Apophallation

Apophallation is the biting off of the penis, known in terrestrial slugs, which are hermaphroditic gastropod mollusks. It has been reported in some species of banana slugs Ariolimax and in the distantly related Deroceras laeve.

In the mating of banana slugs, the penis is inserted into the body of the partner. The penis may become trapped, perhaps because of the action of a special muscle, in which case the penis is gnawed off by either the partner or the owner. No replacement penis grows, but the apophallated slug can mate as a female. It has been proposed that preventing the partner from mating as a male might be adaptive in increasing its allocation of resources to the production of eggs.