Crab

Crabs are decapod crustaceans belonging either to the group Brachyura, the true crabs, or to related groups within the Anomura, such as hermit crabs, marked by a heavily armoured shell, tail segments tucked beneath the body, sideways walking, and a preference for rocky crevices. Despite their similar appearance, crabs are not a single natural group but multiple separate lineages that evolved crab-like traits independently through a process called carcinisation, making them polyphyletic. They range enormously in size, from the pea crab, only a few millimeters wide, to the Japanese spider crab, whose leg span can reach four meters, and most are free-living marine omnivores, though some are herbivores, carnivores or parasites, and some have adapted to freshwater or land. Crabs are commercially significant, making up about 20 percent of all marine crustaceans harvested or farmed worldwide, and hold cultural importance across many civilizations, appearing in Greek and Malay mythology and as the zodiac sign Cancer.