Why this newly discovered species of sea slugs penetrates the central nervous system during mating is unclear, but scientist continue to investigate. (Video courtesy of Rolanda Lange, Johanna Werminghausen and Nils Anthes)

A newly discovered sea slug adds that special something to mating: simultaneous forehead stabbing.

Found on Australia’s Great Barrier Reef, this species of hermaphroditic sea slug­­ — bright yellow, red and white and just a few millimeters long — has the double set of penile organs typical of other members of the Siphopteron genus. Yet the slugs deploy them in a novel way, says marine behavioral ecologist Rolanda Lange of Monash University in Clayton, Australia.

When the as-yet-unnamed slugs mate, one organ delivers the sperm to the female opening on another slug’s body. Seconds after partners position their structures for simultaneous sperm transfer, each slug plunges a needlelike stylet into the area around the other’s eyes, Lange and her colleagues found in a study published this month.

The slugs don’t dodge out of the way. “Maybe they’re preoccupied with inserting their own stylet,” Lange says. The stylets, throbbing in slow pulses, stay inserted for most of the 40 minutes or so of sperm transfer.

In the matings that Lange observed, all slugs stabbed their partners in the head, rather than in a different body zone or a mix of targets, as related slugs do. This head strike drives the stylet into the region of the slug’s central nervous system, and the slow pulses pump compounds from one slug into the other.

Sea slugs intertwine. Their penile stylets can be seen as the long, clear appendages directed at each other's heads. (Johanna Werminghausen)

Just what the slug gains by such injections isn’t clear. There are many other species of animals that slip their mating partner manipulative compounds. These biochemicals make the partner invest extravagantly in egg production, for example, make the partner slow to accept the next mate or simply reduce the chances that sperm will get digested for nutrition instead of used for babies.

Susan Milius, Science News