Any of more than 8,000 species (order Decapoda) of crustacean s having five pairs of legs attached to the thorax.
The shrimps and shrimplike species, which can be as small as 0.5 in. (12 mm) long, have a slender body with a long abdomen, a well-developed fantail, and, often, long, slender legs. The crabs and crablike species, whose claw span can measure 13 ft (4 m), have a flattened body and, frequently, stout short legs and a reduced tail fan. Decapods are primarily marine and are most abundant in shallow tropical waters, but they are commercially valuable throughout the world. Some species (e.g., hermit and crab ; crayfish ; lobster ; shrimp .