Anatomy

In terms of lifestyle there are two distinctly different types of anomurans: false crabs which look and behave similarly to true crabs (Brachyura) and hermit crabs which live in shells. But they are similar anatomically in having the fifth leg much smaller than the other walking legs. In false crabs three pairs of legs are used for walking and in hermit crabs two pairs extend from the shell to move it and its inhabitant about. The abdomen of false crabs is tucked under the body as it is in brachyuran true crabs. Hermit crabs are rarely seen out of the gastropod shells they inhabit. As would be expected their body structure is modified to their lifestyle. All hermit crabs have a twisted soft abdomen protected by an empty shell into which the animal can retreat when threatened. The opening to the shell is closed by the strong claws of the crab to protect the rest of the body and the animal can be almost impossible to dislodge. As the hermit crab grows, it must find a larger shell and in some dense populations, fighting for shells is a frequent event.

Reproduction


Like true crabs, female false crabs usually lay their eggs shortly after copulating but can also store sperm for many months. The eggs are fertilized as they are laid by passing through the chamber holding the sperm. Eggs are carried and brooded in a mass attached to the abdomen. In the case of hermit crabs this is inside the shell. The number of eggs is usually large but depends on the size of the crab. Once developed the egg hatches into a zoea which usually swims off to begin life in the plankton. There it goes through several moults as it grows and develops. Eventually the final larval stage will settle on the sea floor before transforming to a juvenile crab.

Sense Organs

False crabs and hermit crabs have the same sensory organs as true crabs. With long stalked compound eyes they appear to see very well with some species detecting movement 20 or 30 metres away. Like true crabs they have bristles and hairs of varying lengths which act as touch receptors. The bristles can occur all over the body but are most frequently found in clumps on the walking legs. These bristles signal contact with a hard surface simply by bending, while other shorter hairs are sensitive to water currents. Crabs can find food using chemical stimuli. The antennae of false crabs are much longer than those found on true crabs. These have "smell detectors" which detect chemicals that stimulate food searching behaviour. When similar detectors on the legs contact food the chelipeds quickly grasp it and passes it into the mouth. The mouthparts have further receptors which are sensitive to particular chemicals. Anomurans rely on a combination of these sense organs to move around and find food and mates.

Distribution

Hermit crabs range from the deep sea to just above the shore line. In southern Australia hermit crabs and false crabs can be found intertidally but in the tropics some hermit crabs spend considerable time out of the sea. The coconut crab, Birgus latro, of Pacific islands can climb palm trees!