Crab

Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting “tail” (abdomen) (Greek: βραχύς, romanized: brachys = short,[2] οὐρά / οura = tail[3]), usually hidden entirely under the thorax. They live in all the world’s oceans, in fresh water, and on land, are generally covered with a thick exoskeleton, and have a single pair of pincers. Many other animals with similar names – such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs, stone crabs, and crab lice – are not true crabs, but many have evolved features similar to true crabs through a process known as carcinisation.

Crabs are generally covered with a thick exoskeleton, composed primarily of highly mineralized chitin,[4][5] and armed with a pair of chelae (claws). Crabs are found in all of the world’s oceans, while many crabs live in fresh water and on land, particularly in tropical regions. 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 (13 ft).

About 850 species are saltwater crabs, brackish water crabs, freshwater crabs, terrestrial crabs or semiterrestrial crabs.[7]

They are found throughout the world’s tropical and semitropical regions. They were previously thought to be a monophyletic group, but are now believed to represent at least two distinct lineages, one in the Old World and one in the New World.

The earliest unambiguous crab fossils date from the Early Jurassic, with the oldest being Eocarcinus from the early Pliensbachian of Britain, which likely represents a stem-group lineage, as it lacks several key morphological features that define modern crabs.[9][10] Most Jurassic crabs are only known from dorsal (top half of the body) carapaces, making it difficult to determine their relationships.[11] Crabs would radiate in the Late Jurassic, corresponding with an increase in reef habitats, though they would decline at the end of the Jurassic as the result of the decline of reef ecosystems. Crabs increased in diversity through the Cretaceous and represented the dominant group of decapods by the end of the period.