Difference between revisions of "Crab" - New World Encyclopedia

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{{Taxobox_begin | color=pink | name=Crabs}}
 
{{Taxobox_begin | color=pink | name=Crabs}}
 
{{Taxobox_image | image = [[Image:Blue crab on market in Piraeus - Callinectes sapidus Rathbun 20020819-317.jpg|250px]] | caption = ''[[Blue crab|Callinectes sapidus]]''}}
 
{{Taxobox_image | image = [[Image:Blue crab on market in Piraeus - Callinectes sapidus Rathbun 20020819-317.jpg|250px]] | caption = ''[[Blue crab|Callinectes sapidus]]''}}
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*[[Thoracotremata]]
 
*[[Thoracotremata]]
 
{{Taxobox_end}}
 
{{Taxobox_end}}
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The term '''crab''' is often applied to several different groups of short (nose to tail) [[Decapoda|decapod]] [[crustacean]]s with thick exoskeletons, but only members of the '''Brachyura''' are true crabs. Other taxa, such as [[hermit crab]]s, [[porcelain crab]]s and [[king crab]]s are, despite superficial similarities, not crabs at all; rather, they belong to the [[Anomura]] and can be distinguished from true crabs by counting the legs. In Anomura, the last pair of [[pereiopod]]s (walking legs) is hidden inside the [[carapace]], so only four pairs are visible (counting the claws), whereas uninjured true crabs always have five visible pairs.
 
  
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==True crabs==
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{{Taxobox
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True crabs are crustaceans in the [[infraorder]] Brachyura, in the [[Order (biology)|order]] [[Decapoda]]. They have five pairs of walking legs (the first of which is modified into a pair of claws or ''chelae'') and typically a flattened shell. In all but a few crabs (for example, [[Raninoida]]), the [[abdomen]] is folded under the [[cephalothorax]]. The form of the abdomen usually reveals the sex of the crab; males have a narrow abdomen, while females have a much wider abdomen, under which they carry their [[egg (biology)|eggs]]. Crabs are a very diverse group, mostly found in [[saltwater]], but with some groups living in [[freshwater]] or on land. Although famed for their tendency to walk sideways, crabs are in fact able to walk in any direction.
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| color = pink
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Classification within the crabs is traditionally based on the position of the [[gonopore]]s: whether they are found on the legs or on the [[thorax]]. In the two "primitive" sections (sometimes called collectively the "Podotremata"), the gonopores are found on the legs (as in all other decapods); in the [[Heterotremata]], the male gonopores are on the legs, and the female gonopores are on the [[sternum]]; in the [[Thoracotremata]], the gonopores are on the sternum in both males and females.
+
| name = Crabs
 +
| image = Blue crab on market in Piraeus - Callinectes sapidus Rathbun 20020819-317.jpg
 +
| image_width = 250px
 +
| image_caption = ''[[Blue crab|Callinectes sapidus]]''
 +
| regnum = [[Animal]]ia
 +
| phylum = [[Arthropod]]a
 +
| subphylum = [[Crustacean|Crustacea]]
 +
| classis = [[Malacostraca]]
 +
| ordo = [[Decapoda]]
 +
| subordo = [[Pleocyemata]]
 +
| infraordo = '''Brachyura'''
 +
| infraordo_authority = [[Pierre André Latreille|Latreille]], 1802
 +
| subdivision_ranks = Superfamilies
 +
| subdivision =  
 +
*[[Dromiacea]]
 +
**[[Homolodromidae|Homolodromioidea]]
 +
**[[Dromioidea]]
 +
**[[Homoloidea]]
 +
*[[Eubrachyura]]
 +
**[[Raninoidea]]
 +
**[[Cyclodorippoidea]]
 +
**[[Dorippoidea]]
 +
**[[Calappoidea]]
 +
**[[Leucosioidea]]
 +
**[[Majoidea]]
 +
**[[Hymenosomatidae|Hymenosomatoidea]]
 +
**[[Parthenopoidea]]
 +
**[[Retroplumidae|Retroplumoidea]]
 +
**[[Cancroidea]]
 +
**[[Portunoidea]]
 +
**[[Bythograeidae|Bythograeoidea]]
 +
**[[Xanthoidea]]
 +
**[[Belliidae|Bellioidea]]
 +
**[[Potamoidea]]
 +
**[[Psuedothelphusidae|Pseudothelphusoidea]]
 +
**[[Gecarcinucoidea]]
 +
**[[Cryptochiridae|Cryptochiroidea]]
 +
**[[Pinnotheridae|Pinnotheroidea]] *
 +
**[[Ocypodoidea]] *
 +
**[[Grapsoidea]] *
 +
<small>An asterisk (*) marks the crabs included in the clade [[Thoracotremata]].</small>
 +
}}
 +
'''Crabs''' are [[Decapoda|decapod]] [[crustacean]]s of the infraorder '''Brachyura''', which typically have a very short "tail" ([[Greek language|Greek]]: ''brachy'' = short, ''ura'' = tail), or where the abdomen is entirely hidden under the thorax. They are generally covered with a thick [[exoskeleton]], and are armed with a single pair of [[chela]]e ([[claw]]s). Crabs are found in all of the world's oceans; there are also many [[fresh water|freshwater]] and [[terrestrial animal|terrestrial]] crabs, particularly in tropical regions. Crabs vary in size from the [[pea crab]], only a few [[millimetre]]s wide, to the [[Japanese spider crab]], with a leg span of up to 4&nbsp;[[metre|m]]&nbsp;<ref>{{cite web |url=http://oceanlink.island.net/records.html |title=Biggest, Smallest, Fastest, Deepest: Marine Animal Records |publisher=OceanLink |accessdate=September 22 |accessyear=2006}}</ref>.
 +
 
 +
==Anatomy==
 +
True crabs have five pairs of [[arthropod leg|legs]], the first of which is modified into a pair of claws and is not used for locomotion. In all but a few crabs (for example, [[Raninoida]]), the [[abdomen]] is folded under the [[cephalothorax]]. The [[mouthparts]] of crabs are covered by flattened [[maxilliped]]s, and the front of the [[carapace]] does not form a long [[Rostrum (anatomy)|rostrum]]&nbsp;<ref>{{cite journal | quotes=no|author = Dixon, C. J., [[Frederick Schram|F. R. Schram]] & S. T. Ahyong|year=2004|title=A new hypothesis of decapod phylogeny|journal=[[Crustaceana]]|volume=76|issue=8|pages=935–975}}</ref>. The [[gill]]s of crabs are formed of flattened plates ("phyllobranchiate"), resembling those of [[shrimp]], but of a different structure&nbsp;<ref>{{cite journal | quotes=no |author=Taylor, H. H. & E. W. Taylor |year=1992 |title=Gills and Lungs: The Exchange of Gases and Ions |journal=Microscopic Anatomy of Invertebrates |volume=10 |pages=203–293}}</ref>.
  
−
==Sexual dimorphism==
+
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 [[pleopod]]s and is considerably wider&nbsp;<ref>{{cite web |url=http://www.dnr.sc.gov/marine/sertc/Glossary.pdf |publisher=Southeastern Regional Taxonomic Center |title=Glossary of terms for decapods |format=[[Portable Document Format|PDF]]}}</ref>. This relates to the carrying of the [[fertilisation|fertilised]] eggs by the female crabs (as seen in all [[Pleocyemata|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.
−
[[Image:Corystes cassivelaunus.jpg|thumb|250px|''Corystes cassivelaunus'']]
+
 
−
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 is considerably wider, and retains a greater number of [[pleopod]]s. This relates to the carrying of the [[fertilisation|fertilised]] eggs by the female crabs (as seen in all [[pleocyemate]]s). 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 [[omnivore]]s, feeding primarily on [[alga]]e&nbsp;<ref>{{cite journal |quotes=no |title=Natural diet of the crab ''Notomithrax ursus''  (Brachyura, Majidae) at Oaro, South Island, New Zealand |author=Woods, C. M. C. |journal=[[New Zealand Journal of Marine and Freshwater Research]] |year=1993 |Volume=27 |pages=309=315 |url=http://www.rsnz.org/publish/nzjmfr/1993/29.php}}</ref>, and taking any other food, including [[mollusc]]s, [[worm]]s, other [[crustacean]]s, [[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 (biology)|fitness]]&nbsp;<ref>{{cite journal |quotes=no |title=Diet composition influeces the fitness of the herbivorous crab ''Grapsus albolineatus'' |journal=[[Oecologia]] |author=Kennish, R. |volum=105 |issue=1 |year=1996 |pages=22-29 |url=http://www.springerlink.com/content/l7m3368427059312/}}</ref><ref>{{cite journal |quotes=no |title=Diet choice in an omnivorous salt-marsh crab: different food types, body size, and habitat complexity |author=Buck, T. L., G. A. Breed, S. C. Pennings, M. E. Chase, M. Zimmer & T. H. Carefoot |journal=[[Journal of Experimental Marine Biology and Ecology]] |year=2003 |volume=292 |issue=1 |pages=103-116 |url=http://cat.inist.fr/?aModele=afficheN&cpsidt=14847986}}</ref>.
  
 
==Crab fishery==
 
==Crab fishery==
−
Crabs make up 20% of all marine crustaceans caught and farmed worldwide, with over 1&frac12; million [[tonne]]s being consumed annually. Of that total, one species accounts for one fifth: ''[[Portunus trituberculatus]]''. Other important [[taxon|taxa]] include ''[[Portunus pelagicus]]'', several species in the genus ''[[Chionoecetes]]'', the [[blue crab]] (''Callinectes sapidus''), the [[blue land crab]] (''Discoplax hirtipes''), the genus ''[[Charybdis (genus)|Charybdis]]'', the [[edible crab]] (''Cancer pagurus''), the [[Dungeness crab]] (''Cancer magister'') and ''[[Scylla serrata]]'', each of which provides more than 20,000&nbsp;tonnes annually.
+
[[Image:Sorting Crabs Ffionphort.jpg|350px|thumb|left|[[Edible crab]]s being sorted by fishermen at [[Fionnphort]], [[Scotland]]]]
 +
Crabs make up 20% of all marine crustaceans caught and farmed worldwide, with over 1½ million [[tonne]]s being consumed annually. Of that total, one species accounts for one fifth: ''[[Portunus trituberculatus]]''. Other important [[taxon|taxa]] include ''[[Portunus pelagicus]]'', several species in the genus ''[[Chionoecetes]]'', the [[blue crab]] (''Callinectes sapidus''), ''[[Charybdis (genus)|Charybdis spp.]]'', ''[[Edible crab|Cancer pagurus]]'', the [[Dungeness crab]] (''Cancer magister)'' and ''[[Scylla serrata]]'', each of which provides more than 20,000&nbsp;tonnes annually&nbsp;<ref>{{cite web |url=http://www.fao.org/figis/servlet/TabLandArea?tb_ds=Capture&tb_mode=TABLE&tb_act=SELECT&tb_grp=COUNTRY |title=Global Capture Production 1950-2004 |publisher=[[Food and Agriculture Organization|FAO]] |accessdate=August 26 |accessyear=2006}}</ref>.
 +
<br clear="left">
 +
==Evolution and classification==
 +
The infraclass Brachyura contains about 70 [[family (biology)|families]], as many as the remainder of the [[Decapoda]]&nbsp;<ref name="Martin & Davis">{{cite book |url=http://www.nhm.org/research/publications/CrustaceaClassification.pdf |title=An Updated Classification of the Recent Crustacea |author=Martin, J. W. & G. E. Davis |year=2001 |pages=132 pp. |publisher=Natural History Museum of Los Angeles County |format=[[Portable Document Format|PDF]]}}</ref>. 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 [[uropod]]s are absent, having probably evolved into small devices for holding the reduced abdomen tight against the sternum&nbsp;<ref>{{cite journal | quotes=no |author=Guinot, D & J.–M. Bouchard |year=1998 |title=Evolution of the abdominal holding systems of brachyuran crabs (Crustacea, Decapoda, Brachyura) |journal=[[Zoosystema]] |volume=20 |issue=4 |pages=613–694 |url=http://www.mnhn.fr/publication/zoosyst/z98n4a4.html |format=[[Portable Document Format|PDF]]}}</ref>.
 +
 
 +
In most decapods, the [[gonopore]]s (sexual openings) are found on the legs. However, since crabs use the first two pairs of [[pleopod]]s (abdominal appendages) for [[sperm transfer]], this arrangement has changed. As the male abdomen evolved into a narower shape, the gonopores have moved towards the midline, away from the legs, and onto the [[Sternum (arthropod)|sternum]]&nbsp;<ref>{{cite journal | quotes=no |author=De Saint Laurent, M. |year=1980 |title=Sur la classification et la phylogénie des Crustacés Décapodes Brachyoures. II. Heterotremata et Thoracotremata Guinto, 1977 |journal=[[C. R. Acad. Sc. Paris]] |volume=t. 290 |pages=1317–1320}}</ref>. 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 [[Monophyly|monophyletic group]]&nbsp;<ref name="Martin & Davis"/>.
 +
 
 +
The earliest unambiguous crab [[fossil]]s date from the [[Jurassic]], although the [[Carboniferous]] ''Imocaris'', known only from its [[carapace]] is thought to be a primitive crab&nbsp;<ref>{{cite journal | quotes=no |author=[[Frederick Schram|Schram, F. R.]] & R. Mapes |year=1984 |title=''Imocaris tuberculata'', n. gen., n. sp. (Crustacea: Decapoda) fro the upper Mississippian Imo Formation, Arkansas |journal=[[Transactions of the San Diego Society of Natural History]] |volume=20 |issue=11 |pages=165–168}}</ref>. The [[Adaptive radiation|radiation]] of crabs in the [[Cretaceous]] and afterwards may be linked either to the break-up of [[Gondwana]] or to the concurrent radiation of [[Osteichthyes|bony fish]], the main [[predator]]s of crabs&nbsp;<ref>{{cite journal | quotes=no |author=Wägele, J. W. |year=1989 |title=On the influence of fishes on the evolution of benthic crustaceans |journal=J. zool. Syst. Evolut.-forsch. |volume=27 |pages=297–309}}</ref>.
 +
 
 +
About 850 species&nbsp;<ref>{{cite journal | quotes=no |author=Sternberg, R. von & N. Cumberlidge |year=2001 |title=On the heterotreme-thoracotreme distinction in the Eubrachyura De Saint Laurent, 1980 (Decapoda: Brachyura) |journal=[[Crustaceana]] |volume=74 |pages=321–338}}</ref> of crab are freshwater or (semi-)terrestrial species; they are found throughout the world's [[tropic]]al and [[Subtropics|semi-tropical]] regions. They were previously thought to be a closely related group, but are now believed to represent at least two distinct [[Lineage (evolution)|lineages]], one in the [[Old World]] and one in the [[New World]]&nbsp;<ref>{{cite journal | quotes=no |author=Sternberg, R. von, N. Cumberlidge & G. Rodriguez |year=1999 |title=On the marine sister groups of the freshwater crabs (Crustacea: Decapoda: Brachyura) |journal=[[J. Zool. Syst. Evol. Research]] |volume=37 |pages=19–38}}</ref>.
  
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==Other animals==
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==Gallery==
−
A number of animals with a similar shape are commonly called crabs, including the [[crab louse]] (an [[insect]]) and the [[horseshoe crab]] (in the class [[Merostomata]]). The crab louse is the origin of the slang term "crabs" for an infestation of [[pubic lice]].
+
<gallery>
 +
Image:Corystes cassivelaunus.jpg|[[Masked crab]], ''Corystes cassivelaunus''
 +
Image:Liocarcinus vernalis.jpg|''[[Liocarcinus vernalis]]''
 +
Image:Atelecyclus rotundatus museum.jpg|[[Circular crab]], ''Atelecyclus rotundatus''
 +
Image:Gecarcinus ruricola.jpg|The terrestrial [[halloween crab]], ''Geocarcinus ruricola''
 +
Image:Stenorhynchus seticornis 2.jpg|[[Arrow crab]] ''Stenorhynchus seticornis''
 +
Image:Grapsus grapsus.JPG|"Sally lightfoot", ''[[Grapsus grapsus]]''
 +
Image:Thia scutellata.jpg|[[Thumbnail crab]], ''Thia scutellata''
 +
Image:Spider crabs at the Kaiyukan Aquarium in Osaka close.jpg|[[Japanese spider crab]], ''Macrocheira kaempferi''
 +
Image:Ocypode quadrata (2).jpg|[[Ghost crab]], ''Ocypode quadrata''
 +
Image:Fiddler crab.jpg|[[Fiddler crab]], ''Uca pugnax''
 +
Image:Lyreidus tridentatus.jpg|''Lyreidus tridentatus'', a [[Raninidae|raninid]]
 +
Image:Calappa sulcata.jpg|''Calappa sulcata'', a [[Calappa (crab)|shame-faced crab]]
 +
</gallery>
  
−
==[[Superfamily|Superfamilies]]==
+
==Similar animals==
−
[[image:Liocarcinus vernalis.jpg|thumb|300px|''Liocarcinus vernalis'', a crab from the superfamily Portunoidea]]
+
Several other groups of animals are either called crabs or have the term "crab" in their names. These include [[hermit crab]]s, [[porcelain crab]]s and [[king crab]]s, which, despite superficial similarities to true crabs, belong to the [[Anomura]]. The [[United Kingdom|UK]] [[Food Standards Agency]] allows [[king crab]]s to be sold as "crab"&nbsp;<ref>{{cite web |url=http://www.food.gov.uk/multimedia/webpage/namesoffish |title=UK list of legal names for fish species |publisher=[[Food Standards Agency]] |date=[[2003-09-26]]}}</ref>, but this practice is not followed outside the [[food industry]]. Others, such as [[horseshoe crab]]s are much more distantly related. Anomuran "crabs" can be distinguished from true crabs by counting the legs. In Anomura, the last pair of [[pereiopod]]s (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 structure|vestigial]]&nbsp;<ref>{{cite journal |quotes=no |title=Rediscovery of the holotype of ''Paeduma cylindraceum'' (Bell, 1859) and description of a new genus of Hexapodidae (Decapoda, Brachyura) |author=Guinot, D. |journal=[[Zoosystema]] |volume=28 |issue=2 |pages=553&ndash;571 |year=2006 |url=http://www.mnhn.fr/museum/front/medias/publication/8150_z06n2a26.pdf |format=[[Portable Document Format|PDF]]}}</ref>).
−
* [[Bellioidea]]
 
−
* [[Bythograeoidea]]
 
−
* [[Calappoidea]]
 
−
* [[Cancroidea]]
 
−
* [[Cryptochiroidea]]
 
−
* [[Cyclodorippoidea]]
 
−
* [[Dorippoidea]]
 
−
* [[Dromioidea]]
 
−
* [[Gecarcinucoidea]]
 
−
* [[Grapsoidea]]
 
−
* [[Homolodromioidea]]
 
−
* [[Homoloidea]]
 
−
* [[Hymenosomatoidea]]
 
−
* [[Leucosioidea]]
 
−
* [[Majoidea]]
 
−
* [[Ocypodoidea]]
 
−
* [[Parthenopoidea]]
 
−
* [[Pinnotheroidea]]
 
−
* [[Portunoidea]]
 
−
* [[Potamoidea]]
 
−
* [[Pseudothelphuspoidea]]
 
−
* [[Raninoidea]]
 
−
* [[Retroplumoidea]]
 
−
* [[Xanthoidea]]
 
  
−
==External links==
+
==References==
−
* ''Brachyura'' entry at [http://www.itis.usda.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=98276 ITIS Report]
+
{{commonscat|Brachyura}}
−
* ''Brachyura'' entry at [http://animaldiversity.ummz.umich.edu/site/accounts/classification/Brachyura.html#Brachyura Animal diversity web]
+
{{Wikispecies|Brachyura}}
−
* [http://malaysiana.biz/kervin-chong.html Crab Hunt in Kudat, Sabah, East Malaysia]
+
<div class="references-small"><references/></div>
−
*[http://ww2.lafayette.edu/~hollidac/crabgillspot.html Spotlight on:] Osmoregulation and ion transport in crab gills
 
  
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{{credit|62150308}}
+
{{credit|91148907}}
 
[[Category:Life sciences]]
 
[[Category:Life sciences]]

Revision as of 21:58, 1 December 2006

Crabs
Blue crab on market in Piraeus - Callinectes sapidus Rathbun 20020819-317.jpg
Callinectes sapidus
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Crustacea
Class: Malacostraca
Order: Decapoda
Suborder: Pleocyemata
Infraorder: Brachyura

Latreille, 1802

Sections
  • Dromiacea
  • Raninoida
  • Heterotremata
  • Thoracotremata
Crabs
Callinectes sapidus
Callinectes sapidus
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Crustacea
Class: Malacostraca
Order: Decapoda
Suborder: Pleocyemata
Infraorder: Brachyura
Latreille, 1802
Superfamilies
  • Dromiacea
    • Homolodromioidea
    • Dromioidea
    • Homoloidea
  • Eubrachyura
    • Raninoidea
    • Cyclodorippoidea
    • Dorippoidea
    • Calappoidea
    • Leucosioidea
    • Majoidea
    • Hymenosomatoidea
    • Parthenopoidea
    • Retroplumoidea
    • Cancroidea
    • Portunoidea
    • Bythograeoidea
    • Xanthoidea
    • Bellioidea
    • Potamoidea
    • Pseudothelphusoidea
    • Gecarcinucoidea
    • Cryptochiroidea
    • Pinnotheroidea *
    • Ocypodoidea *
    • Grapsoidea *

An asterisk (*) marks the crabs included in the clade Thoracotremata.

Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short "tail" (Greek: brachy = short, ura = tail), or where the 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].

Anatomy

True crabs have five pairs of legs, the first of which is modified into a pair of claws and is not used for locomotion. In all but a few crabs (for example, Raninoida), the abdomen is folded under the cephalothorax. 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

File:Sorting Crabs Ffionphort.jpg
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 [8].

Evolution and classification

The infraclass 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 narower 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 [9].

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

  • Masked crab, Corystes cassivelaunus

  • Liocarcinus vernalis

  • Circular crab, Atelecyclus rotundatus

  • Gecarcinus ruricola.jpg

    The terrestrial halloween crab, Geocarcinus ruricola

  • Arrow crab Stenorhynchus seticornis

  • "Sally lightfoot", Grapsus grapsus

  • Thumbnail crab, Thia scutellata

  • Japanese spider crab, Macrocheira kaempferi

  • Ocypode quadrata (2).jpg

    Ghost crab, Ocypode quadrata

  • Fiddler crab, Uca pugnax

  • Lyreidus tridentatus, a raninid

  • Calappa sulcata.jpg

    Calappa sulcata, a shame-faced crab

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]).

References
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  1. ↑ Biggest, Smallest, Fastest, Deepest: Marine Animal Records. OceanLink. Retrieved September 22, 2006.
  2. ↑ Dixon, C. J., F. R. Schram & S. T. Ahyong (2004). A new hypothesis of decapod phylogeny. Crustaceana 76 (8): 935–975.
  3. ↑ Taylor, H. H. & E. W. Taylor (1992). Gills and Lungs: The Exchange of Gases and Ions. Microscopic Anatomy of Invertebrates 10: 203–293.
  4. ↑ Glossary of terms for decapods (PDF). Southeastern Regional Taxonomic Center.
  5. ↑ Woods, C. M. C. (1993). Natural diet of the crab Notomithrax ursus (Brachyura, Majidae) at Oaro, South Island, New Zealand. New Zealand Journal of Marine and Freshwater Research: 309=315.
  6. ↑ Kennish, R. (1996). Diet composition influeces the fitness of the herbivorous crab Grapsus albolineatus. Oecologia (1): 22-29.
  7. ↑ Buck, T. L., G. A. Breed, S. C. Pennings, M. E. Chase, M. Zimmer & T. H. Carefoot (2003). Diet choice in an omnivorous salt-marsh crab: different food types, body size, and habitat complexity. Journal of Experimental Marine Biology and Ecology 292 (1): 103-116.
  8. ↑ Global Capture Production 1950-2004. FAO. Retrieved August 26, 2006.
  9. ↑ 9.0 9.1 Martin, J. W. & G. E. Davis (2001). An Updated Classification of the Recent Crustacea (PDF), Natural History Museum of Los Angeles County, 132 pp.. 
  10. ↑ Guinot, D & J.–M. Bouchard (1998). Evolution of the abdominal holding systems of brachyuran crabs (Crustacea, Decapoda, Brachyura). Zoosystema 20 (4): 613–694.
  11. ↑ 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.
  12. ↑ 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.
  13. ↑ Wägele, J. W. (1989). On the influence of fishes on the evolution of benthic crustaceans. J. zool. Syst. Evolut.-forsch. 27: 297–309.
  14. ↑ Sternberg, R. von & N. Cumberlidge (2001). On the heterotreme-thoracotreme distinction in the Eubrachyura De Saint Laurent, 1980 (Decapoda: Brachyura). Crustaceana 74: 321–338.
  15. ↑ 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.
  16. ↑ UK list of legal names for fish species. Food Standards Agency (2003-09-26).
  17. ↑ Guinot, D. (2006). Rediscovery of the holotype of Paeduma cylindraceum (Bell, 1859) and description of a new genus of Hexapodidae (Decapoda, Brachyura). Zoosystema 28 (2): 553–571.

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