Dr. Rob Dillon, Coordinator

Thursday, December 6, 2018

To Identify a Physa, 2000

Editor’s Notes – This is #5 in a series on our modern progress toward an understanding of the systematic biology of the North American Physidae.  The present essay will best be appreciated by readers who are familiar with my previous essays on 1971, 1975, 1978, and 1989, linked from footnote [1] below.

This essay was subsequently published as: Dillon, R.T., Jr. (2023c)  To Identify a Physa, 2000.  Pp 177 – 184 in The Freshwater Gastropods of North America Volume 7, Collected in Turn One, and Other EssaysFWGNA Project, Charleston, SC.

When did it dawn on me that the weedy populations of sinistral pulmonates I had called “Physa anatina” as a high school student in 1971 and “Physa hendersoni” as a college student in 1975 and “Physa heterostropha pomilia” when Amy Wethington and I began our research program in 1989 might actually be the same as the Physa acuta invasive across the rest of the known world?  And where did that idea come from?  Here 20 years later, I don’t know.

I probably read my first papers about Old World Physa acuta in connection with research for my book for Cambridge University Press [2].  I remember seeing speculations in the early-1990s [3] that invasive populations of P. acuta in Africa might have originated in America.

And I do remember receiving the copy of Süsswassermollusken [4] from my friend Peter Glöer in early 1995, with the picture of “Physella heterostropha (Say, 1817)” on the cover.  Inside on page 65, Peter figured both Physa acuta at top and Physa heterostropha at bottom, writing “spread by aquarium hobbyists” about the former and “carried from North America” about the latter, noting that the two species “can be confused.”  Peter reported populations of both species throughout Germany.  Hmmm.

The cover of Süsswassermollusken [4]
My best guess is that the idea to test whether American Physa heterostropha (at least) might be the same species as European Physa acuta were born in early 2000, during my correspondence with Dr. Roy Anderson, an amateur of professional caliber working in Northern Ireland.  Roy wanted confirmation of his recent discovery of North American Physa gyrina in the Old Country, which I was gratified to be able to supply [5].  And he also sent me some preserved Physa acuta from Flintshire, the first I had ever personally examined, and I just could not see any difference between his European snails and the Charleston-area populations I had been calling Physa heterostropha.

Amy Wethington and I (with undergraduate Ed Eastman) had performed our first experimental tests of reproductive isolation (RI) among populations of Physa heterostropha and P. gyrina quite early, around 1990, inspired by the mate choice tests not uncommonly undertaken with fruit flies [6].  Amy left Charleston in 1992, but by early 1999, I had developed an NSF proposal to test both prezygotic and postzygotic RI among a variety of physid populations, albeit all American.

Meanwhile, after sojourns in Bloomington and Lexington, Amy had arrived at the University of Alabama to work on her Ph.D. with Dr. Chuck Lydeard.  And in early 2000, Chuck and I hatched a plan to resubmit my freshly-rejected NSF proposal on reproductive isolation in Physa, featuring a graduate research assistantship for Amy.

So the summer of 2000 found Amy travelling all about the United States, collecting Physa for her Ph.D. research.  She visited Philadelphia (the type locality of P. heterostropha), New Harmony (the type locality of P. integra), Douglas Lake Michigan (for an especially well-studied P. integra population), and (of course) Charleston, for our especially, especially well-studied P. heterostropha [7].

And in August of 2000, I set up our first crosses to test for postzygotic reproductive isolation among those four American populations of Physa, working with two excellent College of Charleston undergraduates, Matt Rhett and Tom Smith.  In September our good friend Dr. Philippe Jarne sent us a sample of Physa acuta from France, and in October a sample arrived from Ireland, courtesy of Roy Anderson. 
I wrote, in an October 2000 email to Amy and Chuck in Tuscaloosa, “Our breeding experiments have such a beautiful design that it is impossible to imagine that we simply blundered into it.”  We had three estimates of intraspecific RI: Philadelphia heterostropha x Charleston heterostropha, New Harmony integra x Douglas Lake integra, and French acuta x Irish acuta.  We also had (what I imagined to be) three estimates of interspecific RI: Philadelphia heterostropha with New Harmony integra, New Harmony integra with French acuta, and Philadelphia heterostropha with French acuta.  And (of course) we had our six incross controls [8]. 

Each experiment (and each control) involved ten pairs of snails, so at one point we had (3 + 3 + 6) x 10 = 120 breeding pairs of Physa, each yielding as much as an egg mass per day.  Every embryo had to be counted, and every viable hatchling.  And every cup – not just the adults but their eggs and hatchlings – had to be changed and fed weekly.  Some fraction of the F1 were reared to run gels to verify the outcross, and some additional fraction crossed to confirm F1 fertility.  Tom and Matt worked like field hands.

And what we found was nothing.  No reproductive isolation whatsoever.  No delay in parental maturity, no reduction in parental fecundity, no reduction in F1 survivorship, and no evidence of F1 sterility, in any of those six outcrosses, relative to incross controls.  None of those six populations of Physa could tell each other apart any better than we could.

Looking back on it, our greatest accomplishment in the summer and fall of 2000 may have been the rigor we brought to the documentation of nothing.  Not merely nothing, but really most sincerely nothing.  Which is the most difficult result of all [10].

No RI between Physa acuta and P. virgata [14]
The paper by Dillon, Wethington, Rhett & Smith [11] was published in Invertebrate Zoology in 2002.  In it we spun a charming yarn, hypothesizing that Physa of American origin were introduced by transatlantic shipping into the bustling port of Bordeaux around the turn of the 18th century, to be described from the River Garonne by a Frenchman twelve years before Thomas Say, the first American Conchologist, gave any attention to the crappy little critters here at home.  We called Physa acuta (Draparnaud 1805), now understood as a North American native, invasive on five other continents, “the most cosmopolitan freshwater gastropod in the world.”

The year 2002 also saw the funding of our NSF proposal, “Phylogeny of physid snails (Basommatophora: Physidae) and evolution of reproductive isolation,” now by Lydeard, Dillon, and Ellen Strong.  And the remainder of the physid fauna of the United States (most of it, anyway) followed in (what now seems to be) rapid succession: experiments with Physa gyrina [12] and its cognates in the Midwest [13], P. acuta cognates in the southwest [14], and the surprisingly complex situation with pomilia and carolinae back home in the southeast [9, 15].  I have previously reviewed the phylogeny ultimately proposed by Wethington & Lydeard in 2007 [16], and the summary work we published all together on the evolution of reproductive isolation in the North American Physidae in 2011 [17].

But the 200-year logjam of physid systematics was broken worldwide in the summer of 2000.  And the results ultimately published by Dillon, Wethington, Rhett and Smith in 2002, supplemented by Lydeard and colleagues in 2016 [18], have subsequently inspired a gratifying profusion of follow-up research, including the population genetics of Bousset, Jarne and colleagues [19], the reproductive biology of Janicke, David and colleagues [20], the insights on life history evolution offered by the entire French gang [21], such biogeographical works as those of Albrecht & Vinarski [22] and the recent parasitological survey of Ebbs, Loker, and Brant [23].

I was around ten or twelve years old when freshwater gastropods of the genus Physa first came to my attention, crawling about in marginal pools of the South River behind my house.  I assumed that somebody must be able to identify them, no different from seashells or land snails, but I didn’t know who.  By the age of 20 I was sampling Physa from the Upper New River for my first peer-reviewed publication, and I thought I knew who.  I was a mid-career scientist before I realized that the who who could identify those weedy little things was going to have to be me.

Wisdom is more than knowing what you know, and indeed, more than knowing what you don’t know.  Wisdom is knowing what is knowable and knowing what is known and being able to do the subtraction.


[1] Previous posts in this series:
  • To Identify a Physa, 1971 [8Apr14]
  • To Identify a Physa, 1975 [6May14]
  • To Identify a Physa, 1978 [12June14]
  • To Identify a Physa, 1989 [3Oct18]
[2] Dillon, R. T., Jr. (2000) The Ecology of Freshwater Molluscs.  Cambridge University Press, England. 509 pp. [html]

[3] Brackenbury T & Appleton CC 1991. Effect of controlled temperatures on gametogenesis in the gastropods Physa acuta (Physidae) and Bulinus tropicus (Planorbidae). J. Moll. Std. 57: 461-470. Hofkin B, Hofinger D, Koech D, & Loker E 1992. Predation of Biomphalaria and non-target molluscs by the crayfish Procambarus clarkii: implications for the biological control of schistosomiasis. Ann. Trop. Med. Parasitol. 86: 663 – 670.

[4] Glöer, P., and C. Meier-Brook (1994) Süsswassermollusken.  Deutscher Jugendbund fur Naturbeobachtung.  11.erweiterte Auflage.  136 pp.

[5] Anderson, R. (1996) Physa gyrina (Say), a North American freshwater gastropod new to Ireland, with a key to British Isles Physidae. Irish Naturalists’ Journal 25: 248-253.

[6] Wethington, A. R., E. R. Eastman, and R. T. Dillon.  (2000)  No premating reproductive isolation among populations of a simultaneous hermaphrodite, the freshwater snail Physa.  Pp. 245 - 251 in Freshwater Mollusk Symposium Proceedings (Tankersley, Warmolts, Watters, Armitage, Johnson & Butler, eds.)  Ohio Biological Survey, Columbus.

[7] See last month’s post:
  • Albinism and sex allocation in Physa [5Nov18]
[8] Why this elaborate and labor-intensive design?  If you had asked me 20 years ago, I would have guessed that all six of our outcrosses would return evidence of at least some reproductive isolation, but that the amount between nominal species would be comparable to the amount within.  That’s the result generally obtained with fruit flies.  And in fact, we had already seen evidence of hybrid sterility between what we thought, at the time, were local populations of P. heterostropha.  Those local populations turned out to be bona fide species [9].  And the worldwide invasive, not so much.

[9] For more about our mid-1990s experiments with Physa carolinae, see:
  • TRUE CONFESSIONS: I described a new species [7Apr10]
  • The heritability of shell morphology in Physa h^2 = 0.819! [15Apr15]
[10] Long-time readers may now be able to appreciate, dimly, my reaction to Dr. J. B. Burch’s “Dixie Cup” remark of 2010.  See:
  • The Mystery of the SRALP: Dixie Cup Showdown [2Apr13]
The community of systematic biology drives a speciation ratchet – easily finding differences, never not finding them.  Fame, and perhaps even fortune, accrues to the wanton cataloger of dubious new species, dazzling in their number, rare in their incidence, direly imperiled, and distinguishable only by him, for a fee.  Obscurity at best, opprobrium at worst, is dealt to those of us who devote our careers to cleaning up what can only be a tiny fraction of the mess. 

[11] 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]

[12] Dillon, R. T., C. E. Earnhardt, and T. P. Smith. (2004) Reproductive isolation between Physa acuta and Physa gyrina in joint culture.  American Malacological Bulletin 19: 63 - 68.  [PDF]

[13] Dillon, R. T., and A. R. Wethington. (2006)   No-choice mating experiments among six nominal taxa of the subgenus Physella (Basommatophora: Physidae).  Heldia 6: 41 - 50.  [PDF]

[14] Dillon, R. T., J. D. Robinson, T. P. Smith, and A. R. Wethington (2005) No reproductive isolation between freshwater pulmonate snails Physa virgata and P. acuta.  The Southwestern Naturalist 50: 415 - 422.  [PDF]

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

[16] Wethington, A.R., & C. Lydeard (2007) A molecular phylogeny of Physidae (Gastropoda: Basommatophora) based on mitochondrial DNA sequences.  Journal of Molluscan Studies 73: 241 - 257 [PDF]. For more, see:
  • The Classification of the Physidae [12Oct07]
[17] Dillon, R. T., A. R. Wethington, and C. Lydeard (2011) The evolution of reproductive isolation in a simultaneous hermaphrodite, the freshwater snail Physa.  BMC Evolutionary Biology 11:144 [html] [PDF].  For more, see:
[18] Lydeard C, Campbell D, Golz M. (2016) Physa acuta Draparnaud, 1805 should be treated as a native of North America, not Europe. Malacologia 59:347–50.

[19] Bousset, L., P-Y. Henry, P. Sourrouille, & P. Jarne (2004) Population biology of the invasive freshwater snail Physa acuta approached through genetic markers, ecological characterization and demography. Molec. Ecol., 13: 2023-2036.  Bousset, L., J-P. Pointier, P. David, and P. Jarne (2014) Neither variation loss, nor change in selfing rate is associated with the worldwide invasion of Physa acuta from its native North America. Biological Invasions 16: 1769-1783.

[20] Janicke, T., P. David, and E. Chapuis (2015) Environment-dependent sexual selection: Bateman's parameters under varying levels of food availability.  American Naturalist 185: 756-768. Janicke, T., N. Vellnow, T. Lamy, E. Chapuis, and P. David (2014) Inbreeding depression of mating behavior and its reproductive consequesnces in a freshwater snail. Behavioral Ecology 25: 288 - 299.  Janicke, T., N. Vellnow, V. Sarda and P. David (2013) Sex-specific inbreeding depression depends on the strength of male-male competition.  Evolution 67: 2861-2875.

[21] Chapuis E., Lamy T., Pointier J.-P., Segard A., Jarne P., David P. (2017). Bioinvasion triggers rapid evolution of life-histories in freshwater snails. American Naturalist 190: 694 – 706.

[22] Albrecht C, Kroll O, Moreno Terrazas E, Wilke T. (2008) Invasion of ancient Lake Titicaca by the globally invasive Physa acuta (Gastropoda: Pulmonata: Hygrophila). Biol Invasions. 11:1821–6. Vinarski MV. (2017) The history of an invasion: phases of the explosive spread of the physid snail Physella acuta through Europe, Transcaucasia and Central Asia. Biol Invasions 19:1299–314.

[23] Ebbs, E. T., E. S. Loker and S. V. Brant (2018) Phylogeny and genetics of the globally invasive snail Physa acuta Draparnaud 1805, and its potential to serve as an intermediate host to larval digenetic trematodes.  BMC Evolutionary Biology 18: 103.

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