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Crab
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Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" ( = short, ????/?ura = tail), or where the reduced abdomen is entirely hidden under the thorax. They are generally covered with a thick exoskeleton, and armed with a single pair of chelae (claws). 6,793 species are known. Crabs are found in all of the world's oceans. Additionally, there are also many freshwater and terrestrial crabs, particularly in tropical regions.
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Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" ( = short, ????/?ura = tail), or where the reduced abdomen is entirely hidden under the thorax. They are generally covered with a thick exoskeleton, and armed with a single pair of chelae (claws). 6,793 species are known. Crabs are found in all of the world's oceans. Additionally, there are also many freshwater and terrestrial crabs, particularly in tropical regions. Crabs vary in size from the pea crab, only a few millimetres wide, to the Japanese spider crab, with a leg span of up to 4 m.
Evolution and classification
The infraorder Brachyura contains about 93 families , as many as the remainder of the Decapoda. The evolution of crabs is characterised by an increasing robustness of the body, and a reduction in the abdomen. Although many other groups have also undergone similar processes of carcinisation, it 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 decapodes, 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.
The earliest unambiguous crab fossils date from the Jurassic, although the Carboniferous Imocaris, known only from its carapace is thought to be a primitive crab. The radiation of crabs in the Cretaceous and afterwards may be linked either to the break-up of Gondwana or to the concurrent radiation of bony fish, the main predators of crabs.
About 850 species of crab are freshwater or (semi-)terrestrial species; they are found throughout the world's tropical and semi-tropical regions. They were previously thought to be a closely related group, but are now believed to represent at least two distinct lineages, one in the Old World and one in the New World.
Behaviour
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 for the best hiding holes.
Diet
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 .
Culinary use
Crabs are prepared and eaten as a dish in several different ways all over the world. Some crabs are eaten whole, including the shell, as soft-shell crab, with other crabs it's just the claws and/or legs that are eaten. The latter is particularly common for larger crabs, such as the snow crab.
In some regions spices are used to improve the culinary experience of eating crab. In East Asia the chilli crab is an example of a heavily spiced dish. In Maryland, blue crab is often eaten with Old Bay Seasoning.
With the British dish dressed crab, the meat is extracted from all over the crab and placed inside the hard shell. On the other side of the Atlantic, an American way to take advantage of the crab meat is by extracting it and adding a flour mix, creating a crab cake.
Crabs are also used in soups, like in bisque, a global dish of French origin.
Fishery
Crabs make up 20% of all marine crustaceans caught and farmed worldwide, with over 1½ million tonnes being consumed annually. Of that total, one species accounts for one fifth: Portunus trituberculatus. Other important taxa include Portunus pelagicus, several species in the genus Chionoecetes, the Blue crab (Callinectes sapidus), Charybdis spp., Cancer pagurus, the Dungeness crab (Cancer magister) and Scylla serrata, each of which provides more than 20,000 tonnes annually .
Cultural influences of the crab
The Moche people of ancient Peru worshipped nature, especially the sea. They placed emphasis on animals and often depicted crabs in their art.
The Cancer Constellation is one of the 12 Zodiac signs.
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