If you’re an aficionado of sea slugs, you’re probably used to weird and cringe-worthy sex acts. When two of these hermaphrodite animals penetrate each other at the same time like an R-rated yin-yang symbol, you won’t bat an eyelid. When several of them link up in a long mating chain, you’ll have seen it all before. When one amputates its disposable penis after sex only to uncoil a spare, your only reaction will be, “Meh.”

Still, when two sea slugs penetrate each other in the head while having sex, even the most jaded expert might do a double-take.

Rolanda Lange certainly did. An evolutionary biologist from the University of Tubingen, she is well-versed in sea slugs and their sexual antics. Recently, she found a new species while diving at Lizard Island, Australia. It’s just 2 to 3 millimetres long, and looks like an undersea orchid. Its body is white and surrounded by translucent petal-like flanges, rimmed with vivid reds and yellows. It doesn’t have a formal name yet. For now, Lange simply calls it Siphopteron species 1.

Every individual is a hermaphrodite with both male and female genitals. When they have sex, they can simultaneously penetrate each other, with penises that extend to their whole body length. “They are relatively well-endowed, says Lange.

The penises are also forked. One branch ends in a cone-shaped structure called the penile bulb, which is ringed by small spines. It goes inside the partner’s female genital opening, and delivers sperm. The other branch ends in a fiendish spine called the penile stylet. It stabs straight into the partner’s forehead, and pumps fluid from the prostate gland. So, during sex, each slug gets a dose of sperm in the usual place, and an injection of prostate fluid just above its eyes. This goes on for just over 40 minutes.

“You may imagine I was quite excited and surprised to find out they reciprocally injected into their partners’ head!” says Lange.

View Images Sea slug sex. H = head; pa = parapod; pb = penile bulb; pst = penile stylet; t = tail lobe. Credit: Lange et al, 2013. Royal Society

Back in her lab, she filmed 16 sexual encounters between these sea slugs, and showed that they always aim for the head with their penile stylets. She even spotted one mating circle, in which three slugs stabbed/inseminated each other at the same time.

This wince-inducing brand of sexual puncturing is called “traumatic insemination”. It’s widespread throughout the animal kingdom. Bed bugs famously do it. At least one species of spider does it. Many worms and snails do it. It’s not new to see traumatic insemination among snails and slugs, says Ronald Chase from McGill University. “What is new is the documentation of species-specific injection sites,” he says.

Lange found that some Siphonopteron sea slugs are flexible, aiming for somewhere on the body or foot. Another new species always goes for just next to the genital opening. But Species 1 is the only member of the group that only and always aims for the head. Lange calls it “cephalo-traumatic secretion transfer”; the colloquial version isn’t printable on a National Geographic site.

The obvious question: Why? The short answer: No one knows.

In many species that practice traumatic insemination, females have ways of storing a male’s sperm so that they can choose between many suitors. The males bypass this choice by injecting their sperm into unusual places.

In some cases, the males traumatically inject other fluids to give their sperm a helping hand. The male common garden snail does this by shooting a female with a love dart, which delivers a hormone that stops her from digesting his sperm. “We think it is plausible to assume something similar may be going on in our species,” says Lange. The injections could compel the female into keeping the male’s sperm around.

But: the head! Siphonopteron penises are so long and flexible that they could inject prostate fluids anywhere on their partners. Why does this one species go for the head, when its close relatives don’t?

Lange thinks that it’s aiming for the brain, either injecting near it or directly into it. Many parasites use chemical injections to control their victims’ control their behaviour. The emerald cockroach wasp, for example, turns cockroaches into docile pets by stinging them in their brains. But there’s no record of an animal using similar injections to control a sexual partner’s behaviour. Perhaps this sea slug is the first?

“[That’s] completely speculative,” says Chase. “The paper presents no evidence of neural manipulation, only the possibility based on the site of injection.”

“After head injection, the slugs do become calmer,” adds Lange. “But they have that in common with [similar species] which do not inject into the head. My educated guess is that it does not manipulate observable behaviour, but rather influences something internal, such as sperm storage.”

Lange now wants to use high-resolution scanners to work out exactly where penile stylets are going, and whether they are hitting any nerve clusters. She also wants to analyse the prostate fluids, and work out whether the slugs behave differently after the injections.

Reference: Lange, Werminghausen & Anthes. 2013. Cephalo-traumatic secretion transfer in a hermaphrodite sea slug. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2013.2424