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The Astonishing Sex Lives Of Slugs

Two "banana slugs" curling up...
Two "banana slugs" curling up...
Kerstin Viering

BERLIN - Chances are, you don't know anyone with professional interests quite like biologist Heike Reise. She and her colleagues at Germany's Senckenberg Museum of Natural History in Görlitz are studying the sex life of slugs, also known as "naked snails" for their lack of shells.

Every year the scientists catch and observe primarily field slugs (genus Deroceras), which apparently have a richly eccentric sex life, even by snail standards.

The researchers leave the slugs — hermaphrodites possessed of both male and female sex organs — on their own for a few days in individual receptacles. "After about a week they’re ready to mate," Reise says. The scientists record their activity on camera. They put two slugs together and film their sexual activity from various angles in red light. "The snails can’t see this, so they are not disturbed," Reise explains.

Such research on the molluscs would hardly be possible without filming because while snails are not known for their swiftness in general, there is nothing sluggish about their sex lives. They spend hours engaged in foreplay, but the act of sex is very quick. A penis will suddenly emerge, and it takes less than a second to ejaculate. Way too fast, in other words, for the eyes of curious researchers -- so video makes it possible "to analyze exactly the interaction of the genitalia," Reise says.

What the sex tapes reveal depends very much on the snails themselves. It appears that each of the 100 or so presently known field slug species has its own sexual preferences, "and these behaviors are so typical that you can tell the different species apart by whichever behavior they exhibit," says Reise.

This is particularly helpful because field slugs are otherwise difficult to distinguish. There are differences of size and color, but both can vary widely within the same species. It’s more conclusive to examine their penises, which differ in build depending on the species, although even this doesn’t necessarily guarantee accurate identification. That's why researchers analyze coupling behaviors.

Reise and her colleagues have already been able to identify several as yet unidentified slug species. One of these successes involves a slug that can now be found all over the world and that has made itself a very unpopular pest in vegetable patches. For a long time scientists thought this was the Deroceras panormitanum, originally from southern Italy, but a few were starting to suspect that several species were being mistaken for them. But few were absolutely certain — until Reise and her colleagues came along and discovered that the coupling behaviors of the southern Italian slugs were massively different from those of the cosmopolitan pest now known as Deroceras invadens.

Using genetic markers, the Senckenberg researchers intend next to tackle the question of whether all the vegetable-eating pests in the world belong to this new species or whether there are in fact more unidentified species out there. Reise and colleagues are also hoping their research will yield answers to how the slugs proliferate, why they are so successful, and how to better combat them.

Fascinating — and a little kinky

But the motivating force of snail sex research for many scientists is sheer curiosity, and astonishment, at the bizarre ways of nature. They can only shake their heads in wonder at the love games of the great, grey slug Limax maximus, for example. During copulation, these 10- to 20-centimeter-long slugs twine themselves around each other as they hang from a tree and create a spiral with their intertwined penises, transferring sperm from tip to tip.

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Two Limax Maximus slugs mating - Photo: John from Tulsa

For other slug species, coupling can be even more challenging. The black Limax cinereoniger is a case in point. It can take them up to 20 minutes to get their 10-centimeter penises untangled. Then there are the problems encountered by Limax redii, native to the southern Alps, which has a penis anywhere from 15 to 85 centimeters long. Yet they somehow manage to untangle themselves after sex.

The North American yellow banana slug (Ariolimax) doesn’t always manage, however, to withdraw its penis out of the female sex organ, and when that happens at least two species take drastic measures: they either bite their own penis off, or let their partner do it, and then go on to live out the rest of their days as a female. But scientists have also recorded such amputations when there was technically no need for them, and they want to discover the reasons why.

Other snail species have a predilection for rough sex, where one partner gets hurt. These include Helix pomatia (variously called the Burgundy snail, Roman snail or escargot), which actually rams a harpoon-like love dart into its partner’s body.

Researchers have pondered the function of this sharp calcareous weapon for years. It is now clear that it is an injection of a hormone-like secretion to facilitate the path of sperm to egg. A practice that plays a major role in the love games of Deroceras species — whereby one partner secretes a substance onto the back of the other after the sex act — could have a similar function, Reise and colleagues believe. "We think field slugs are manipulating their partners hormonally to make sure that the offspring is theirs,” she says.

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