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'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; 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 ^[1].

Contents

Anatomy

Diet

Crab fishery

Evolution and classification

Gallery

Similar animals

Other

References

Anatomy

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 ^[2]. The gills of crabs are formed of flattened plates ("phyllobranchiate"), resembling those of shrimp, but of a different structure ^[3].
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 ^[4]. 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, feeding primarily on algae ^[5], 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 ^[6][7].

Crab 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 ^[8].

Evolution and classification

The infraorder Brachyura contains about 70 families, as many as the remainder of the Decapoda ^[9]. 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 ^[10].
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 ^[11]. 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 ^[12]. 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 ^[13].
About 850 species ^[14] 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 ^[15].

Gallery

Similar animals

Several other groups of animals are either called crabs or have the term "crab" in their 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" ^[16], but this practice is not followed outside the food industry. Others, such as horseshoe crabs are much more distantly related. 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 ^[17]).

Other

The Moche people of ancient Peru worshipped nature, especially the sea.^[18] They placed emphasis on animals and often depicted crabs in their art. ^[19]

References

1. Biggest, Smallest, Fastest, Deepest: Marine Animal Records
2. A new hypothesis of decapod phylogeny, Dixon, C. J., F. R. Schram & S. T. Ahyong, , , Crustaceana, 2004
3. Gills and Lungs: The Exchange of Gases and Ions, Taylor, H. H. & E. W. Taylor, , , Microscopic Anatomy of Invertebrates, 1992
4. Glossary of terms for decapods
5. Natural diet of the crab ''Notomithrax ursus'' (Brachyura, Majidae) at Oaro, South Island, New Zealand, Woods, C. M. C., , , New Zealand Journal of Marine and Freshwater Research, 1993
6. Diet composition influeces the fitness of the herbivorous crab ''Grapsus albolineatus'', Kennish, R., , , Oecologia, 1996
7. Diet choice in an omnivorous salt-marsh crab: different food types, body size, and habitat complexity, Buck, T. L., G. A. Breed, S. C. Pennings, M. E. Chase, M. Zimmer & T. H. Carefoot, , , Journal of Experimental Marine Biology and Ecology, 2003
8. Global Capture Production 1950-2004
9. An Updated Classification of the Recent Crustacea, Martin, J. W. & G. E. Davis, , , Natural History Museum of Los Angeles County, 2001,
10. Evolution of the abdominal holding systems of brachyuran crabs (Crustacea, Decapoda, Brachyura), Guinot, D & J.–M. Bouchard, , , Zoosystema, 1998
11. Sur la classification et la phylogénie des Crustacés Décapodes Brachyoures. II. Heterotremata et Thoracotremata Guinto, 1977, De Saint Laurent, M., , , C. R. Acad. Sc. Paris, 1980
12. ''Imocaris tuberculata'', n. gen., n. sp. (Crustacea: Decapoda) fro the upper Mississippian Imo Formation, Arkansas, Schram, F. R. & R. Mapes, , , Transactions of the San Diego Society of Natural History, 1984
13. On the influence of fishes on the evolution of benthic crustaceans, Wägele, J. W., , , J. zool. Syst. Evolut.-forsch., 1989
14. On the heterotreme-thoracotreme distinction in the Eubrachyura De Saint Laurent, 1980 (Decapoda: Brachyura), Sternberg, R. von & N. Cumberlidge, , , Crustaceana, 2001
15. On the marine sister groups of the freshwater crabs (Crustacea: Decapoda: Brachyura), Sternberg, R. von, N. Cumberlidge & G. Rodriguez, , , J. Zool. Syst. Evol. Research, 1999
16. UK list of legal names for fish species
17. Rediscovery of the holotype of ''Paeduma cylindraceum'' (Bell, 1859) and description of a new genus of Hexapodidae (Decapoda, Brachyura), Guinot, D., , , Zoosystema, 2006
18. Benson, Elizabeth, The Mochica: A Culture of Peru. New York, NY: Praeger Press. 1972
19. Berrin, Katherine & Larco Museum. ''The Spirit of Ancient Peru:Treasures from the Museo Arqueológico Rafael Larco Herrera.'' New York: Thames and Hudson, 1997.