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Portunidae (swimming crabs; class Malacostraca, order Decapoda) Family of crustaceans in which the body is round, transversely oval, or square, with the anterior of the carapace not being narrowed. The fifth pair of legs is modified for swimming, being shaped as broad, flattened paddles which move in a figure-of-eight pattern. Some species use the modified legs for digging. The portunids are the most powerful and agile swimmers of all crustaceans and can swim forward, backward, and sideways with great rapidity. Males attend the females before the moulting period and carry them around beneath the body. The male releases the female to allow her to moult, and copulates with her soon after.

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