Crab (assorted)

Crabs are decapod crustaceans: either the Brachyura, the "true crabs", or various groups within the closely related Anomura, such as hermit crabs and porcelain crabs. This group entry covers sightings where the species was not recorded. The World Register of Marine Species (WoRMS) lists Brachyura and Anomura as accepted infraorders.

Wikipedia says crabs are not a single natural group: the crab body form, with a heavily armoured shell and the tail segments concealed under the body, evolved several times. It says they vary in size from the pea crab, a few millimetres wide, to the Japanese spider crab, with a leg span of up to 4 m.

A conservation status is not given here, because this entry covers a whole group.

Wikipedia describes a habit of hiding in rocky crevices, and says a substantial number of species are adapted to freshwater or other non-marine habitats.

Where Divers Find Them

In Tulamben, Bali, Blue Season Bali says the small crevices and cracks of the Drop Off make hiding places for crabs, and names crabs among the animals of Seraya Secrets, with the boxer crab among its better known ones. Neptune Scuba Diving describes a resident population of decorator crabs coming out from cracks on night dives at the USAT Liberty Wreck.

Wikipedia says many crabs can run swiftly sideways, though others walk forwards and some can swim. It says many are free-living marine omnivores, feeding on a mixture of algae, small animals such as molluscs, polychaete worms and other crustaceans, and detritus, while others are specialist herbivores or carnivores and some are parasitic.

Underwater Encounter Behavior

The sources cited here do not describe how crabs react to divers.