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Xanthidae (mud crabs; class Malacostraca, order Decapoda) A family of crabs several species of which have neurotoxins contained in their exoskeletons. Mud crabs are oval to hexagonal, and broadened anteriorly. Many are very fecund, Menippe mercenaria (stone crab) producing 3–6 million eggs in less than 70 days. This species can be a serious predator of oysters, each crab taking on average up to 219 small and large oysters per year. Some species of mud crabs live on coral reefs, and three or four species of Xantho occur in European waters.