There are two types of hermit crabs – land and marine and about 500 different species of hermit crabs around the world. Most hermit crab species live on the ocean floor, but many live on land. Female land-based hermit crabs return to the sea to breed.
The hermit crab is a crustacean. The front half of the crab is covered with a hard exoskeleton (hard outer skin). The long abdomen however has a softer exoskeleton. For this reason, hermit crabs use salvaged empty shells which they live in to protect their abdomen. As the crab grows in size, it must find a larger shell to provide this protection. Several hermit crab species, both terrestrial and marine, use “vacancy chains” to find new shells: when a new, bigger shell becomes available, hermit crabs gather around it and form a kind of queue from largest to smallest. When the largest crab moves into the new shell, the second biggest crab moves into the newly vacated shell, thereby making its previous shell available to the third crab, and so on.
Hermit crabs depend upon properly fitting shells for protection from predators, mating success and reproduction. Where there is a shortage of shells, land hermit crabs have been found attempting to shelter themselves in items they find on the shores such as glass jars and bottle tops. This is the reason why people are discouraged from picking shells.
Hermit crabs vary widely in colour, from red to brown to purple, with stripes, dots, and other patterns. They have ten jointed legs; the front two legs have large, grasping claws (called pincers or chelipeds) and the rear pair of legs are very small. They have a flattened body, sensory antennae, two eyes located at the ends of stalks, and a soft, twisted abdomen (which the hermit crab keeps hidden inside its shell).
Hermit crabs are omnivores (eating plants and animals) and scavengers (eating dead animals that they find). They also eat worms, plankton, and organic debris.
(Photo credit: Cas-Eikenaar)