Any species of sluggish marine crab in the widely distributed family Majidae (or Maiidae). Spider crabs have a beak-shaped head; thick, rounded body; and long, spindly legs. They use a mucuslike mouth secretion to fasten algae, sponges, and other organisms to the hairs, spines, and knobby projections covering the body. Most species are scavengers, especially of carrion. Their size varies greatly. The body of the European long-beaked spider crab (Macropodia rostrata) is less than 0.5 in. (1 cm) in diameter, whereas the Japanese giant crab (Macrocheira kaempferi), whose outstretched claws can measure 13 ft (4 m) from tip to tip, is perhaps the largest known arthropod.

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