Dr. Rob Dillon, Coordinator

Wednesday, April 7, 2010

TRUE CONFESSIONS: I Described a New Species

Editor's Note.  This essay was subsequently published as: Dillon, R.T., Jr. (2019b) TRUE CONFESSIONS: I described a new species.  Pp 193-198 in The Freshwater Gastropods of North America Volume 2, Essays on the Pulmonates.  FWGNA Press, Charleston.

... or, to deflect at least a fraction of the calumny, two accomplices and I described a new species. A pdf of the recent description of Physa carolinae by Wethington, Wise, and Dillon can be downloaded from note (1) below.

The existence of dark, slender populations of Physa in the Charleston area was first called to our attention in the 1980s by the late Julian Harrison, a colleague on The College faculty, primarily a herpetologist but an excellent all-round naturalist. The sample he brought us came from a shallow, swampy pond on James Island, in suburban Charleston. I told him these were simply "Physa heterostropha," which is what I called all the local Physa populations twenty years ago.

A couple years later Amy Wethington and I discovered a second population of slender, dark Physa in an agricultural ditch on Johns Island, about 10-15 km south of the city. We were working on a population genetic study designed to evaluate barriers to dispersal among sea islands (2), and needed to find a Physa population in an extensive region of sod farms to complete our (rather tightly specified) sampling grid. On the map, this part of Johns Island looked most unpromising. Sod farms are heavily fertilized and irrigated, ditched and drained into collecting ponds, brutally hot in the summer and exposed in the winter. But sure enough, in a damp and weedy ditch in the middle of Johns Island [right, below], Amy and I found another population of strikingly dark, slender Physa.

Intriguingly, this dark slender morphology seemed to have a strongly heritable component. Lab lines of the “Johns Island Physa” retained their distinct appearance to the second and third generations in culture. In the mid-1990s I did some experiments (as yet unpublished, shame on me) to estimate the heritability of shell shape (six linear measures) by regressing F1 hybrids between the Johns Island line and our standard (fatter) Physa lines on their mid-parent values. The heritability of shell morphology was strikingly high, but I digress.

The F1 hybrids derived from that experiment failed to reproduce. In retrospect, these observations probably influenced the first set of studies Amy and I designed to test reproductive isolation in Physa (3). We studied two populations of Physa heterostropha, two populations of P. integra, and two populations of P. acuta because (we imagined) that we’d find some reproductive isolation within species, as well as reproductive isolation between nominal species. We didn’t find any reproductive isolation among any of these six populations, of course, prompting us to synonymize most of the world’s Physa populations under the single nomen, Physa acuta.

Wait, wait! Does Rob Dillon have the arrogance to assert that dozens (scores?) of specific physid nomina (4) recognized by the entire community of systematic biologists around the world for 200 years are all synonyms of a single, variable, cosmopolitan Physa acuta, while in some ditch ten kilometers south of Charleston lives a bona fide undescribed species that only Rob Dillon can recognize? Let’s back up and get a fresh start.

The taxonomy of the North American Physidae in currency when Amy and I began our research program in the late 1980s was that of George Te, as reproduced in Burch’s "North American Freshwater Snails." Had I sent my dark, slender Physa to Te while he was still active in the 1970s, I feel fairly certain that he would have identified them as "Physella hendersoni" (5), the type locality of which is in Yemassee, SC, just 80 km west of Charleston. Burch's figure 677, labeled "Physella (Costatella) hendersoni ssp," does indeed appear to depict a slender shell quite similar to that borne by our Johns Island population.

But in another of those serpentine turns for which freshwater malacology is so famous, “hendersoni” was originally described by Clench (1925) as a subspecies of Physa pomilia Conrad (1834). Te considered pomilia to be a subspecies of P. heterostropha while holding hendersoni distinct. Working with snails sampled from their type localities, however, our 2007 breeding studies demonstrated both that hendersoni and pomilia are conspecific, as Clench originally suggested, and that pomilia/hendersoni most certainly is reproductively isolated from heterostropha/acuta (6).

And our dark, slender physids (which we began calling "Physa Species A" about ten years ago) are reproductively isolated both from pomilia/hendersoni and from acuta/heterostropha. The paper immediately preceding the description of these populations as Physa carolinae is a Dillon (solo) work documenting F1 hybrid sterility between carolinae and acuta, and both sexual isolation and apparent hybrid inviability between carolinae and pomilia (7).

So yes, as embarrassing as it looks – the research group responsible for synonymizing the physid fauna of the entire continent from about 40 nominal species down to maybe ten (8) is now asserting that we have discovered a Physa species overlooked by everybody, in our own back yards.

Physa carolinae seems seasonally common and widespread throughout the Atlantic Coastal Plain, ranging at least from Virginia to Georgia. We do not have any original field observations further south, but a glance through the collections of the Florida Museum of Natural History a couple years ago suggested to me that Physa carolinae may also be widespread in Florida, museum lots generally catalogued under the specific nomen "hendersoni."

Throughout its range, P. carolinae is most commonly found in swamps, ditches, and other waters of an intermittent or vernal character. It seems to be a southeastern ecological analogue of Aplexa – the two taxa converging on each other in habitat, life history, and morphology.

This Saturday just past Amy, John Wise, and I were featured in the Charleston newspaper as "Snail Sleuths - CofC Researchers Find Lowcountry Species" (9, 10). In addition to some cutesy quotes about the pace of snail research being - well - slow, I found myself saying something like this to the reporter: "We send scientists all over the world, and we don't know the slugs under our own trash cans." If anybody on this list is aware of any funding agencies that might be responsive to such an appeal, please bring them to our attention at your earliest convenience.

And keep in touch,


(1) Wethington, A.R., J. Wise, and R. T. Dillon (2009) Genetic and morphological characterization of the Physidae of South Carolina (Pulmonata: Basommatophora), with description of a new species. The Nautilus 123: 282-292. [PDF]

(2) Dillon, R.T., and A.R. Wethington (1995) The biogeography of sea islands: Clues from the population genetics of the freshwater snail, Physa heterostropha. Systematic Biology 44:401-409. [PDF]

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

(4) Wethington, Wise & Dillon listed 19 nomina in their P. acuta synonymy, including heterostropha, integra, the western virgata, and the more tropical cubensis. Also listed was P. natricina, about which I offered an entire essay on 12Mar08.

(5) I swapped several letters with George Te in 1976, while I was still an undergraduate at Virginia Tech. He identified the Physa acuta I sent him from the New River as "P. hendersoni," and my Physa gyrina as "P. pomilia."

(6) Dillon, R. T., J. D. Robinson, and A. R. Wethington (2007) Empirical estimates of reproductive isolation among the freshwater pulmonates Physa acuta, P. pomilia, and P. hendersoni. Malacologia 49: 283 - 292. [PDF]

(7) Dillon, R. T. (2009) Empirical estimates of reproductive isolation among the Physa species of South Carolina (Pulmonata: Basommatophora). The Nautilus 123: 276-281. [PDF]

(8) Wethington, A. R. & C. Lydeard (2007) A molecular phylogeny of Physidae (Gastropoda: Basommatophora) based on mitochondrial DNA sequences. J. Molluscan Stud. 73: 241 - 257. [PDF]

(9) Snail Sleuths: CofC researchers find lowcountry species
Charleston Post & Courier 3April2010

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(10) And I'm now starring on YouTube! Check it out: