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Sea Slugs Stab Mates Between The Eyes to Get Them In The Mood

Photo of 2 sea slugs.
These as-yet unnamed sea slugs stab their mates between the eyes with a needle-like appendage to get their partners in the mood. Photograph by Johanna Werminghausen and Dr Rolanda Lange, University of Tuebingen.

The Marquis de Sade had nothing on this critter—a sea slug that makes out by stabbing potential mates between the eyes and, researchers suspect, drugging them.

Besides having a penis, the sea slug—an as-yet undescribed species from Australia’s Great Barrier Reef—is also armed with a hypodermic device for injecting chemicals that possibly make its partners more willing. (See “Why Sea Slugs Dispose of Their Own Penises.”)

That’s according to scientists who today described the marine slug’s “exceptional traumatic mating behavior” in the journal Proceedings of the Royal Society B.

A mating tactic never before documented in animals, the head injections are thought to contain compounds that manipulate the victim in a way that makes reproductive success more likely, the study team reported.

A Chemical Romance

The sex drug may act, for instance, to prevent the recipient from digesting “the donor’s sperm and instead use it to fertilise the current egg batch,” study leader Rolanda Lange, from Monash University in Victoria, Australia, explained in an email.

She noted that the common garden snail, Cornu aspersum, employs a similar strategy in which snails shoot so-called love darts at their mating partner. Fired into the body, the darts are coated with a substance that allows more of the shooter’s sperm to survive.

Given sea slugs are rather promiscuous, another possibility is that the injected dose could prolong a sea slug’s postcoital recovery time before mating again.

This would give “a sperm competition benefit to the current male,” said Lange, an evolutionary biologist.

Since sea slugs are hermaphrodites, possessing both male and female reproductive parts, each partner gives as good as it gets. During their traumatic tryst, they simultaneously attempt to penetrate the other’s head. (Learn about sea slugs in National Geographic magazine.)

Love Hurts

While such needlelike sex organs, or stylets, are present in other sea slugs from the same genus (Siphopteron), the study found that only the head-injecting species went specifically for the eye region.

The team’s observations of closely related species during their violent courtships found they stabbed much more haphazardly over the body, or at the female genital opening.

The team suspects the head-injecting sea slug deliberately targets the central nervous system that sits behind the forehead. If so, this could put it in the same sinister league as brainwashing parasites that manipulate their host animal’s behavior to their own reproductive ends.

One thing does seem certain: there’s no need for Cupid’s arrow when you’ve got a hypodermic needle. (See pictures of sea slugs “In Living Color” in National Geographic magazine.)