Snails and sex change, take 2

When last we spoke, I wrote about the size-advantage hypothesis and a little about the strange lives of Crepidula, the group of slipper limpets that I study. Since I ought to be writing a manuscript on an unrelated topic, this is clearly the time to follow up that post.

I would be remiss if I didn't say that Stephen J. Gould has a fantastic essay on sex change ("Sex and Size"), including quite a bit on Crepidula, in his collection The Flamingo's Smile.  Go to the library and read it. The genus has also been the subject of quite a bit of internet ink already, at Deep Sea News, The Other 95%, and from the Collin lab blog, among many others. You should also check out the Collin lab blog for a nice explanation of the scientific name for Crepidula fornicata, one of my study species (hint: not what it sounds like...or is it?). They've also got a fabulous post about this whole group of snails.

But the point of my post is to write about the work I did several summers ago in collaboration with my then-labmate, Dr. J. Matt Hoch. We combined our forces to come up with a project investigating sex change (related to the subject of J. Matt's dissertation) in my study system (C. fornicata), and in particular, investigating how the optimal size at sex change might vary with the population density.

From my last post, you may recall that there is, in theory, an optimal size for sex change to occur. But this size might not be fixed within the species, and indeed we know from decades of previous work that sex change can happen at a range of sizes. Using the hypothesis that there was a chemical produced by other C. fornicata individuals that affected sex change, we expected that the population density (number of other snails around) would play an important role in determining that optimal size.

Crepidula fornicata at one of our low-density field sites. There's a small stack there in the middle, surrounded by green algae. Also pictured: barnacles and mud snails. Photo credit A. Cahill

We tested this idea by going out into the field and measuring snails from populations of different densities. We measured their size and recorded their sex, and from there were able to determine the average size at sex change in each population. We (J. Matt) then did some statistical modeling to test which of several factors was most important in determining that size. The upshot is that the most important things in determining whether a snail is male or female are its size (no surprise there) and the sizes of the other snails in its stack (its immediate environment). Although the population density did show up in the models, explaining some variation in the data, it wasn't very important. We ultimately published  the result in the journal Marine Biology.

Incidentally, that was my first contribution both to the scientific literature and to the century-old literature on this group of snails.