A tiny sea slug found on Australia's Great Barrier Reef stabs its sexual partners through the head with a specialized probe, apparently to inject secretions that influence its partners' behaviour after mating.

Credit: ROLANDA LANGE

Rolanda Lange of Monash University in Melbourne, Australia, and her colleagues observed 16 matings between 20 individuals of a newly discovered sea slug (Siphopteron sp.) that has a two-part penis. In all cases, seconds after the animal had inserted its penile bulb into a sexual partner to transfer sperm, it stabbed the other part — a specialized needle-like structure (pictured) — into the head of its mate.

Related sea-slug species are known to inject prostate secretions in a similar manner, but not to the head. The authors suggest that this species is targeting the neural ganglia near the injection site, and that the secretions manipulate the behaviour of the sperm receiver.

Proc. R. Soc. B http://doi.org/p33 (2013)