Dr. Rob Dillon, Coordinator

Monday, March 11, 2019

Pleurocera shenandoa n.sp.

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019c) Pleurocera shenandoa n.sp.  Pp 101 - 108 in The Freshwater Gastropods of North America Volume 3, Essays on the Prosobranchs.  FWGNA Press, Charleston.

One of the recurring subthemes we have developed in this blog, over its 20 year history, has been the roll of serendipity in science.  The study of Pleurocera clavaeformis described in our posts of February 2007 and March 2011 [1] and the study of P. semicarinata reported in our post of July 2014 [2] were both designed to confirm cryptic phenotypic plasticity in populations of pleurocerid snails using genetic variance at allozyme-encoding loci.  The sets of populations involved in both of those studies were geographically widespread, which meant that I needed to sample some additional sets of well-characterized pleurocerid populations as controls to calibrate the expected levels of genetic divergence among my sets of experimental populations.

An element of serendipity was introduced into both studies when the genetics of the “well-characterized” control populations turned out to be as surprising as the experimental populations.  The additional research this necessitated, for P. simplex (reported in Sept – Nov 2016) [3] and P. canaliculata (reported in June 2013) [4] yielded additional insights into the evolutionary biology of an enigmatic group of organisms that becomes more fascinating to me every time I step in the creek.

So, there were actually two sorts of controls for the studies referenced above, calibration standards and mobility standards.  In the very first study of allozyme polymorphism I ever published [5], way back in 1980, I set Pleurocera simplex population WYTH as a mobility standard, defining the mobility of the most common allozyme band in that population as 100 SDEMM (standard Dillon electrophoretic mobility millimeters.)  And in every study of allozyme polymorphism in pleurocerid populations I have published since that date, a total of 13 in all, I have included a population linked somehow to my 1980 population WYTH, calibrating every allele at every locus by its mobility relative to that standard.
Figure 4 of Dillon [2]
So in both my 2013 study of cryptic phenotypic plasticity in Pleurocera canaliculata [4], and in my related 2014 study of P. semicarinata [2], my mobility standard was population SV.  This was the same population of P. semicarinata I called “PINE” in my 1980 paper, which I ran beside population WYTH thirty-five years previous, hence calibrating the mobilities of all allozyme bands in the 2013 and 2014 papers in units of SDEMM.

Now look at Figure 4 of my 2014 study, reproduced above.  “Standard” Virginia semicarinata population SV is more genetically similar to P. canaliculata than to any member of the (nominally conspecific) semicarinata/livescens/obovata cluster.  In fact, my P. semicarinata “standard” seems to be the most genetically divergent population in the entire study!

What is this semicarinata population from Virginia I called SV in 2014, which I called PINE in 1980?  And what made me think that it was Pleurocera semicarinata in the first place?

Let’s flip the calendar back another five years to 1975, when the young Rob Dillon was a sophomore at Virginia Tech, blissfully bumping around on the backroads of the upper New River drainage with boots, nets and buckets thrown in the back of a state pickup truck.  You may recall, from my essay of May 2014 [6], that my undergraduate research thesis was entitled “Factors in the distributional ecology of upper New River mollusks (Va/NC).”

Goodrich’s papers [7,8] were available in the university library in 1975, but this was before the publication of Burch’s EPA key [9], so I had no pictures.  Nor (of course) any access to reference collections.  So, I confess that I simply guessed.  Goodrich listed just two species of Goniobasis from the area, Goniobasis proxima as a trans-Appalachian inhabitant of the highlands of North Carolina and G. simplex in the Bluestone River of West Virginia, apparently captured over from the Tennessee drainage.  That suggested to me that my softwater species in the New River must be G. proxima, and my hardwater species must be G. simplex.

Even a college sophomore knew crappy science when he saw it, and he was not happy about that, at all.  I do remember thinking, 45 years ago, that there seemed to be at least TWO hardwater species of Goniobasis in the upper New River drainage in addition to the single softwater species but had no idea how to start solving the problem.

A method to start solving the problem presented itself in grad school at the University of Pennsylvania.  My advisor at the ANSP, Dr. George Davis, had a big grant to work out the systematics of unionid mussels with the brand-new technique of allozyme electrophoresis, and was generous enough to allow me to bring my samples of pleurocerids into his lab.  And absorbed onto the little paper wicks applied onto the butt ends of the first gels I ever ran was the proteinaceous goo of ground-up Goniobasis from the upper New River drainage.

Our gels clearly showed one species in the softwater and two species in the hard [5].  And by now I did have a research collection at my fingertips.  The softwater species was (indeed) Goniobasis proxima, upon which I focused my dissertation, and most of the rest of my career, and one of the two hardwater species was (indeed) Goniobasis simplex.  That other hardwater species seemed to match Goniobasis semicarinata.
Figure 2 of Dillon & Davis [5]
Of course, this was just shell-matching.  That is all that pleurocerid taxonomy had ever been, as of 1980.  And Goodrich [7] gave the range of Goniobasis semicarinata as “Tributaries of Ohio River, Scioto River to Big Blue River, Indiana; Licking River to Salt River in Kentucky,” saying nothing about Virginia at all.  I assumed at the time, and continued to assume for many years, that the range of G. semicarinata must extend from Ohio through West Virginia up some 200 km of Kanawha River drainage into the upper New River basin, unbeknownst to Goodrich.

The 1980 paper I published with George Davis ultimately involved two populations of G. simplex, six populations of G. proxima, and the four populations we identified as “Goniobasis semicarinata,” shown on the bottom row of our Figure 2 as reproduced above.  Image (k) depicts population “PINE” from Little Pine Run, a tributary of the New River in Pulaski County, which (renamed “SV”) was ultimately to serve as a mobility standard for my 2013 & 2014 studies of cryptic phenotypic plasticity in canaliculata [4] and semicarinata [2].  In addition to two other “semicarinata” populations from the New River drainage, we also included population “ROA” from Mill Creek in Montgomery County (image J), a tributary of the North Fork Roanoke River.

Yes, the Roanoke River is an Atlantic drainage.  And in subsequent years, as the FWGNA survey extended throughout all the Atlantic drainages from Georgia to the New York line, I discovered populations of what I called Goniobasis or Pleurocera semicarinata north down the length of the Great Valley of Virginia, in tributaries of the Roanoke, James, and Shenandoah Rivers as far north as Waynesboro, my home town [10].

This went on for many years.  And at no time, from my initial judgement call in 1980 to maybe perhaps 2010, had I ever studied at any length a bona fide, Midwestern population of semicarinata on the hoof.

In 2010 the FWGNA project began to devote an increasing amount of our attention to the freshwater gastropod fauna of Ohio River drainages.  This led to the population genetic surveys that ultimately disentangled the evolutionary relationships between what had historically been called Goniobasis semicarinata, G. livescens, Pleurocera canaliculata, P. acuta, and Lithasia obovata.  Perhaps the most striking result of those studies was the tremendous ecological adaptability of what we now call Pleurocera semicarinata – populations now known to inhabit the entire range of waterbodies in the Midwest, from the smallest creeks to the grandest rivers, including lake shores of both sand and rock.  In addition to this, or perhaps because of this, populations of bona fide P. semicarinata demonstrate tremendous plasticity of shell morphology.

That plasticity certainly extends to cover the shell morphology demonstrated by what I had been calling semicarinata in the Great Valley of Virginia.  But 30 years of field experience had impressed upon me that Virginia populations seem to be entirely restricted to small creeks.  They do not extend into larger rivers or lentic bodies of water of any sort, nor do they demonstrate anywhere near the range of shell phenotypic plasticity.  There also seemed to be body color differences – the Midwestern populations typically demonstrating a brighter orange coloration and the Virginia populations darker.  I’m not entirely convinced of this, but it is worth mentioning.

And in 2014 we extended the FWGNA survey into the Ohio drainages of West Virginia.  There really are no Pleurocera populations of any species inhabiting any tributaries of the Kanawha river through most of the state.  The Virginia populations I had been calling semicarinata appear to be isolated by over 200 km of uninhabited waters from the nearest bona fide semicarinata population in Ohio.

So all that, together with the genetic results shown in 2014 Figure 4 reproduced above, combined to force me into describing a new species, in the appendix of the first hardcopy volume of the FWGNA project, coming soon [11].  This new species, Pleurocera shenandoa, is locally quite common and widely distributed throughout a long-settled part of the world, where one might reasonably expect the biota to be well-known and well-characterized.

It is not.  Perhaps the primary theme of my 20 years of blog posts, and the entire set of four FWGNA volumes to be published soon, is that even here in the 21st century, in the home of the best science the world has ever known, we remain stunningly ignorant of even the most commonplace.  We spend billions of dollars shooting space probes to the moons of Jupiter, and not a nickel to understand the little brown snails in the creeks behind our own houses.  Shame on us all.


[1] Dillon, R. T. Jr. (2011)  Robust shell phenotype is a local response to stream size in the genus Pleurocera (Rafinesque 1818). Malacologia 53: 265-277. [PDF] See:
  • Goodrichian taxon shift [20Feb07]
  • Goodbye Goniobasis, Farewell Elimia [23Mar11]
[2] Dillon, R. T., Jr.  (2014) Cryptic phenotypic plasticity in populations of the North American freshwater gastropod, Pleurocera semicarinata.  Zoological Studies 53:31. [PDF] See:
  • Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]
[3] Dillon, R. T., Jr (Dillon, R. T. (2016) Two reproductively isolated populations cryptic under Pleurocera simplex (Say, 1825) inhabiting Pistol Creek in Maryville, Tennessee.  Ellipsaria 18(2): 15-16. [PDF]
  • The cryptic Pleurocera of Maryville [13Sept16]
Dillon, R. T. & J. D. Robinson (2016) The identity of the "fat simplex" population inhabiting Pistol Creek in Maryville, Tennessee.  Ellipsaria 18(2): 16-18. [PDF]
  • The fat simplex of Maryville matches type [14Oct16]
Dillon, R. T. (2016)  Match of Pleurocera gabbiana (Lea, 1862) to populations cryptic under P. simplex (Say, 1825).  Ellipsaria 18(3): 10 - 12.  [PDF]
  • One Goodrich missed: The skinny simplex of Maryville is Pleurocera gabbiana [14Nov16]
[4] Dillon, R. T., S. J. Jacquemin & M. Pyron (2013) Cryptic phenotypic plasticity in populations of the freshwater prosobranch snail, Pleurocera canaliculata.  Hydrobiologia 709: 117-127.  [PDF]  See:
  • Pleurocera acuta is Pleurocera canaliculata [3June13]
  • Pleurocera canaliculata and the process of scientific discovery [18June13]
[5] Dillon, R.T. and G.M. Davis (1980) The Goniobasis of southern Virginia and northwestern North Carolina: Genetic and shell morphometric relationships. Malacologia 20: 83-98. [PDF]

[6] Sweet, gauzy memories of my college days:
  • To identify a Physa, 1975 [6May14]
[7] Goodrich, C. (1940) The Pleuroceridae of the Ohio River system.  Occas. Pprs. Mus. Zool. Univ. Mich. 417: 1-21.

[8] Goodrich, C. (1942) The Pleuroceridae of the Atlantic Coastal Plain.  Occas. Pprs. Mus. Zool. Univ. Mich. 456: 1-6.

[9] This is a difficult work to cite.  J. B. Burch's North American Freshwater Snails was published in three different ways.  It was initially commissioned as an identification manual by the US EPA and published by the agency in 1982.  It was also serially published in the journal Walkerana (1980, 1982, 1988) and finally as stand-alone volume in 1989 (Malacological Publications, Hamburg, MI).

[10] Sweet, gauzy memories of Waynesboro, VA.  Well, not so much:
  • The Clean Water Act at 40 [7Jan13]
[11] Dillon, R. T., Jr. (2019) Description of a new species of freshwater snail (Caenogastropoda: Pleuroceridae) from the Great Valley of Virginia.  Appendix 1 in Dillon, Ashton, Reeves, Smith, Stewart & Watson, The Freshwater Gastropods of North America, Volume I.  Atlantic Drainages, Georgia through Pennsylvania.  FWGNA Publishing, Charleston, SC.

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