The Life of Animals | Crab | Generally Crabs are 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 millimeters wide, to the Japanese spider crab, with a leg span of up to 4 meters (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.
The earliest unambiguous crab fossils date from the Jurassic, although Carboniferous Imocaris, known only from its carapace, may be a primitive crab. Often Crabs show marked sexual dimorphism. Often males have larger claws, a tendency the which is particularly pronounced in the fiddler crabs of the genus Uca (Ocypodidae). In fiddler crabs, males have one Claw the which is greatly enlarged and the 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.
Some crabs, Notably the Portunidae and Matutidae, are also capable of swimming. Crabs are Mostly active animals with complex behavior patterns. Crabs growing niche to be aggressive males Towards Often one another and fight to gain access to females. 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. The evolution of crabs is characterized 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 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
