Crabs are decapod scavangers of the infraorder Brachyura, which commonly have a short anticipating "tail" (stomach area) (Greek: βραχύς/ brachys = short, οὐρά/ οura = tail), generally totally covered up under the thorax. They live in all the world's seas, in new water, and ashore, are by and large secured with a thick exoskeleton and have a solitary pair of hooks. Numerous different creatures with comparable names –, for example, loner crabs, ruler crabs, porcelain crabs, horseshoe crabs and crab lice – are no.
Crabs are by and large secured with a thick exoskeleton, made fundamentally out of calcium carbonate, and outfitted with a solitary pair of chelae (paws). Crabs are found in the greater part of the world's seas, while numerous crabs live in new water and ashore, especially in tropical areas. Crabs shift in size from the pea crab, a couple of millimeters wide, to the Japanese insect crab, with a leg compass of up to 4 meters (13 ft). Around 850 types of crab are freshwater, physical or semi-physical species; they are found all through the world's tropical and semi-tropical districts. They were already thought to be a monophyletic gathering, yet are currently accepted to speak to no less than two particular heredities, one in the Old World and one in the New World. The most punctual unambiguous crab fossils date from the Jurassic, albeit Carboniferous Imocaris, known just from its carapace, may be a primitive crab. The radiation of crabs in the Cretaceous and subsequently may be connected either to the separation of Gondwana or to the simultaneous radiation of hard fish, crabs' primary predators.
