How crabs are doing it

Crabs are Decapods- four pairs of legs and one pair of claws makes ten extremities. But unlike other Decapods such as shrimps, crayfish and lobsters, the body of crabs is wider than long and the reduced tail (pleon) is folded forward under the thorax. The photo on the left shows the flat crab pleon (white arrow), which is lifted to expose the male gonopods (black arrows). These two pairs of gonopods are transformed pleopds. The slender gonopods are lying within the hollow, larger gonopods to build a syringe-like injector for the spermatophores. In this special case, the animal probably is a Carcinus aestuarii, a close relative of Carcinus maenas.

The tail is also covering the sexual organs in female crabs. Thus, male and female animals lift their tails and mate belly to belly. This decapodal missionary style is shown in the image on the left. The upper animal is female. The other animal below is the male, which can be recognized by the fused tail segments. Both photos were taken in 2005 in the Helgoland rock-bottom tidal flat.

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