land, they are still tied to the water for survival. First, all females must return to the ocean’s edge to lay their eggs, releasing them onto the sand or rocks to be carried away by the tide.

Secondly, there is a common belief that these crabs actually breathe using lungs. They have gills like all crabs, and must stay close to a water source for oxygen. Periodically they will dip their claws into the water and soak these gills.

Land crabs can utilize the shells of washed up marine snails or the shells of terrestrial species. Unfortunately, in captivity they are often subjected to the indignity of wearing the decapod version of a clown suit.

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Other diaries in this series can be found here.

Originally posted to Mark H on Fri Feb 29, 2008 at 06:55 PM PST.

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