Crab

True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" Greek brachys short, 2 ura tail 3 , or where the reduced abdomen is entirely hidden under the thorax. Other animals, such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs and crab lice, are not true crabs.

Crabs are generally covered with a thick exoskeleton, and armed with a single 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 millimetres wide, to the Japanese spider crab, with a leg span of up to 4 160 metres 13 ft .

About 850 species of crab are freshwater, terrestrial or semi-terrestrial species 5 they are found throughout the world's tropical and semi-tropical 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 Jurassic, although Carboniferous Imocaris, known only from its carapace, may be a primitive crab. 7 The radiation of crabs in the Cretaceous and afterward may be linked either to the break-up of Gondwana or to the concurrent radiation of bony fish, crabs' main predators.

Crabs often show marked sexual dimorphism. Males often have larger claws, 9 a tendency which is particularly pronounced in the fiddler crabs of the genus Uca Ocypodidae . In fiddler crabs, males have one claw which is greatly d and which is used for communication, particularly for attracting a mate. 10 Another conspicuous difference is the form of the pleon abdomen in most male crabs, this is narrow and triangular in form, while females have a broader, rounded abdomen. 11 This is due to the fact that female crabs brood fertilised eggs on their pleopods.

Crabs typically walk sideways 12 a behaviour which gives us the word crabwise . This is because of the articulation of the legs which makes a sidelong gait more efficient. 13 However, some crabs prefer to walk forwards or backwards, including raninids, 14 Libinia emarginata 15 and Mictyris platycheles. 12 . Some crabs, notably the Portunidae and Matutidae, are also capable of swimming.