sand crab, crabs

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. 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.