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Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" ( = short, ουρά/οura = 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, many crabs live in freshwater 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 .

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 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. 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.

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.

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.

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 over 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 species are eaten whole, including the shell, such as soft-shell crab; with other species just the claws and/or legs are eaten. The latter is particularly common for larger crabs, such as the snow crab.

In some regions spices improve the culinary experience. In Asia, masala crab and chilli crab are examples of heavily spiced dishes. In Maryland, blue crab is often eaten with Old Bay Seasoning.

For the British dish Cromer crab, the meat is extracted and placed inside the hard shell. One American way to prepare crab meat is by extracting it and adding a flour mix, creating a crab cake.

Crabs are also used in bisque, a global dish of French origin.

Live crabs are often boiled. Norwegian scientists addressed this ethical issue by stating that crabs don't feel pain. However, later research suggests that crustaceans are indeed able to feel and remember pain, although the latter point is not an issue in cooking.

Fisheries

Crabs make up 20% of all marine crustaceans caught, farmed, and consumed worldwide, amounting to 1½ million tonnes annually. One species accounts for one fifth of that total: Portunus trituberculatus. Other commercially 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 yields more than 20,000 tonnes annually .


Cultural influences of the crab

Moche vessel representing a crab


The constellation Cancer is one of the 12 signs of the Zodiac.

John Bevis first observed the Crab nebula and its resemblance to the earthly creature in 1731. The Crab pulsar lies at the nebula's center.

The Moche people of ancient Perumarker worshipped nature, especially the sea. They often depicted crabs in their art.

Western cultures have been influenced by the crab towards the game crab soccer, where players rest and move on an inverted all-fours pose.

Gallery

Image:Corystes cassivelaunus.jpg|Masked crab, Corystes cassivelaunusImage:Liocarcinus vernalis.jpg|Liocarcinus vernalisImage:Atelecyclus rotundatus.jpg|Circular crab, Atelecyclus rotundatusImage:Gecarcinus quadratus (Nosara).jpg|The terrestrial halloween crab, Geocarcinus quadratusImage:Stenorhynchus seticornis 2.jpg|Arrow crab Stenorhynchus seticornisImage:Grapsus grapsus.JPG|"Sally Lightfoot", Grapsus grapsusImage:Thia scutellata.jpg|Thumbnail crab, Thia scutellataImage:Spider crabs at the Kaiyukan Aquarium in Osaka close.jpg|Japanese spider crab, Macrocheira kaempferiImage:Ocypode_quadrata_(Martinique).jpg|Ghost crab, Ocypode quadrataImage:Fiddler crab.jpg|Fiddler crab, Uca pugnaxImage:Lyreidus tridentatus.jpg|Lyreidus tridentatus, a raninidImage:Hepatus epheliticus.jpg|Hepatus epheliticus, a calico crab

See also



References

  1. Walters, Martin & Johnson, Jinny. The World of Animals. Bath, Somerset: Parragon, 2007.
  2. "Crab (animal)," Encarta Encyclopedia. 2005 ed.
  3. The Miles Kelly Book of Life. Great Bardfield, Essex: Miles Kelly Publishing Ltd., 2006.
  4. Scientists say lobsters feel no pain
  5. Crabs 'feel and remember pain' suggests new study
  6. Benson, Elizabeth, The Mochica: A Culture of Peru. New York, NY: Praeger Press. 1972
  7. Berrin, Katherine & Larco Museum. The Spirit of Ancient Peru:Treasures from the Museo Arqueológico Rafael Larco Herrera. New York: Thames and Hudson, 1997.



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