Any member of the crab family Portunidae (order Decapoda, class Malacostraca). In these crabs, the hindmost pair of legs (5th pair) are flattened into paddles for swimming. The family includes the blue crab (Callinectes sapidus), an edible crab of the Atlantic coast of North America; the velvet crab, Portunus sp of the Atlantic and Pacific oceans and the Mediterranean Sea; and Scylla serrata (mangrove crab) of the Indo-Pacific region; also called swimming crab .