The front half of a hermit crab is covered with a hard exoskeleton like that of most other crabs. The long abdomen has a softer exoskeleton, which can adapt to fit into a spiraled snail shell. A crab presses its abdomen, fourth and fifth pairs of legs and uropods against the shell's inner wall by contracting its longitudinal muscles. The large left uropod (appendage at the end of its abdomen) hooks to the center post of the shell. They use their large left claws for defense, for holding onto tree limbs and for balance. The smaller right claw and the next pair of appendages, which also have claws, are used for collecting and passing food and water to the mouth. There are rough surfaces that help hold the crab in the shell. Hermit crabs have reduced gills and their moist gill chambers have highly vascularized areas for gas exchange.
Hermit crab species come in a range of sizes, from a fraction of an inch (a few millimeters) to nearly the size of a coconut.
Many different species of land hermit crabs live in tropical areas of the Indo-Pacific region, the western Atlantic and the western Caribbean. Land hermit crabs live close to the shoreline and must have access to both land and water. They use pools and crevices of sea water to wet their gills and the interiors of their shells, and they reproduce and spend their early stages in water. Other hermit crab species are entirely aquatic.
Hermit crabs have stalked eyes with acute vision and two pairs of antennae. They use the longer pair for feeling and the shorter, feathery pair for smelling and tasting. They also have sensory hairs that are part of the exoskeleton. They use these hairs and their antennae as vibration sensors.
Hermit crabs are omnivorous scavengers. They eat whatever they find, although not the former occupants of the shells they use.
At the Smithsonian's National Zoo, land hermit crabs are fed crab food and assorted fruits and vegetables.
Hermit crabs mate in seawater. Before mating, the male holds the female with one claw and taps or strokes her with the other or pulls her back and forth. Both crabs emerge partially from their shells to mate. They place their stomachs together to mate. The male has a long, tubular penis with which he puts sperm into the female. After the eggs hatch, the larvae go through several aquatic life stages and molts. When adulthood is reached, the crabs migrate to shore for a terrestrial life.
As a hermit crab grows, it molts, shedding its exoskeleton and creating a new, larger one to accommodate its larger body. A crab molts by building up enough water pressure in its body to split its old shell. Some crabs leave their shell and bury themselves in sand to molt. Some species store water in their shell before molting, and remain in the shell throughout the molt, which may take from 45 to 120 days. You can tell that a crab is freshly molted when it has a clean, fresh, bluish color. A crab may eat its molted shell, possibly for its calcium, vitamins and minerals.
Land hermit crabs can live about 10 years but have been known to live to 11 years old in human care.
Hermit crabs are extremely popular pets, and can be easily found in pet stores. But hermit crabs do not breed in human care—they return to the ocean to breed and live out the first part of their lives. As a result, all hermit crabs purchased through pet shops come from the wild. Also, land hermit crabs are at risk of habitat loss, as the mangroves and coastal areas where they live are taken over and developed by humans.