Physa progress

F. C. Baker called Physa heterostropha “the most misunderstood mollusk in America.” Without question, the entire family Physidae has been a source of considerable frustration to me personally, since I collected my first batch as a child 30-40 years ago. The doggone things are everywhere, and all 40+ nominal species of American Physa pretty much look identical.

Now I’m happy to report that progress is being made. In the most recent issue of Invertebrate Biology, Amy Wethington, two undergraduates and I report that the most common species of Physa in North America is Physa acuta. Here’s the reference:

Dillon, R.T. Jr., A.R. Wethington, J.M. Rhett & T.P. Smith (2002) Populations of the European freshwater pulmonate Physa acuta are not reproductively isolated from American Physa heterostropha or Physa integra. Invert. Biol. 121:226-234. [PDF]

Yes, it appears that the most misunderstood mollusk in America, together with its more northern cognate, is not different from a Physa discovered in southern France in 1805 - a Physa which has since spread throughout the Old World, down to South Africa and out to Japan. Although it seems fairly clear that Physa acuta is a North American native, it was apparently first described in Europe. I think P. acuta may be the most common freshwater mollusk in the world.

This is the but the first salvo in an all-out frontal assault on the bastion of the Physidae. This spring Chuck Lydeard, Ellen Strong and I have been awarded a $400k three-year grant from the NSF to review the systematics of the entire family. Chuck and Amy will focus on a big sequencing effort, Ellen will work on the anatomy, and I’ll keep cranking out the breeding studies.

We’ve already collected almost a year of data on crosses involving the other major group of physids, those that Burch puts in the subgenus Physella. (Physa acuta, integra, and heterostropha are in the subgenus Costatella.) We’ve got good results from Physa gyrina gyrina, Physa gyrina aurea, Physa microstriata, Physa utahensis, and a population from Zion National Park. We’ve also piddled with Physa parkeri and Physa ancillaria in the past year. Just this spring we’ve begun new experiments with the Costatella group, including Physa virgata and P. hendersoni. You’ll hear more about all this research in Charleston next month.

So what’s the bottom line? The number of biological species of physids has probably been overestimated by an order-of-magnitude. The critters show tremendous phenotypic plasticity, in shell, life history, and any other respect you could name. The Charleston meeting will feature several talks focusing on phenotypic plasticity in pulmonate snails as well. We’ll keep you posted as this research progresses.

P.S. - You will notice that we do not use “Physella” as a genus. Jack Burch (1992) has moved Physella back down to subgenus rank, and we agree. See Burch & Jung’s “Freshwater Snails of the University of Michigan Biological Station Area” (Walkerana 6:85) for more.