This is a fine red crab. Crabs live in all the world's oceans, in freshwater and on land. They are generally covered with a thick exoskeleton and have a single pair of pincers on the front. Crabs first appeared during the Jurassic Period. Crabs vary in size from the pea crab, a few millimeters wide, to the Japanese spider crab, with a leg span up to 4 m. Crabs typically walk sideways[, because of the articulation of the legs which makes a sidelong gait more efficient. Some crabs walk forward or backward. Some crabs are also capable of swimming, some also have their last pair of walking legs flattened into swimming paddles. Crabs are mostly active animals with complex behaviour patterns such as communicating by drumming or waving their pincers. Crabs tend to be aggressive toward one another, and males often fight to gain access to females. On rocky seashores, where nearly all caves and crevices are occupied, crabs may also fight over hiding holes. Some crabs dig burrows in sand or mud, which they use for resting, hiding, and mating, and to defend against intruders.