Stacks of Sex-Changing Sea Snails

This could be happening right in your back yard! The picture above shows the common but oft-ignored sea snails Crepidula fornicata (best scientific name around*), also called slipper-shell snails or just slippers. In New England, if you step out onto a rocky beach or wade into the ocean you may happen upon these humble creatures. And, while they may not be as charismatic as, say, sea angels or the Atlantic wolffish (Crepidula don’t even move for about 95% of their lives), there’s more to these critters than meets the eye.

For starters, the reason they live on top of each other in that weird looking stack (there are at least 6 individuals in the picture) is because they don’t move around as adults. So, once they find a mate in the vast ocean it’s worth hanging on for dear life.

But what if one of these snails finds a partner but they’re both males? Time for a sex change! One becomes a female, which causes the other to remain male. These snails are “protandrous sequential hermaphrodites,” which is a big sciency way to say that they get the best of both worlds – they all start their lives as males, and eventually they will all become females. How?

The large snails at the bottom of a stack are always female and the small snails at the top are always male. As the larger, older females die, the next largest member of the stack switches sex from male to female… and on it goes. Not so boring after all.

But it’s not just their life-style that’s interesting; they’re also an important member of our coastal ecosystems. Unlike most snails that eat seaweed or scrape algae off of rocks, Crepidula make their living by filtering food particles out of the water. So, they often compete for food with commercially important species like mussels and oysters. When not competing they’re getting eaten themselves, by important New England species like crabs and sea stars. Some parasitic sponges and snails use Crepidula as a host, and even when they die their old shells provide homes for a host of species, some of which you may have had as a kid.

And, while Crepidula may be a natural part of our marine ecosystem in New England, they’re a shockingly successful invasive species along some European coastlines. In a few areas you can find up to 9,000 of them in a square meter! And they’re starting to have an effect on some important fisheries there.

Susceptible to pollution and high temperatures (not to mention the potential threat of ocean acidification on their microscopic larvae) in New England, it’s difficult to predict the fate of this species in the years to come. But their ability to successfully colonize new environments all over the world offers hope that they’ll also be resilient to the effects of climate change.

So, although they may not look that much different from a rock, Crepidula are one of the many New England creatures that make our oceans special, and worth fighting for. Who knows, one day soon you may be introduced to them, perhaps with a little garlic butter.

* Though we think that this species has one of the best names around, it’s probably just a happy coincidence. It was most likely named fornicata due to its arched shape (fornicata = arched) that the longer stacks form.

Casey Diederich is a 5th year PhD candidate in Tuft’s University’s Biology Department, and is conducting his research on slipper-shell snails. We are thrilled to have Casey guest blogging for us about some of the more fascinating animals in our ocean. Watch for his close-up look at plankton, coming soon. You might be surprised at how interesting and important these little guys are! – Ed.

Photo credit: Paul J. Morris via Wikimedia Commons