Crab

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 visible pairs.

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 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.

Related Topics:
Sexual dimorphism - Pleopod - Fertilised - Eggs - Pleocyemate - Pereiopod

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