Clownfish do it, wrasses do it, and sometimes even chickens do it. Animals that spontaneously change sex are probably more common than you think. After all, the ability to change the sex ratio of a species based on environmental factors makes way more sense than the system we've got going on.

A new study from the Smithsonian Tropical Research Institute examined the ol' sex switcheroo in one species in particular and found a surprising trigger: touch.

It makes sense for bigger tropical slipper limpets (Crepidula marginalis) to become female. Because eggs take more energy to produce than sperm, a bigger mollusk will be more capable of reproductive success as a mom than a small one. In fact, females of the species play host to multiple males that live on their backs. But how does a snail's body know that the time is right to make that change? Some kind of inter-animal communication must occur that "tells" a snail that it has reached the right relative size to become a mom.

STRI staff scientist Rachel Collin and former intern Allan Carrillo-Baltodano, now a pre-doctoral student at Clark University, found that the snail seems to use physical contact with other members of its species to guide sex change. This came as a surprise to Collin and Carrillo-Baltodano, who were testing the hypothesis that the snails rely on waterborne chemical signaling to guide the changes.

"Slipper snails don't move around much, so you don't really think of them having complex reactions to each other," Collin said in a statement. "But this study shows that there is more going on there than we thought."

To test the waterborne hypothesis, the researchers placed paired snails in water and separated some of them with mesh. The water could still pass through, allowing for any chemical signals passed along that way to go about their business. But in the groups without mesh, the snails also touched.

When pairs of snails were in direct contact with one another, the larger snail was quicker to transition from male to female - and the smaller snail was more likely to stay male.

The researchers aren't entirely sure what the mechanism behind this strange cue is. It's possible that chemical signaling is still the key, but that certain chemicals are passed back and forth by direct contact. But the results are a reminder that even sedentary, seemingly boring sea snails can give scientists a big surprise.

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