Sea slug mating features a stab in the head

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

Found on Australia’s Great Barrier Reef, the new 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 Siphopteron slugs. Yet the new 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, the slugs each insert a second organ, a needlelike stylet. Each slug plunges it like a syringe into the area around the other’s eyes, Lange and her colleagues report November 12 in the Proceedings of the Royal Society B.

The slugs about to be injected 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 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.

Just what the slug (for now called Siphopteron species 1) gains by such injections isn’t clear yet. 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, or 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.

Partner manipulation “seems to be part and parcel of the mating ritual in many, if not most, hermaphroditic animals,” says evolutionary biologist Menno Schilthuizen of Leiden University in the Netherlands. In July in PLOS ONE, he and his colleagues described an Everettia snail from Borneo with the first example of what’s called a love dart — a device that delivers manipulative compounds in snail mating — that has hollow inner channels. Unlike other snails that fire these love darts, chemical-coated calcium spikes, into each other during sperm transfer, the Everettia snail has evolved darts with internal fluid-carrying channels that create natural hypodermic needles.

Snail love darts don’t strike a consistent target region as the Siphopteron species 1’s stylet does. The new paper focuses on the consistent forehead targeting but what Schilthuizen finds more exciting is the paper’s observation that species 1 and four other Siphopteron slugs differ considerably in injection sites. “Copulatory injection itself is widespread,” he says, but “its manner varies as much as all other things sexual.”

LINKED IN   Two hermaphrodite Siphopteron slugs — only a few millimeters long — mate. The heads have a shieldlike structure, and two winglike flaps curl around their sides. Mating starts with intertwining when each partner sticks out a penis. Then the slugs insert the penis into an opening on the right side of the partner and then drive sharp stylets into each other’s foreheads.

Credit: © Rolanda Lange, Johanna Werminghausen, Nils Anthes