Crab, any short-tailed member of the crustacean order Decapoda (phylum Arthropoda)—especially the brachyurans (infraorder Brachyura), or true crabs, but also other forms such as the anomurans (suborder Anomura), which include the hermit crabs. Decapods occur in all oceans, in fresh water, and on land; about 10,000 species have been described.Unlike those of other decapods (e.g., shrimp, lobster, crayfish), crabs’ tails are curled under the thorax, or midsection. The carapace (upper body shield) is usually broad. The first pair of legs is modified into chelae, or pincers.
Most crabs live in the sea; even the land crabs, which are abundant in tropical countries, usually visit the sea occasionally and pass through their early stages in it. The river crab of southern Europe (the Lenten crab, Potamon fluviatile) is an example of the freshwater crabs abundant in most of the warmer regions of the world. As a rule, crabs breathe by gills, which are lodged in a pair of cavities beneath the sides of the carapace, but in the true land crabs the cavities become enlarged and modified so as to act as lungs for breathing air.Walking or crawling is the usual mode of locomotion, and the familiar sidelong gait in the common shore crab is characteristic of most members of the group. The crabs of the family Portunidae, as well as some others, swim with great dexterity by means of their flattened paddle-shaped legs.
