Tropical slipper limpets are pretty unremarkable. They spend most of their lives sitting in one spot under a rock filtering their food out of the water. When it comes time for the limpets to mate, though, things get interesting. 

Compared to their body size, male limpets have some of the most impressive penises in the animal kingdom—almost as long as the snail’s entire body. They don’t hang on to these massive members forever, though. Slipper limpets begin their lives as males and turn into females as they age and grow—a process called sequential hermaphroditism. Their penises slowly shrink and then disappear while their female organs develop. 

A recent study in The Biological Bulletin shows that this dramatic change is set off by a simple touch.

Plenty of animals—including some starfish, crustaceans, and fish—change sex at some point in their lives. The timing of a sex change has to do both with an animal’s size and the company it keeps. Bigger animals can produce and carry more eggs than smaller ones, while sperm production is less size-dependent, so the switch usually happens when an animal reaches a certain size. For many sex-changing animals, there’s also a social factor, and the animals in a mating group or population can “tell” each other when it’s the best time to switch. 

Chemical cues are used by sea snails and other mollusks to warn one another about predators, recognize each other, and choose sites to live, among other things. Rachel Collin, a biologist at the Smithsonian Tropical Research Institute, figured that limpets used similar chemical signals to prompt each other's sex changes, too.

To test that idea, she and intern Allan Carrillo-Baltodano collected small male slipper limpets at Playa Chumical on Panama’s Pacific coast. They paired the snails off and placed them in different cups of seawater that allowed them different degrees of access to any chemical cues their cup mates might release. In some cups, the snails were free to roam around and come in contact with each other. In others, a mesh barrier kept them on separate sides of the cup. Some of the barriers had a finer mesh that only allowed water through, while others were a little wider and let the snails touch each other through the screen. Still other pairs were separated by fine mesh, but the snails were switched from one side of the cup to the other every week, letting them come in contact with their cup mate’s pedal mucus, the gluey substance snails and slugs use to help them move around.

When the limpets were allowed total access to each other, the larger one in each pair grew faster and changed sex sooner than their counterparts that were separated from their partners, while smaller, free-roaming partners delayed their own sex changes longer than the smaller snails in the separated pairs. The snails that were separated by the fine mesh and those that switched sides in their cups changed sex on the same, slower schedules, while the ones that were separated by the wider mesh fell in the middle.

Collin and Carrillo-Baltodano said they expected the sex change to be triggered by chemical cues since limpets are sedentary, but their results suggest that the change “requires some kind of physical interaction that is lost when the snails are separated.” For now, they aren’t sure what it is about physical contact that sets the change in motion. It might have to do with the way one snail positions itself relative to another, or a certain type of physical stimulation. Chemical signals could also still be at work, but ones that are contact-based instead of waterborne.