The term crab is often applied to several different groups
of short (nose to tail) decapods with thick exoskeletons,
but only members of the Brachyura are true crabs; other
taxa, such as hermit crabs, porcelain crabs, king crabs,
and horseshoe crabs are, despite superficial similarities,
not crabs at all. Hermit crabs, king crabs and porcelain
crabs belong to the Anomala and can be distinguished from
true crabs by counting the legs - in Anomala, the last
pair of pereiopods (walking legs) is hidden inside the
carapace and so only four pairs are visible (counting
the claws), whereas uninjured true crabs always have five
They are crustaceans in the suborder Brachyura, in the
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 eggs. Crabs are a very diverse group, mostly found
in salt water, 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.
Classification within the crabs is traditionally based
on the position of the gonopores: 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.
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 pleopods. 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.
Soft Shell crab
Queen crab ( snow, opilio)
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