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any of approximately 20 species of shore crabs (order Decapoda of the class Crustacea). O. quadratus, the beach crabs noted for their running speed, occur on dry sand above the high-tide mark on the western Atlantic coast from New Jersey to Brazil. The crab, sandy or whitish in colour, has claws of unequal size and rather hairy legs. The back, or carapace, is nearly rectangular in shape and about 3.75-5 cm (about 1.5-2 inches) across. Long stalks support the eyes. The crab lives in burrows sometimes 1 m (3.3 feet) deep. Sand fleas are an important part of the diet.