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any member of the family Portunidae (order Decapoda of the class Crustacea). In these animals, the fifth (hindmost) pair of legs are flattened into paddles for swimming. The family includes the blue crab (q.v.), an edible crab of the Atlantic coast of North America; the velvet crab, Portunus, of the Atlantic and Pacific oceans and the Mediterranean Sea (see ); and Neptunus of the Indian and Pacific oceans.