In its native Pacific waters, the sea slug Siphopteron makisig looks tiny and delicate, like a bud of colored glass. But in reality this slug is a mirror-image mating machine, as modeled by the pair in the photograph above.
Like most sea slug species, S. makisig is a hermaphrodite, endowed with both male and female reproductive organs that it uses at the same time during mating. But unlike other sea slugs, it tops off trysts with unusually targeted stabbing.
The sex starts normally enough. To fertilize eggs developing in each slug’s female parts, the other slug deposits sperm with its penis. Actually, only half of its penis, which has two prongs: one that delivers sperm with its bulbous end, and the other tipped with a syringe-like stylet (and sometimes called hypodermic genitalia). During the sex act, each slug stabs the other with the stylet, which delivers prostate fluid likely bearing hormones. Evolutionary biologist Rolanda Lange says the fluid may “increase the fecundity of a sea slug’s own sperm, or inhibit that deposited by previous partners.”