Watching snail sex could help scientists see evolution in real-time

Science has not exactly been kind to the rough periwinkle.

For one, taxonomists keep forgetting the species, known to malacologists as Littorina saxatilis, exists. It’s been “discovered” so many times that The Guardian once called it “the most misidentified creature in the world.”

This year, the tiny sea snail suffered another insult after scientists made an embarrassing discovery: they’re bad in bed. Male rough periwinkles mount a female for anywhere from a minute to two hours, and researchers assumed that short mountings weren’t enough for the male to ejaculate. But according to a new study published last month in the Journal of Molluscan Studies, that particular step takes less than a minute.

“You have to study something that makes you laugh,” Samuel Perini, a postdoctoral researcher at the University of Trento and the lead author of the paper, told Inverse. Watching snails have sex isn’t everyone’s idea of a good time, but Perini says it’s helping scientists understand how the struggle to mate plays a key role in evolution.

How they did it — In two separate trials, the scientists rounded up a total of 38 young females from the shores of Saltö, an island off the coast of Sweden in the Baltic Sea, and raised them in captivity for 10 months until they were old enough to breed. They then returned to the island and captured 60 males, making sure to check for a “fully developed penis.”

Back at the lab, they partnered each female with two males about three-quarters her size — a lab-tested ratio that means an ideal match for snails — and waited for one male to assume a position atop the female’s shell. “It's a bit hidden when the penis gets inserted,” says Perini. Since the scientists couldn’t observe it directly, the process of insemination had to be measured another way.

Perini and his co-authors interrupted the pairs after different lengths of time, from one minute to thirty minutes, and separated the females into their own tanks. After giving the females some time to rest, about two weeks, the researchers cracked open their shells and counted the number of fertilized eggs. To their surprise, they found that all the females had similar amounts of fertilized eggs, no matter how long the male had stayed on top.

The authors considered a few reasons why males might finish up so quickly. The snails are abundant on their home shores, and in a population with no shortage of females, males might want to mate fast and move on. And for sea snails, getting caught in flagrante delicto is compromising in more ways than one: it exposes the pair to hungry predators.

But these explanations leave another important question unanswered: If the male finishes his business in less than a minute, why does he stay mounted for so long? The answer might lie in what biologists call “sexual conflict,” a push-and-pull between the opposing interests of male and female snails.

This isn’t the first time Littorina saxitilis has, err, come under the microscope. Perini’s principal investigator, ecologist Kerstin Johannesson, has been studying the species for decades, and her lab previously found that while females of other snail species leave behind a scent trail for males to follow, rough periwinkles apparently don’t. This could be the female’s way of thinning the ranks — there are so many male snails that she might get overwhelmed if they all knew where to find her.

Why it matters — Snails are popular subjects for scientists studying the battle of the sexes, in part because they’re easy to work with. “They don’t go anywhere very fast…so they’re pretty easy to keep track of,” says Adam Chippindale, a professor of evolutionary genetics at Queen’s University in Ontario, Canada. “They also have some crazy sexual conflict.” Chippindale wasn’t involved in the current research.

Early research on sexual conflict tended to emphasize the way males compete for female attention and try to keep other males away. Chippindale studies fruit flies, which try to woo females by hoarding and guarding resources like food. The males of some other species, including mice, go so far as to seal up the female’s reproductive tract with semen that hardens like cement into “a kind of cork.”

But the field has gone through a kind of “feminist critique,” says Chippindale, as scientists have learned just how much power females wield during reproduction, especially once the male has left the scene. “They're not just like an empty vessel, where if a male puts in more tickets in the lottery, he has a better chance of winning.” Depending on the species (and how much she likes the male), a female might send special enzymes to kill the sperm, or she might simply “boot it out.”

Of course, males try to get around these tactics. Unlike the rough periwinkle, male fruit flies don’t start transferring sperm immediately after mounting. Instead, they spend the first 8 or 9 minutes injecting a special semen, powered up with proteins that among other uses, can modify the female’s pheromones, making her smell more like a male and therefore less attractive to other males. And if the female has recently mated, the proteins might even eject the previous male’s sperm.

Female rough periwinkles mate with as many as 23 males at a time, and they likely have their own ways of choosing the “fittest” sperm, says Perini. With such stiff competition, the males might be sticking around for so long to ensure their sperm wins out, perhaps by transferring proteins or injecting more semen to dilute the sperm from previous mates.

What’s next — Next up, Johannesson’s lab plans to try a similar experiment with a different population of rough periwinkles in Spain — the same species, but with some genetic variations that might make for different mating habits. The work is part of a broader effort to understand the processes that create new species, including sexual selection.

As they try to outdo each other, males and females can develop new behaviors and even new body parts, after enough time. The competition “could generate such a strong force…that it could split one species into two in a very short time,” says Perini. For scientists, watching snails have sex is one way to see evolution in action.