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. Crabs have a soft body covered with a hard shell. 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
thumb|300px|left|Anatomy of a [[fiddler crab|alt=Anatomical drawing of crab, labeling walking legs, carapace, eye, antenna, rostrum chelipeds, propodus, carpus, and merus]]
The
infraorder Brachyura contains about 93
families, as many as the remainder of the
Decapoda.
The evolution of crabs is characterized 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.
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
thumb|Crab Masala from [[Karnataka, India|alt=Photo of cooked crab in bowl of soup]]
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.
Fishery
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
alt=Photo of round ceramic with reliefs of the crab shell and claws
The
Moche people of ancient
Peru worshipped nature, especially the sea. They often depicted crabs in their art.
The
Cancer Constellation is one of the 12
Zodiac signs.
Western cultures have been influenced by the crab towards the game
Crab soccer, where players rest and move on an inverted all-fours pose. Unlike crabs, however, they do not move sideways.
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.
Gallery
