Crab

For other uses, see Crab (disambiguation).

Crabs
Callinectes sapidus
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Subphylum: Crustacea Class: Malacostraca Order: Decapoda Suborder: Pleocyemata Infraorder: Brachyura Linnaeus, 1758
Superfamilies
Section Podotremata Cyclodorippoidea Homolodromioidea Dromioidea Homoloidea Raninoidea Section Eubrachyura Subsection Heterotremata Dorippoidea Calappoidea Leucosioidea Majoidea Hymenosomatoidea Parthenopoidea Retroplumoidea Cancroidea Portunoidea Bythograeoidea Xanthoidea Bellioidea Potamoidea Pseudothelphusoidea Gecarcinucoidea Cryptochiroidea Subsection Thoracotremata Pinnotheroidea Ocypodoidea Grapsoidea

Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (Greek: brachy = short, ura = tail), or where the reduced abdomen is entirely hidden under the thorax. They are generally covered with a thick exoskeleton, and are armed with a single pair of chelae (claws). 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.Biggest, Smallest, Fastest, Deepest: Marine Animal Records. OceanLink. Retrieved on September 22, 2006.

Anatomy

250px

True crabs have five pairs of legs, the first of which are modified into a pair of claws and are not used for locomotion. In all but a few crabs (for example, Raninoida), the abdomen is folded under the cephalothorax in the adult stage. The mouthparts of crabs are covered by flattened maxillipeds, and the front of the carapace does not form a long rostrum.Dixon, C. J., F. R. Schram & S. T. Ahyong (2004). A new hypothesis of decapod phylogeny. Crustaceana 76 (8): 935–975.  The gills of crabs are formed of flattened plates ("phyllobranchiate"), resembling those of shrimp, but of a different structure.Taylor, H. H. & E. W. Taylor (1992). Gills and Lungs: The Exchange of Gases and Ions. Microscopic Anatomy of Invertebrates 10: 203–293.  They can also be the size of a pea, or even smaller.

Most crabs show clear sexual dimorphism and so can be easily sexed. The abdomen, which is held recurved under the thorax, is narrow in males. In females, however, the abdomen retains a greater number of pleopods and is considerably wider.Glossary of terms for decapods (PDF). Southeastern Regional Taxonomic Center. This relates to the carrying of the fertilised eggs by the female crabs (as seen in all pleocyemates). In those species in which no such dimorphism is found, the position of the gonopores must be used instead. In females, these are on the third pereiopod, or nearby on the sternum in higher crabs; in males, the gonopores are at the base of the fifth pereiopods or, in higher crabs, on the sternum nearby.

Diet

Crabs are omnivores, some feeding primarily on algae, others taking any type of food, including mollusks, worms, other curstaceans, 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.

Crab fishery

Edible crabs being sorted by fishermen at Fionnphort, Scotland

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 Global Capture Production 1950-2004. FAO. Retrieved on August 26, 2006..

Crab larva (megalopa)

Evolution and classification

The infraorder Brachyura contains about 93 familiesSystema Brachyurorum: Part 1. An Annotated checklist of extant Brachyuran crabs of the world Raffles Bulletin of Zoology, 2008, 286pp., as many as the remainder of the Decapoda.Martin, J. W. & G. E. Davis (2001). An Updated Classification of the Recent Crustacea (PDF), Natural History Museum of Los Angeles County, 132 pp..  The evolution of crabs is characterised by an increasing robustness of the body, and a reduction in the abdomen. Although 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.Guinot, D & J.–M. Bouchard (1998). Evolution of the abdominal holding systems of brachyuran crabs (Crustacea, Decapoda, Brachyura) (PDF). Zoosystema 20 (4): 613–694. 

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.De Saint Laurent, M. (1980). Sur la classification et la phylogénie des Crustacés Décapodes Brachyoures. II. Heterotremata et Thoracotremata Guinto, 1977. C. R. Acad. Sc. Paris t. 290: 1317–1320.  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.Schram, F. R. & R. Mapes (1984). Imocaris tuberculata, n. gen., n. sp. (Crustacea: Decapoda) fro the upper Mississippian Imo Formation, Arkansas. Transactions of the San Diego Society of Natural History 20 (11): 165–168.  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.Wägele, J. W. (1989). On the influence of fishes on the evolution of benthic crustaceans. J. zool. Syst. Evolut.-forsch. 27: 297–309. 

About 850 speciesSternberg, R. von & N. Cumberlidge (2001). On the heterotreme-thoracotreme distinction in the Eubrachyura De Saint Laurent, 1980 (Decapoda: Brachyura). Crustaceana 74: 321–338.  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.Sternberg, R. von, N. Cumberlidge & G. Rodriguez (1999). On the marine sister groups of the freshwater crabs (Crustacea: Decapoda: Brachyura). J. Zool. Syst. Evol. Research 37: 19–38. 

Gallery

Similar animals

Several other groups of animals are either called crabs or have the term "crab" as part of their common names. These include hermit crabs, porcelain crabs and king crabs, which, despite superficial similarities to true crabs, belong to the Anomura. The UK Food Standards Agency allows king crabs to be sold as "crab",UK list of legal names for fish species. Food Standards Agency (2003-09-26). but this practice is not followed outside the food industry.

Anomuran "crabs" can be distinguished from true crabs by counting the legs. In Anomura, the last pair of pereiopods (walking legs) is hidden inside the carapace, so only four pairs are visible (counting the claws), whereas uninjured true crabs generally have five visible pairs (in the family Hexapodidae, the last pair of pereiopods is vestigial Guinot, D. (2006). Rediscovery of the holotype of Paeduma cylindraceum (Bell, 1859) and description of a new genus of Hexapodidae (Decapoda, Brachyura) (PDF). Zoosystema 28 (2): 553–571. ).

Others, such as horseshoe crabs, are much more distantly related.

Cultural influences of the crab

The Moche people of ancient Peru worshipped nature, especially the sea. Benson, Elizabeth, The Mochica: A Culture of Peru. New York, NY: Praeger Press. 1972 They placed emphasis on animals and often depicted crabs in their art.Berrin, Katherine & Larco Museum. The Spirit of Ancient Peru:Treasures from the Museo Arqueológico Rafael Larco Herrera. New York: Thames and Hudson, 1997.

References

Wikimedia Commons has media related to:

Brachyura

Wikispecies has information related to:

Brachyura

See also

This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia