Hermit crabs are decapod crustaceans of the super family Paguroidea, not closely related to true crabs. Most hermit crabs salvage empty seashells to shelter and protect their soft abdomens, from which they derive their name. There are about five hundred known species of hermit crabs in the world, most of which are aquatic, living at a range of depths from shallow coral reefs and shorelines to deep sea bottoms, although some species are terrestrial. A number of species, most notably king crabs, have abandoned seashells for a free-living life; these species have forms similar to true crabs and are known as carcinised hermit crabs. Hermit crabs live in the wild in colonies of 100 or more, and do not thrive in smaller numbers.
As hermit crabs grow, they must exchange their shell for a larger
one. Since intact gastropod shells are not an unlimited resource,
there is frequently strong competition for the available shells,
with hermit crabs fighting over shells. The availability of empty
shells depends on the abundance of the gastropods and hermit crabs,
but most importantly on the frequency of organisms that prey upon
gastropods but leave the shells intact. A hermit crab with a shell
which is too tight cannot grow as fast as hermit crabs with
well-fitting shells, and is more likely to be eaten.
However, there are times when hermit crabs size up each other's shells and trade shells. As expected, these house swaps do not occur very often.
Also, having sea anemones on shells can be very useful for hermit crabs because they can scare away fish and other marine animals. Sea anemones also have an advantage of eating leftovers when the hermit crab is finished eating the food. Furthermore, sea anemones can be transferred to new shells with the hermit crab.
The various species range in size from the Pacific hermit crab, which rarely grows larger than a peach to Coenobita brevimanus which can approach the size of a coconut. The shell-less hermit crab Birgus latro is the world's largest terrestrial invertebrate. Terrestrial hermit crabs begin their lives in the sea but, through a series of moults, develop the ability to breathe air. After the last developmental moult, the young hermit crab will drown if left in water for an indefinite period of time. Their link with the sea is never entirely broken, however, as hermit crabs carry a small amount of water in their shells at all times to keep their abdomen moist and their modified gills hydrated. It is believed that C. brevimanus is the species of Coenobita best adapted to life on land.
The fossil record of in situ hermit crabs using gastropod shells stretches back to the Late Cretaceous. Before that time, at least some hermit crabs used ammonites' shells instead, as shown by a specimen of Palaeopagurus vandenengeli from the Speeton Clay, Yorkshire, UK from the Lower Cretaceous. The reproductive organs of hermit crabs are located near and just below the crab’s heart and open to the outside at the base of the last pair of walking legs in the male. In the female, they’re located at the base of the middle pair of walking legs. Female hermit crabs usually lay their eggs shortly after copulating, however they can also store sperm for many months. The eggs are fertilized as they are laid by passing through the chamber holding the sperm. The eggs are carried and hatched in a mass attached to the abdomen inside the shell. The number of eggs is usually large, but depends on the crab’s size. The developing crabs go through four stages, two of which (the baupilus and protozoea) occur while still in the egg. Most crabs hatch at the third stage, the zoea. This is a larvae stage wherein the crab has several long spines, a long narrow abdomen, and large fringed antennae. The fourth stage of development is the magelops. Hermit crabs are usually born in the ocean, near the shore. Because of this, hermit crabs cannot reproduce in captivity. After the crabs are born, they move inland away from the water, where they search for abandoned shells to inhabit. Hermit crabs then begin growing and developing through a process call molting. In this process, the crabs shed their exoskeleton. During this, the crabs are extremely vulnerable and inactive, and usually find protection by burrowing in the ground. It takes around 10 days for their new exoskeleton to harden, and during this period the crab is able to regenerate any lost or broken claws or legs. A hermit crab can molt as often as every other month when they’re young, or every 18 months when they’re older. The life span of the hermit crab in the wild is up to 30 years.
This Hermit Crab Page is Copyright The Animal Web Guide © 2004 - 2009 Chuck Ayoub