The Mechanism of Sea Slugs (Pseudobiceros hancockanus)Penis Fencing

Genital "Handshakes"

Mechanism refers to the physical structures within an individual that make a particular behavior possible. For sea slugs engaged in penis fencing, those mechanisms are everted penises and sperm - the process by which the mechanisms contribute to the behavior is described below.

Members of this species are hermaphroditic; each individual has both sperm-producing testes and egg-producing ovaries. They duel over which individual will carry the eggs and which individual will fertilize the eggs through a behavior called “penis fencing” where the individuals will try to inject sperm into each other while avoiding having sperm injected into them. The mechanism by which this occurs is primarily through an everted penis and through the sperm’s ability to move through the body. This mechanism is important and significant because in hermaphroditic individuals there has been a resulting evolutionary arms race between mechanisms better adapted to blocking advances versus injecting - the results of which are evident not only in the flatworms and sea slugs described here but also in leeches.

Sea slugs penis fencing

Two sea slugs (Pseudobiceros hancockanus) rear up to begin penis fencing. The individual on the right clearly displays an everted penis. (Source: National Geographic)