Everything about Crab totally explained
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.
Diet
Crabs are 6 legged omnivores, some feeding primarily on algae, others taking any type of food, including mollusks, 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.
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 .
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 other groups have also undergone similar processes of
carcinisation, it's 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 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
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're 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.
Gallery
Image:Corystes cassivelaunus.jpg|Masked crab, Corystes cassivelaunus
Image:Liocarcinus vernalis.jpg|Liocarcinus vernalis
Image:Atelecyclus rotundatus.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 raninid
Image:Hepatus epheliticus.jpg|Hepatus epheliticus, a calico crab
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.
Further Information
Get more info on 'Crab'.
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