Hermit Crab, a land- or water-dwelling crustacean. Unlike true crabs, hermit crabs have soft, vulnerable abdomens. For protection from predators, many hermit crabs seek out abandoned shells, usually snail shells. When a hermit crab finds one of the proper size, it pulls itself inside, leaving several legs and its head outside the shell. (A hermit crab has five pairs of legs, but not all of them are fully developed.) A hermit crab carries the shell wherever it goes. When it outgrows its shell, it switches to a larger one. Most adult hermit crabs are from 1/2 inch (13 mm) to 4 3/4 inches (121 mm) long. Living on the seashore, in tidepools, and on the sea bottom in deeper water, hermit crabs scavenge their food.

The hermit crabThe hermit crab carries a snail shell on its back for protection.

The giant and striped hermit crabs are commonly found along the Atlantic coast of the United States and in the West Indies. Land hermit crabs are collected in Florida and the West Indies and sold as pets. The coconut, or robber, crab, native to the South Pacific, lives in a burrow rather than a shell.

Which Crab Carries Its Home on Its Back?

A hermit crab is a crab that does not have its own hard shell. A hermit crab carries a discarded snail shell on its back for protection.

A hermit crab’s soft body is naturally flexible and can twist easily to fit into the spiral interior of a snail’s shell. As a hermit crab becomes bigger, it outgrows its shell and shops for another.

Sometimes, hermit crabs carry other organisms, such as sea anemones, on their shells. These hitchhikers help camouflage the crabs. When a hermit crab changes shells, often the anemone will transfer to the new shell and continue along with its crab friend.

In places where discarded shells are hard to come by, hermit crabs may cover themselves with pieces of bamboo or coconut shell. Others hide among coral reefs.

How Do Some Hermit Crabs Decorate Their Shells?

Land hermit crabs are picky about their shells. They seem to “shop” for the perfect shell. Once they find one they like, they move in and simply enjoy it.

Cousins of land hermit crabs include more than 800 kinds of marine hermit crabs. At least one kind—the Dardanus crab—moves into a shell and then decorates it.

Dardanus crabs often place sea anemones on their shells, which works well for both animals. The hermit crab gets a little extra protection from other sea creatures, because sea anemones are poisonous, which makes other creatures wary of them. The anemones get leftover food from the crab and a free ride.

How Can You Tell When a Hermit Crab Is About to Molt?

Some hermit crabs give no sign that they are ready to molt. But others may look different and act strange. Here are some of the signs of molting you might notice in a hermit crab:

Its eyes may get cloudy.

It may eat and drink more than usual and grow a sac of fat. Then, it may stop eating and become less active.

It may dig a lot.

If your hermit crab shows several of these signs, place it in a warm, humid tank by itself. Supply food and water dishes and 6 inches (15 centimeters) of such bedding as moist sand. If your crab is molting above ground, spray it lightly with mist. Otherwise, leave it alone.

After it has molted and grown a new exoskeleton, it will reemerge. Let your crab eat its old exoskeleton for the nutrition it provides.

What Kinds of Shells Do Hermit Crabs Like?

All land hermit crabs need new shells to fit them as they grow. For that reason, you should supply your hermit crab with many shells in various sizes.

Different hermit crabs like different kinds of shells, and they also enjoy “shopping” for shells. Hermit crabs will spend a lot of time checking out new shells, so replace them from time to time.

Purple pinchers like roundish shells with circular openings, smooth insides, and a coil leading off to the right. They usually prefer short coils so they do not have too much shell to drag around. Pet purple pinchers have no preference between unpainted, natural shells and decorated shells.

Different kinds of hermit crabs prefer other kinds of shells. Ecuadorians, for example, like shells with wide, oval openings. Be sure to choose your shells carefully so your crabs will be happy with the selection.

What Is in That Shell?

A hermit crab has two pairs of antennae and round eyes on the ends of eyestalks. Hermit crabs have 10 legs, but only 6 legs show. These front 6 legs are known as walking legs. Hermit crabs keep their 4 back legs inside their shell. The back legs are much smaller than the walking legs.

The front pair of legs ends in claws, or pinchers. The left front leg has a large pincher, which the crab uses for moving around and defending itself. When the animal is hiding inside its shell, it uses this pincher to seal off the shell’s opening. The right front leg has a smaller pincher, which the crab uses to eat and drink. Both front pinchers have thick layers of exoskeleton.

Who Is the World’s Largest Land Crab?

Most hermit crabs are small, but the coconut crab is not like most crabs. This land crab, which lives on islands in the western Pacific and Indian oceans, can grow to about 3 feet (0.9 meters) from claw to claw. An adult coconut crab can weigh as much as 9 pounds (4 kilograms).

Of course, it can be hard to find a shell big enough to fit a coconut crab, but that does not bother this animal. After it outgrows the largest shell it can find, a coconut crab goes through a metamorphosis, or change. Unlike other hermit crabs, the coconut crab grows a shell of its own. This shell forms when the coconut crab is about a year old.

Coconut crabs breathe through modified gills, but cannot breathe in the water. They burrow in limestone, coral rock, or in undergrowth on land. Still, coconut crabs need to lay their eggs in water, and they need water to keep their gills wet.