VIDEO: Slug penetrates mate’s forehead during sex

By Wes Judd 28 November 2013
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An Australian sea slug uses a spike to inject fluid into the head of its mate during intercourse, researchers have found.

THERE ARE MANY WEIRD mating practices in the animal kingdom, but researchers have discovered a particularly unique case in a Pacific Ocean-dwelling sea slug.

These 2-3mm long, brightly coloured hermaphroditic slugs (Siphopteron quadrispinosum) are found on Australia’s Great Barrier Reef as well as the waters around New Guinea and Hawaii. They have forked penises, roughly the length of their bodies.

During 40-minute mating sessions, the slugs reciprocally push one branch of the penis into each other’s female genital opening and release sperm, while the other branch penetrates their partner’s head, very close to the eye.

Here they use a sharp spike to inject a chemical secretion from the slug version of a prostate gland. The researchers suspect this may have an effect on the central nervous system, which increases the chance of a successful fertilisation for the slug injecting the secretion.

The discovery was revealed today in the journal Proceedings of the Royal Society B. It came out of research at Lizard Island, Queensland, by scientists from Monash University, the University of Queensland and the University of Tuebingen in Germany.

Intercourse between Great Barrier Reef sea slugs. (Credit: Dr Rolanda Lange)

Weird animal sex among slugs

There are other male animals, such as the common garden snail and some species of salamander, that are known to inject a kind of prostate fluid into the female before mating, but it is the location of this injection that makes the sea slug so special.

“As far as I know, the injection into the head of a mate is entirely unique to these slugs,” says Dr Rolanda Lange of the University of Tuebingen, lead researcher on this study.

In addition, every other species that releases prostate secretion prior to mating does so to increase the male’s reproductive success. However, due to the location of the sea slug’s injection and the fact that each slug is both male and female, scientists suspect it isn’t that simple.

Rolanda guesses that the slugs purposefully aim for the brain when injecting, which would imply that the secretion has a neurological function, related to reproductive success.

“In certain animals, there’s a conflict of mating interesting – a sexual conflict,” she told Australian Geographic. “Each sex invents mechanisms to get their way through, which may consequently hurt their partner. Neurologically is one of those ways.”

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