Brachyura
Grey swimming crab Liocarcinus vernalis
Scientific classification
Unrecognized taxon (fix):	Reptantia
Infraorder:	Brachyura Linnaeus, 1758
Sections and subsections
Dromiacea Raninoida Cyclodorippoida Eubrachyura Heterotremata Thoracotremata

Crab in action on a sea urchin

Crabs are a form of decapods (having eight walking legs and two grasping claws), along with lobsters, crayfish and shrimps. Crabs form an order within the decapods, called the Brachyura. Their short body is covered by a thick exoskeleton.

They are an extremely successful group, found all over the world. They are basically heavily armored shell-breakers. Most crabs live in sea-water, but there are some who live in fresh water, and some who live on land. The smallest are the size of a pea; the largest (the Japanese spider crab) grows to a leg span of 4 metres. About 7,000 species are known.