Hermit crabs are decapod crustaceans of the superfamily Paguroidea.They are not closely related to true crabs. Most species of hermit crabs have long soft abdomens which are protected from predators by the adaptation of carrying around a salvaged empty seashell, into which the whole crab's body can retract. Most frequently hermit crabs utilize the shells of sea snails, marine gastropod mollusks. The tip of the hermit crab's abdomen is adapted to clasp strongly onto the columella of the snail shell. As the hermit crab grows in size it has to find a larger shell and abandon the previous one. This habit of living in a second hand shell is what gave rise to the popular name "hermit crab", which is a reference to the idea of a hermit living alone in a small cave.--------------- There are about five hundred known species of hermit crabs in the world, most of which are aquatic and live in saltwater at depths ranging from shallow coral reefs and shorelines to deep sea bottoms. However in the tropics, a number of species are terrestrial, and some of these are quite large, for example, Coenobita clypeatus. A number of other species, most notably king crabs, have abandoned seashells for a free-living life; these species have forms which are more similar to true crabs, and are known as carcinized hermit crabs. Hermit crabs live in the wild in colonies of 100 or more, and do not thrive in smaller numbers.