This sea slug’s mating ritual ends with a stab to the head

In its native Pacific waters, the sea slug Siphopteron makisig looks tiny and delicate, like a bud of coloured glass. But in reality this slug is a mirror-image mating machine, as modelled 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 fertilise 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.”

When other animals stab partners during sex (aka traumatic insemination), they spear various body parts. But in a study Lange co-authored, she identifies S. makisig as “the first known instance” of an animal stabbing its partner between the eyes—perhaps the better to influence the central nervous system.

Description

S. makisig, a marine gastropod, is less than a quarter inch long, with yellow and red markings on its translucent white exterior.

Habitat

The sea slug lives on sand beds at ocean depths from about 20 feet to about 90 feet and commonly nestles within microalgal formations.

Range

The mollusc has been identified in waters off the Philippines, Australia, and Indonesia.

This story appears in the March 2022 issue of National Geographic magazine.