Admin riporto la versione di wikipedia inglese, buona lettura!FONTE:
From Wikipedia, the free encyclopedia
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (Greek: βραχύς / brachys = short, οὐρά / οura = tail), or where the reduced abdomen is entirely hidden under the thorax. Many other animals with similar names – 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 metres (13 ft).
About 850 species of crab are freshwater, terrestrial or semi-terrestrial species; 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. 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, 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 enlarged and which is used for communication, particularly for attracting a mate. 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. This is due to the fact that female crabs brood fertilised eggs on their pleopods.
Crabs typically walk sideways (a behaviour which gives us the word crabwise). This is because of the articulation of the legs which makes a sidelong gait more efficient. However, some crabs walk forwards or backwards, including raninids, Libinia emarginata and Mictyris platycheles. Some crabs, notably the Portunidae and Matutidae, are also capable of swimming.
Crabs are mostly active animals with complex behaviour patterns. They can communicate by drumming or waving their pincers. Crabs tend to be aggressive towards 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.
Crabs are omnivores, feeding primarily on algae, and taking any other food, including molluscs, worms, other crustaceans, fungi, bacteria and detritus, depending on their availability and the crab species. For many crabs, a mixed diet of plant and animal matter results in the fastest growth and greatest fitness.
Crabs are known to work together to provide food and protection for their family, and during mating season to find a comfortable spot for the female to release her eggs.
FONTE IMMAGINE: http://en.wikipedia.org/wiki/File:Carpilius_convexus_is_consuming_Heterocentrotus_trigonarius_in_Hawaii.jpg
Live crabs are often boiled. In 2005, Norwegian scientists concluded that lobsters cannot feel pain. However, later research suggests that crustaceans are indeed able to feel and remember pain.
The infraorder Brachyura contains 6,793 species in 93 families, as many as the remainder of the Decapoda. The evolution of crabs is characterised by an increasingly robust body, and a reduction in the abdomen. Although many other groups have undergone similar processes, carcinisation is most advanced in crabs. The telson is no longer functional in crabs, and the uropods are absent, having probably evolved into small devices for holding the reduced abdomen tight against the sternum.
In most decapods, the gonopores (sexual openings) are found on the legs. However, since crabs use the first two pairs of pleopods (abdominal appendages) for sperm transfer, this arrangement has changed. As the male abdomen evolved into a narrower shape, the gonopores have moved towards the midline, away from the legs, and onto the sternum. A similar change occurred, independently, with the female gonopores. The movement of the female gonopore to the sternum defines the clade Eubrachyura, and the later change in the position of the male gonopore defines the Thoracotremata. It is still a subject of debate whether those crabs where the female, but not male, gonopores are situated on the sternum, form a monophyletic group.
Numbers of extant and extinct (†) species are given in brackets. The superfamily Eocarcinoidea, containing Eocarcinus and Platykotta, was formerly thought to contain the oldest crabs; it is now considered part of the Anomura.
Dromioidea (147, 85†)
Homolodromioidea (24, 107†)
Homoloidea (73, 49†)
Section Raninoida (46, 196†)
Section Cyclodorippoida (99, 27†)
Aethroidea (37, 44†)
Calappoidea (101, 71†)
Cancroidea (57, 81†)
Carpilioidea (4, 104†)
Cheiragonoidea (3, 13†)
Corystoidea (10, 5†)
Dairoidea (4, 8†)
Dorippoidea (101, 73†)
Eriphioidea (67, 14†)
Goneplacoidea (182, 94†)
Hexapodoidea (21, 25†)
Leucosioidea (488, 113†)
Majoidea (980, 89†)
Palicoidea (63, 6†)
Parthenopoidea (144, 36†)
Pilumnoidea (405, 47†)
Portunoidea (455, 200†)
Potamoidea (662, 8†)
Pseudozioidea (22, 6†)
Retroplumoidea (10, 27†)
Trapezioidea (58, 10†)
Xanthoidea (736, 134†)
Grapsoidea (493, 28†)
Ocypodoidea (304, 14†)
Pinnotheroidea (304, 13†)
Both the constellation Cancer and the astrological sign Cancer are named after the crab, and depicted as a crab. John Bevis first observed the Crab Nebula and its resemblance to the animal in 1731. The Crab pulsar lies at the centre of the nebula.
The Moche people of ancient Peru worshipped nature, especially the sea, and often depicted crabs in their art. In Greek mythology, Karkinos was a crab that came to the aid of the Lernaean Hydra as it battled Heracles.
FONTE IMMAGINE: http://commons.wikimedia.org/wiki/File:Halloween_crab.jpg