The Western Pyrgulopsis: 151 planets in the sky?

I love retirement for many reasons, not the least of which is that it is a good excuse.  I very rarely submit grant proposals or papers to scientific journals anymore, and so very rarely volunteer to review anybody else’s.  I do not attend meetings because – you know – fixed income!  And as for serving on committees, boards, and panels, I have paid my dues.

Pyrgulopsis neritella (A,B) and landyei (C-F). From [1]

But on 28Sept26 I received a courteous email from a Dr. Jacob Burkhart from the US Fish & Wildlife Service, Midwest Regional Office, out in Minnesota somewhere, I think.  He explained that he was working on a team that is completing a “Species Status Assessment for several Pyrgulopsis springsnail species found in the Steptoe Valley of Nevada,” and asked me if I “would be willing to provide a peer review” of their “proposed treatment of springsnail taxonomy.”  And my rusty old armor cracked open just a bit.  His situation was just so pathetic.

So here is my reply, verbatim:

“Well, OK.  One of my favorite quotes in the history of the world comes from Dr. Seuss’ Cat in the Hat: ‘This mess is so big, and so deep and so tall, We cannot pick it up. There is no way at all.’ But I suppose it is my duty to help you try.”

So on 2Oct26 my new friend Jacob sent me a 6-page document entitled “NV Springsnails Taxonomy Summary For Peer Review."  In this document he and two FWS colleagues briefly reviewed their motivation, which turned out to be rooted in that “Megapetition” from the Center for Biological Diversity we reviewed way back in [14July09], see also [18May11].  Jacob and colleagues then soldiered bravely through thirty years of scattered, disorganized, ambiguous, confusing and contradictory gobbledygook, ultimately resolving to “evaluate the following entities for the purposes of this SSA report: P. landyei, P. neritella, P. sulcata, P. sterilis, and P. CS28.”  Bless their hearts.

 

And along with his little document for review, Jacob attached a huge bolus of supporting materials, including an important paper by Mike Young and five colleagues [2] that I will confess to you right now, I had been cravenly avoiding.

 

In early August I had received a comment from Mike at the bottom of my [7Sept22] post to this blog, “Just 125 of Pyrgulopsis in the American West.”  See note [3] about my species count.  I had prophesied that “One day, I feel sure, somebody will come behind (Bob Hershler) and test the 125 (Pyrgulopsis species) that remain.”  And I had concluded “I cannot imagine when, or by whom.  Not it.”

 

In August, Mike Young had answered, “We did it. And it appeared yesterday in Western North American Naturalist.”  To that comment I responded, “Great news! Could you send me a pdf?”  Alas, his answer turned out to be no.  And my effort to download a copy for myself was defeated by a paywall.

 

One of the very few things I do not love about retirement is that I have lost my access to most of the scientific literature being published today.  Sure, I could impose on colleagues in higher education to download a pdf for me, and occasionally I do.  Or, I could kick the Pyrgulopsis can down the road, and pray that the Son of Man might Return in all His Glory before I ever saw it again.

P. neritella (1.3 mm), P. landyei (1.4 mm). From [1]

Mike Young had been most cooperative when I reviewed his 2021 paper on Physa [5] back in 2024, and I knew him to be a hard worker, with excellent technical skills.  I had been impressed both by the quality and the quantity of the data he had gathered for that research effort, especially by the rigor with which he documented every nucleotide in every sequence, and every point to the right of the decimal in every sample site.  He was breathtakingly naïve regarding the biology of his study organisms, and pristinely ignorant of the published literature.  Consequently, he was utterly unable to state an hypothesis, much less test one.  But I was, and in his gigantic dataset I discovered a light sprinkling of rare cases bearing directly on the point of important hypotheses.

 

It was a lot of work to mine the diamonds from Mike Young’s mountain of slag, however, and I just was not sure I heard the call to do it again.  The Pyrgulopsis mess is today an order of magnitude worse than the Physa mess was at the dawning of my career, and going in the wrong direction.  Burch’s 1980 Bible [6] listed 16 species of Pyrgulopsis/Fontelicella in North America, and 65 species and subspecies of Physa.  Today we [7] count seven good, biological species of Physa [8], and 151 species of Pyrgulopsis.

 

Don’t get me wrong.  Science is the construction of testable hypotheses about the natural world.  To the extent that our current hypothesis about the western Pyrgulopsis is testable, it is science.  It is the intellectual equivalent of looking into the heavens and describing 151 planets, wandering randomly across the night sky [9].

 

Now here was Mike Young’s Pyrgulopsis paper open before me, with all 13 of his supplements in a zipped folder beside, and no sound of the trump, and I was clean out of excuses.  So, I dove in.  And I have never, in a professional career spanning over 50 years, laid eyes upon a more splendid dataset, a more cringeworthy analysis, or a more amazing result in the long and glorious history of freshwater gastropod population genetics [10].

 

First, the splendid dataset.  Mike and his colleagues sampled 4,270 individual Pyrgulopsis from 644 locations.  All were sequenced for the mitochondrial COI gene.  And “from each COI clade exhibiting a marked level of divergence and represented by more than one individual” Mike Smith sequenced one or more of three additional genes, mitochondrial ND1, nuclear ITS2 and nuclear 28S. 

 

To these 4,270 lines of data Mike and colleagues added 1,362 Pyrgulopsis sequences previously published, mostly of COI but a smattering of others.  And his entire 5,632-line [11], 22-column data set he made available as an excel spreadsheet in his Supplementary Material 2.  Splendid.

The 644 sample sites of Young et al. [2]

Regarding Mike Young’s analysis, it was, in two words, baroquely arbitrary.  Young characterized his species concept as “phylogenetic… requiring reciprocal monophyly” while specifying “an additional requirement for species-level rather than population-level divergence,” which is so obviously circular one wonders how such reasoning could be entertained in Western thought.  The application of this concept turned out to require such mental gymnastics such as: 

“Requiring reciprocal monophyly for the P. pilsbryana species complex would have resulted in recognizing all specimens as members of the type species, despite many populations exhibiting levels of divergence that were characteristic of valid species among other members of this genus.”

And how, one might wonder, did Mike Young determine that ‘characteristic’ level of species divergence, which seems to have been violated in what he decided a priori must be a P. pilsbryana “complex?”

The best way to summarize Mike Young’s baroquely-arbitrary analysis is to look back from a sample of his results.  The figure below is a tiny slice off the top of Young’s Figure 2, which is intended to relay the results for his Pyrgulopsis pilsbryana “species complex.”  At left is the maximum-likelihood COI tree showing six clusters.  The seven rows of boxes at right relay the results of his “identification of candidate species.”  This little subset of seven nominal species includes four previously-described (bedfordensis, blainica, ignota, and fresti), the daggers by ignota and fresti indicating that they are “not considered part of the pilsbryana complex,” apparently interlopers from some other “complex” unspecified.  There are also two candidate species as yet undescribed (CS21 and CS22), and one “informal group” sampled from Brush Creek “not assigned to a candidate species.”

I do not know how the six COI clusters at left map onto the seven candidate species at right.  Has the “informal group” from Brush Creek been subtracted from the gene tree?  We’ll let that go and soldier on.

Young et al. [2] Figure 2, detail.

Each of the seven rows has 11 columns, each column corresponding to one “species delimitation method.”  For a key to the 11 such methods, see Young’s complete Figure 2 caption at footnote [12].  Orange boxes indicate that this particular COI clade was delimited as a species, gray means no, white means no data, and an asterisk indicates “more than one taxon within the clade.”

 

And the main result I would like to highlight is this.  Six of the seven “candidate species” shown in this tiny slice off the top of Young’s results were in fact subsequently listed in his Table 1 as “valid,” all but that "informal group" from Brush Creek.  This despite the fact that no data were available for anything other than COI sequence for bedfordensis, blainica, fresti, or the as-yet-unnamed CS21 whatsoever.

 

Mike Young’s baroquely-arbitrary classification system ultimately devolved to ad hoc binning on the basis of one, single character.  On COI sequence alone [13], he and his colleagues ultimately listed 21 Pyrgulopsis species as “candidates for synonymy” and identified 17 “previously unrecognized clades” that “were delimited as candidate species,” for a net decrement of 4 species.  Our 151-planet hypothesis is now a 147-planet hypothesis.

 

I belong to an elite generation of evolutionary biologists who are old enough to have been influenced by Sneath & Sokal's numerical taxonomy [15], but not so old as to have forgotten it.  And vividly I remember how systematic biologists of that age used to rage against “single character classifications.”  Even given a genetic basis for the character being analyzed, it was universally understood fifty years ago that populations are the unit of evolution, and universally appreciated that populations demonstrate genetic variation both within and among, and with but a single character any interspecific variance that might be attributable to reproductive isolation cannot be recognized.

 

Note that I said above, “To the extent that our current hypothesis about the western Pyrgulopsis is testable, it is science.”  The bottom line is that Mike Smith’s cringeworthy analysis does not constitute a test.

But stepping back from the species hypothesis entirely, and simply understanding a COI sequence as the single genetic marker that it is.   We have now arrived at Mike Young’s amazing result, and the grand finale of the present essay.  Quoting him verbatim: 

“And given the fairly frequent co-occurrence of taxa—at 100 of the 1101 sites in this study, which is a minimum estimate—more thorough collecting and analysis of specimens from single sites is also warranted.”

Holy crap, could that be true?  The model of the Western Pyrgulopsis I have always borne in my mind has been one of complete allopatry.  One single nominal species per spring, never any more, right?  But returning to Mike’s big Supplement #2 spreadsheet, sorting by Location and running an eager finger down the first 1,000 rows or so, I counted six sample sites at which more than a single COI haplotype was discovered.  Wow.  That is quite simply, and quite profoundly, amazing.

 

Mike labeled each COI haplotype as a different “delimited species.”  And in so doing, he unwittingly committed an act of science.  He constructed an hypothesis about the natural world that can actually be tested.  Next month, we test it.

 

Notes:

 

[1] Hershler, R. (1998) A systematic review of the hydrobiid snails (Gastropoda: Rissooidea) of the Great Basin, western United States. Part I. Genus Pyrgulopsis. The Veliger 41: 1-132.

 

[2] Young, Michael K.; Pilgrim, Kristine L.; Miskow, Eric S.; Lundskog, Chante L.; Perez, Kathryn E.; and Schwartz, Michael K. (2026) "Molecular refinement of the taxonomy of Pyrgulopsis springsnails in western North America," Western North American Naturalist: Vol. 86: No. 2, Article 10.  Available at: https://scholarsarchive.byu.edu/wnan/vol86/iss2/10

 

[3] In my Bob Hershler essay of [6July22] I counted N = 126 species of Pyrgulopsis in the American West by paging through his (2017) “Annotated Checklist” [4].  Apparently molluscabase.org lists N = 151.  I have no insight to offer regarding the origin of this discrepancy.

 

[4] Hershler & Liu (2017) Annotated Checklist of Freshwater Truncatelloidean Gastropods of the Western United States, with an Illustrated Key to the Genera.  US Bureau of Land Management Technical Note 449: 1 – 142.

 

[5] Young, M.K., R. Smith K.L. Pilgrim, and M.K. Schwartz (2021) Molecular species delimitation refines the taxonomy of native and nonnative physinine snails in North America.  Scientific Reports 11: 21739. https://doi.org/10.1038/s41598-021-01197-3.  For my review, see:

• The SRALP and the SRNLP: A new hope [14May24]
• The SRALP and the SRNLP: Physa acuta were found [11June24]
• The twelve phascinating Physa of Bliss [2July24]
• Cytoplasmic male sterility in the Snake River Physa [7Aug24]

[6] 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 a stand-alone volume in 1989 (Malacological Publications, Hamburg, MI).

 

[7] The clarity brought to the Physa of North America is in large part due to the many years of laboratory and field effort logged by myself and my students, the likes of which I could never hope to match with Pyrgulopsis in what remains of my sojourn in this World of Woe.  It is true that a smattering of samples have passed across my lab bench in recent years.  But I must confess, I have never seen a Pyrgulopsis on the hoof.  So, let’s give that jumbo 28-oz can of Bush’s Baked Pyrgulopsis another swift kick, okay? No?

 

[8] The confirmed species of Physa in North America are: acuta, carolinae, gyrina, jennessi, pomilia, vernalis, and zionis.  One might also count Aplexa elongata as number 8. The jury is still out on ancillaria/parkeri. 

 

[9] Without the entry of a single new datum into evidence, Galileo and every other astronomer peering into the night sky of the seventeenth century immediately discarded Ptolomy’s geocentric model of the heavens and embraced Copernicus’ heliocentric model.  Two reasons.  First, the heliocentric theory is powerful, yielding strong predictions about the behavior of planets as yet undiscovered and capable of rejecting planets that cannot exist.  And second, the heliocentric theory is elegant.  I do not understand what has happened to our profession in these latter generations, but I do know this.  Most of the faculty in the university science departments and research institutes of today would not know a theory – elegant, ugly or otherwise – if it bit them on the bottom.

 

[10] Mike Young’s discovery of significant sympatry in the western Pyrgulopis fauna scored 94.1 international amazingness units, edging out the 93.2 iau scored by Bianchi and colleagues back in 1994.  For a complete ranking of the top five, see:

• The third-most amazing research results ever published for the genetics of a freshwater gastropod population [1].  And the fourth-most amazing, too. [3Mar22]

[11] The excel spreadsheet downloadable as Supplementary Material 2 does indeed contain 5,632 lines, but only 5,631 are data, line #1 being the column labels.  Apparently the gene sequence(s) from one individual Pyrgulopsis have been lost, for an error rate of 0.018%

 

[12] The complete caption of Figure 2, from [2]:  "Identification of candidate species for most members of the Pyrgulopsis pilsbryana species complex, structured following highly supported clades (branches with red dots and all collapsed nodes have bootstrap support >85) from the maximum-likelihood tree for the COI gene fragment. Taxa in bold are hypothesized candidate species; those marked with a dagger are not considered part of this complex (see text). Each column depicts the results of 1 species delimitation method (A, ASAP; P, Poisson tree processes; S, statistical parsimony networks) for mitochondrial data (COI, ND1, or the concatenated sequences [CON]) or diagnosability (D) and substantial molecular support (B, bootstrap support ≥85) for nuclear data (ITS2 or 28S). Cell fill denotes whether a COI clade was delimited as a species (orange, yes; gray, no; white, no data). A cell with an asterisk denotes that more than 1 taxon within the clade was delimited. Vertically oriented labels indicate informal group membership for specimens not assigned to a candidate species."

 

[13] Well, to be completely accurate.  One of the species Mike proposed for synonymy was P. vinyardi, under P. gibba.  That confirms the result Hsiu-Ping Liu and I published in the gray literature back in 2021 [14], finding no evidence of disequilibrium between COI and penial morphology.  So in the single particular case of P. vinyardi, the evidence of synonymy is more than just mtDNA sequence.

 

[14] Liu, H-P, and R. T. Dillon, Jr. (2021) Resolving the species status of Surprise Valley Pyrg (Pyrgulopsis gibba) and Vineyard Pyrg (Pyrgulopsis vinyardi).  Report to Stantec Environmental Consulting. FWGNA Circular 6: 1 – 5. [pdf]  For a review, see:

• Just 125 species of Pyrgulopsis in the American West [7Sept22]

[15] Sneath, P.H.A, and R.R. Sokal (1973) Numerical Taxonomy: The Principles and Practice of Numerical Classification.  W. H. Freeman. 573 pp.  

The peculiar pleurocerids of the Interior Highlands III: Pleurocera canaliculata lawrencii

Subspecies are populations of the same species in different geographic locations, with one or more distinguishing traits [1].  They are races given a Latin name.  And those “distinguishing traits” are subjective – they reside in the eye of the beholder.  They might, or might not, have a genetic basis.

My readership will be familiar, I trust, with the subspecies of Pleurocera canaliculata as recognized by the FWGNA project.  In 2013 we published the discovery that the populations of pleurocerid snails bearing gracile, acutely-conical shells inhabiting waters of the United States from New York to Kansas previously identified as “Pleurocera acuta” are conspecific with populations bearing much heavier, more robust shells identified as Pleurocera canaliculata, with not one but two announcements on this blog in June of that year [2].

The subspecies of Pleurocera canaliculata

My colleagues and I coined the term “cryptic phenotypic plasticity” to describe this phenomenon, because the evidence did not suggest to us that the basis for the acuta/canaliculata distinction was heritable.  But we saved Rafinesque’s (1824) nomen “acuta” as a subspecies under Thomas Say’s (1821) canaliculata because the gracile, acutely-conical shell form and the heavy, more-robust shell form are, both in our judgment and in the judgment of many generations of malacologists who came before us, “distinguishing traits.”

The geographic range of Pleurocera canaliculata, considering all subspecies together, is huge.  Quoting chapter and verse out of the Burch Bible [3], the range of just what was historically identified as Pleurocera acuta is “Ohio River streams and tributaries; Great Lakes and tributaries; Mississippi River and westward to Nebraska and Kansas; through the Erie Canal into the basin of the Hudson River; Cumberland and Duck rivers, Tennessee.”  And that’s not even including the other subspecies – the heavily shelled typical form and the extremely slender and gracile pyrenellum of middle Tennessee and North Alabama.

Focusing in 1939 on just “the Mississippi drainage exclusive of The Ohio,” which really isn’t much of a focus at all, Goodrich [4] gave the range of P. acuta as “lakes and streams of Minnesota to streams of Louisiana on the western side of the Mississippi, and in the St. Croix River on the east side to the Illinois River.”  In that vast region he synonymized (without comment) four previously-described species under acuta (Raf 1831): elevata Say 1821(!), alexandrensis Lea 1845, haleiana Lea 1845, and “lawrencei” Lea 1869.  Goodrich also recognized two subspecies in the region: lewisii Lea 1862 and his own hinkleyi Goodrich 1921.

OK, jumping forward from the ancient history to the modern.  As regular readers of this blog might recall, in October of 2021 I spent a productive week working in Shi-Kui Wu’s beautifully-curated collections at the University of Colorado in Boulder, preparing for our recently-debuted Freshwater Gastropods of Missouri web resource.  And it was there that I first laid eyes on the shells borne by populations that Goodrich and all subsequent authors have identified as Pleurocera acuta from anywhere west of the Mississippi River.

Shells of adults from the Meramec River.

And they simply looked different.  Their shell morphology is unusually diverse, in spire height, in relative body whorl height, and especially in whorl angulation.  Typical Pleurocera canaliculata shells of all three subspecies previously known to me demonstrate a strong angulation – sometimes described as a keel – low on the whorl, at the posterior end of the aperture.  Indeed, that strong angulation manifests itself in an aperture outline classically described as “auger-shaped” or “trapezoidal.”  A trapezoidal aperture was the key character used by every authority from Isaac Lea [5] to Jack Burch to distinguish the entire genus Pleurocera from the genus Goniobasis or “Elimia,” a distinction we now understand to have been entirely artificial [6].  More evidence of which anon.

Some of the shells borne by a minority of individuals Goodrich and subsequent authors identified as “Pleurocera acuta” inhabiting streams in the Missouri Ozarks do indeed bear strong angulations low on their body whorls, especially when young.  See (A) in the sample from the Meramec River (Jefferson Co, MO 0925-02) above and below.  But most do not, especially in adulthood.  Rather, the shells born by most of the nominal “Pleurocera acuta” of the Ozarks demonstrate entirely rounded whorls, with little or no angulation or keel in evidence.

Meramec River YOY juveniles

Another unique feature of the shell morphology demonstrated by Ozark populations traditionally identified as “Pleurocera acuta” is the occasional presence of spiral cords.  The seven shells figured below were specially selected from a sample of about 40 from the Gasconade River (Maries Co, MO 0925-05) to demonstrate such sculpture.  In some shells a central cord develops so strongly that it might be described as a mid-whorl keel, unique in populations of P. canaliculata of any subspecies, in my long experience.  No malacologist of the classical school would ever identify shells such as those figured below as belonging to a gastropod of the genus Pleurocera.  From 1862 to 1980, classically-minded malacologists would have called those “Goniobasis.”  After 1980, most would have identified them as “Elimia.”

Pleurocera shells from the Gasconade R.

So in 1869 our old buddy Isaac Lea published a brief Latinate description of “Goniobasis lawrencii” from “Washita River, near Hot Springs, Arkansas, Dr. Lawrence” [7].  This he followed with a more complete English description and figure of Goniobasis “lawrencei” (note respelling) in 1874 [8].

One might feel a twinge of sympathy [9] for the good Dr. Lea, in his valiant effort to describe a shell so variable in its featurelessness: smooth, subcylindrical, rather thick, spire raised, whorls flattened.  In his remarks he focused on what “not one of the ten specimens” sent to him had, which was an apex.  But he speculated that “The upper whorls may in perfect specimens be found to be folded or carinate.”

George Tryon [10] omitted Goniobasis lawrencii entirely from his 1873 monograph of the North American “Strepomatidae,” probably because Lea’s full description and figure had not as of that date found publication. Calvin Goodrich caught it in 1927, however, respelled it “lawrenci,” and dispatched it with a single line, “The types are Pleurocera acuta Raf” [11].  He mentioned the taxon again in his 1937 catalog of Pleurocera acuta synonyms exclusive of the Ohio, now spelling it “lawrencei,” as we noted five paragraphs above [4].  But the nomen disappeared from the published literature thereafter.

From Lea [8]

And alas, continuing with variations on the theme of disappearance.  The “Washita River near Hot Springs,” spelled Ouachita River by modern cartographers, was completely impounded by a series of dams in the 20th century, beginning with the modest Remmel Dam in 1924, continuing with the impressive Carpenter Dam in 1932 and concluding with the mammoth Blakely Mountain Dam, closed in 1952. The only pleurocerid populations surviving in the Ouachita River today are the common Ozarkian P. potosiensis, hanging on in the shallow riffles above the Blakely Mountain Dam Impoundment.

 

Pleurocerid populations matching Lea’s figure nevertheless survive today scattered in tributaries of the Saline and Arkansas Rivers immediately north of the Ouachita, and become quite common and widespread in the White River and its tributaries draining the Ozark Highlands of Arkansas, and through Missouri tributaries such as the Gasconade and Meramec as figured above.

 

These populations bear shells sufficiently distinct from the other subspecies of P. canaliculata (acuta, pyrenellum, and the typical form s.s.) to warrant recognition at the subspecific level, Pleurocera canaliculata lawrencii (Lea 1869).

 

The FWGNA Project, coming into the present essay, recognized seven pairs, seven triplets, and one quartet of subspecies among the 127 valid biological species of freshwater gastropods in our 23-state study area.  Here we promote one of those seven triplets to become our second subspecific quartet. Most of the distinguishing traits among most of these 15 total sets, I feel most certain, are mostly ecophenotypic in their origin.  They are examples of “cryptic” phenotypic plasticity, because in identifying the populations bearing those traits as different species or even genera, the true origin of the variance was hidden by previous generations of Malacologists.

 

On the other hand.  One of the most influential papers I read during the course of my mostly-forgotten graduate education was published by Francisco Ayala and colleagues in 1974, “Genetic differentiation during the speciation process in Drosophila” [12].  Studying pairs of subspecies, semispecies, sibling species, non-sibling species and conspecific populations, Ayala was able to demonstrate a clear correlation between taxonomic divergence and genetic divergence, as estimated using the new technique of allozyme electrophoresis.  And in 1980, my buddy Steve Chambers was able to replicate Ayala’s findings in the enigmatic pleurocerid fauna of Florida and South Georgia [13].

 

It seems likely to me that the shell characters I have used in the present essay to resurrect Isaac Lea’s nomen “lawrencii” as a subspecies of P. canaliculata may indeed have a heritably genetic basis.  The regionalization of lawrencii populations to the Ozark Highlands, uplifted in the late Paleozoic and isolated from the center of pleurocerid diversity in the southern Appalachians by the late Cretaceous Mississippi embayment, suggests both the motive and the opportunity for genetic divergence.

 

And perhaps speciation?  Populations of Pleurocera canaliculata bearing both the typical and the acuta shell morphology are widespread in the Mississippi River and tributaries (such as the Black River) draining flatlands immediately to the east of the range of lawrencii.  Might those ranges touch?  Might the disparate forms of P. canaliculata hybridize, might they blend, might some reproductive isolation have evolved?  We’ll keep an eye peeled.

 

Notes:

 

[1] For further elaboration on the concept of the subspecies, see:

• What is a subspecies? [4Feb14]
• What subspecies are not [5Mar14]

[2] 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].  For a review, see:

• Pleurocera acuta is Pleurocera canaliculata [3June13]
• Pleurocera canaliculata and the process of scientific discovery [18June13]

[3] 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).

 

[4] Goodrich, C. (1939) Pleuroceridae of the Mississippi River basin exclusive of the Ohio River system.  Occasional Papers of the Museum of Zoology, University of Michigan 406: 1 – 4.

 

[5] Aperture shape was the primary character Isaac Lea used in 1862 to distinguish Trypanostoma (“auger-shaped”) from Goniobasis (“subrhomboidal”).  Trypanostoma was a junior synonym of Pleurocera.  For more see:

• A House Divided [10May20]

[6] Dillon, R. T. (2011) Robust shell phenotype is a local response to stream size in the genus Pleurocera (Rafinesque 1818). Malacologia 53: 265-277 [pdf].  For more, see:

• Goodbye Goniobasis, Farewell Elimia [23Mar11]

[7] Lea, I. (1869) Descriptions of six new species of fresh water shells.  Proceedings of the Academy of Natural Sciences of Philadelphia 21: 124 – 125.

 

[8] Lea, I. (1874) Supplement to Isaac Lea’s paper on Unionidae. Journal of the Academy of Natural Sciences of Philadelphia (Second Series) 8: 55 – 69.

 

[9] The one who is authoring the present essay, however, does not:

• Isaac Lea drives me nuts [5Nov19]

[10] Tryon, G. W. (1873) Land and Freshwater shells of North America Part IV, Strepomatidae.  Smithsonian Miscellaneous Collections 253: 1 - 435.

 

[11] Goodrich, C. (1927) Some misplaced pleurocerids. Nautilus 41: 57 – 62.

 

[12] Ayala, F. J., M. L. Tracey, D. Hedgecock & R. C. Richmond (1974) Genetic differentiation during the speciation process in Drosophila.  Evolution 28: 576-592.

 

[13] Chambers, S. M. (1980) Genetic divergence between populations of Goniobasis (Pleuroceridae) occupying different drainage systems.  Malacologia 20: 63 – 81.  For more about my buddy Steve and a review of his groundbreaking research on the North American Pleuroceridae, see:

• Fred Thompson, Steve Chambers, and the pleurocerids of Florida [15Feb17]

The peculiar pleurocerids of the Interior Highlands II: Pleurocera simplex ozarkensis

There is no substitute for field experience.  You can track the modern scientific literature of malacology with relentless vigilance, pore over the historic monographs, paw through the national collections, dissect those snails, measure ‘em, drag their proteins through a gel or sequence every nucleotide in their genome.  All those data are just pieces in a puzzle scattered across your table top.  They will not fit together until you wade knee-deep into the river and look down at a population of the living organisms in their natural environment.  And again, in the creek upstream.  And again, in the lake downstream.  And repeat, next drainage over.  You cannot know them, until you walk with them.

So, on August 31, 2025, I loaded my pickup and set off on a two-week field trip to the Interior Highlands.  My itinerary was a zigzag path south from Jefferson City through the Missouri Ozarks, west across the top of Arkansas and into eastern Oklahoma.  I then planned to cut a giant S-shaped curve back across central Arkansas to cover the Arkansas and Ouachita drainages in one swell foop.  The weather was lovely, the water was low, I met a lot of nice people along the way [1], and had a wonderful adventure.

Christopher Rogers at the Little Lee River, OK.

And the rivers into which I waded those two weeks were strange unto my eyes.  Quite in contrast with the lotic environments of the southern, eastern, and midwestern United States of my long experience, typical streambeds in the Interior Highlands are composed of white, baseball-sized cobble.  And at low water, such as I found in early September of last year, I often found myself stomping dryshod across 40 yards of cobble to reach a two-inch trickle.

But when the rains come, Heaven help the hindmost!  Way back in the woods through which I had stomped to reach all those dry riverbeds last September was everywhere evidence of the floods of May 2025, tangles of storm-washed brush still wrapped around tree limbs 10 feet above my head.  The lotic environments of the Interior Highlands are, in one word, flashy.  In two words, very flashy.  They are a hybrid of an eastern river, and a western dry wash.

And my goodness, Pleurocera potosiensis populations where omnipresent in every river, stream, and mean little trickle I visited my first few days out, sampling south through the Ozarks.  I was heading toward a cluster of four sites that Russ Minton and his colleagues [2] had sampled for their 2017 study of 16S mtDNA sequence divergence in P. potosiensis, which they designated M6, M7, M8, and M10.

Last month we reviewed Minton’s 2017 study in considerable detail, reproducing his Figure 1 map of study sites across the Ozark Highlands of three states.  At left is an enlargement of the little red dashed-line box I drew at the top of last month’s three-state map.  Minton sequenced a total of nine snails from the four populations inhabiting the little rectangle of the Missouri Ozarks have I extracted here, demonstrating two very different haplotypes, five of the typical haplotype (“X”) and four of a second haplotype (about 8% different) that I called Z.  And the middle 40% of Minton’s big gene tree, highlighting those nine haplotypes, is extracted and reproduced below.

So, coming down from the north, I arrived first at Minton’s M10, Clifty Spring.  And the environment didn’t strike me as any different from typical for an Ozark Highland stream – a wide bed of dry white cobble with a trickle of water running through.  I found the spring itself more like a side-pool, a colder pool of clear water at the edge of the stream bed.  And its snail population no different from that of any other Pleurocera potosiensis I had seen in previous days.  Both of the snails Minton had sequenced from that spring in 2017 demonstrated the X haplotype.

It was at Minton’s site M6, as I came near Damascus, that suddenly there shined ‘round about me a light from heaven, and I fell to the earth, and heard a voice saying unto me, “You ain’t in the Ozarks anymore.”  The little stream running under Missouri Road C at site M6 is called Tick Creek on USGS maps, although in Minton’s paper it is referred to as “Trek Creek.”  It was ice cold, crystal clear, bank-full and stable.  And it was rich, and it was productive!

Detail from Minton et al. [2] Fig. 4

The date was September 4, 2025.  And for four days I had been stomping through dreadfully unproductive streams - 90% plain bare dry rock, and 10% plain bare wet rock.  I found nothing green – no moss, no macrophyte, no algae, not even any visible periphyton.  Indeed, I found very little silt, and almost no mud.  Approximately zero visible organic matter of any sort. After four days of this, I had become numb to it.

And suddenly, in tiny little Tick Creek under tiny little Missouri Road C, I had stumbled upon such a lovely, productive, stable little stream that it transported me spiritually out of the dusty Ozarks and back home to the green, rolling hills of Old Virginny.

And my eyes were opened.  And behold, the pleurocerid snails grazing therein were not Pleurocera potosiensis of the Interior Highlands.  They were Pleurocera simplex of the Southern Appalachians.  The shells were not strikingly different.  They seemed to have somewhat larger body whorls relative to expectation for potosiensis, and were entirely missing that strong peripheral keel one often sees in potosiensis, especially on the juvenile shell.

The most striking feature of the Tick Creek population was the black body color, which one never sees in potosiensis.  Black body is a signal feature of P. simplex populations in smaller eastern streams of my long experience.  Pleurocera simplex populations tend to lose that distinctive body coloration in larger streams, and indeed, the subspecies P. simplex ebenum of Cumberland drainages is typically brown/orange of body.  But if you see black in East Tennessee, it’s simplex.

So, referring back to Minton’s map and gene tree. The genetic contrast between the population of snails inhabiting typical Ozark stream M10 and those inhabiting rich, productive stream M6 was as stark as the environmental difference.  All three of the snails sequenced from Tick Creek M6 [3] demonstrated haplotype Z.

The peculiar pleurocerids of Tick Creek (M6).

Driving further south I next arrived at Minton’s site M7, the Missouri Road T bridge over Little Piney Creek.  And there I found another very typical-looking population of P. potosiensis, inhabiting another very typical-looking Ozarkian stream of bare white cobble, like Site M10.  And just as at Site M10, both of the snails Minton had sequenced from site M7 demonstrated haplotype X.

The confirmation I needed for my hypothesis I found at Minton’s site M8, Mill Creek at the State Recreation Area.  There I discovered a side-spring, marked “flowing well” on USGS topo maps, issuing a gorgeous, productive, ice-cold crystal-clear spring run perhaps 100 m long into Mill Creek.  Standing knee deep in Mill Creek, looking upstream with that "flowing well" spring run to my right, I found a mixture of typical-looking brown-bodied P. potosiensis and black-bodied snails indistinguishable from eastern P. simplex.  And at that site, Minton had sampled two snails, one of which proved to bear haplotype X the other of which proved to bear haplotype Z.

The snails at site M8 clearly do not constitute a single randomly-breeding population, but rather an admixture of two populations demonstrating at least some reproductive isolation.  Those are distinct biological species [4].  One must be Pleurocera potosiensis, and the other must be something else.  My hypothesis is that the brown-bodied pleurocerids in Mill Creek bear haplotype X and are correctly identified as P. potosiensis, and that the black-bodied pleurocerids in the side spring bear haplotype Z and are correctly identified as P. simplex.

Dr. R. Ellsworth Call (1856 – 1917) was born in Brooklyn but spent most of his life in The Heartland, teaching in secondary schools across Illinois, Missouri, Iowa, Kentucky, and Indiana [5].  He is best known to friends of the FWGNA Project for the six-part series of papers he published on the Mollusca of Kansas 1885 – 1887 [6], as well as his 1886 collaboration with Henry Pilsbry to describe the genus Pyrgulopsis [7].

And chief among the many and wondrous contributions R. Ellsworth Call made to American malacology in 1886 was his “Description of a new strepomatid mollusk of the Genus Goniobasis,” published in the Bulletin of the Washburn College Laboratory of Natural History, way out in Topeka [8].  In that paper he described the shell of Goniobasis ozarkensis with a large paragraph of adjectives and adverbs including “globosely elongated, faintly bicarinate, body whorl large, obtusely angulate at the periphery.”  The animal he described as “black above.”  The type locality was “Blue Spring, in the Ozark Mountains, Shannon County, Missouri.”  That’s less than 60 miles south of Minton’s Mill Creek site M8.

Goniobasis ozarkensis Call 1886 [9]

We mentioned last month that Goodrich [10] considered Call’s ozarkensis one of the four subspecies of potosiensis, and as such the nomen was transmitted to the present day by Burch [11].  No, those populations are not a subspecies of potosiensis.  At Minton’s Mill Creek site M8 there is clear evidence of reproductive isolation between a population that matches Call’s 1886 description of ozarkensis and a population of typical P. potosiensis.  Let’s transfer Call’s nomen from subspecific level under potosiensis to subspecific level under simplex: Pleurocera simplex ozarkensis (Call 1886).

As a generality, the shell of Pleurocera potosiensis bears a mid-whorl angulation, much more pronounced in juveniles, typically becoming obsolete in adults.  The shell is usually tan, brown, or golden, sometimes bearing bands of darker color.  The body color is never black.   Populations of P. potosiensis are common in rivers and streams throughout the Interior Highlands.

By contrast, the shell of P. simplex ozarkensis never bears a mid-whorl angulation, although sometimes faint carina are evident.  Both shell and body are darker, sometimes black.  Populations of P. simplex ozarkensis are restricted to richer, more stable springs, streams and rivers of the Interior Highlands with significant groundwater input.

Continuing on my journey with my eyes opened by the Damascus-Road experience of Thursday afternoon, September 4, I was able to recognize nine additional populations of P. simplex ozarkensis in the springs and streams I sampled further south through the Interior Highlands, all the way to Hot Springs, Arkansas [12].  And I found plenty of additional evidence of reproductive isolation.  Indeed, I found another mixed population in the Big Piney River about 30 miles south of Mill Creek M8, where “Boiling Spring” bubbles up right on the river margin, spitting a big population of simplex into a huge population of potosiensis, both populations retaining distinctive features [14].

And ultimately, I arrived in Eastern Oklahoma, to dip my boot toe into Sallisaw Creek, the home of Minton’s population OK, the pleurocerid that had launched my 3,400 mile, 15-day Odyssey.  And there I found the pleurocerid population mixed.  My readership will recall that Minton sequenced three individuals from his population OK, and that all three returned the simplex haplotype Z.  I myself could distinguish both species in Sallisaw Creek, mostly simplex, but a significant fraction of potosiensis.  And I will level with you.  Some of the pleurocerid snails in Sallisaw Creek I could not sort to species, with any degree of certainty.

Pleurocera potosiensis and Pleurocera simplex ozarkensis are effectively a sibling species pair.  Often in my 14 days of wading the creeks and rivers of the Interior Highlands, I reached down and drew back a pleurocerid snail I could not firmly identify one way or the other.  It seems quite likely to me that potosiensis and simplex hybridize.  Heaven knows we’ve amply documented that phenomenon in pleurocerid species more different than this pair [15].

And now, turning a practiced eye back on the ten paratypic shells that R. Ellsworth Call illustrated in his original 1886 description, I think it quite likely that shell #2 and shell #9 are P. potosiensis.  I think the population Call sampled at Blue Spring, Shannon CO, Missouri, may have been mixed [16].

Many independent lines of evidence suggest strongly that Pleurocera simplex, like most of the pleurocerids of Eastern North America, evolved in the Paleozoic Era, with the Appalachian Orogeny.  The hypothesis is inescapable that Pleurocera potosiensis evolved from a population of P. simplex after the Interior Highlands were separated by the Cretaceous embayment.

And were Charles Darwin alive today, I feel sure he would hasten to suggest that natural selection is a very likely cause.  At some point in the last 65 my, some population of P. simplex apparently adapted to a flashy, low-nutrient riverine environment, speciated, and spread downstream throughout the Ozark/Ouachita Highlands, leaving its ancestral P. simplex population, indeed hundreds of ancient P. simplex populations, stuck upstream in the creeks.  Grist for a future post, perhaps?  Stay tuned.

 

Notes:

 

[1] In addition to my friends Randy Sarver and Dave Michelson of the MoDNR, whom I acknowledged last month, special shout-outs here to Christopher Rogers with the Grand River Dam Authority in Tahlequah, Oklahoma, and Kyle Gustafson and his students at Arkansas State University in Jonesboro.

 

[2] Minton, R.L., B.L. McGregor, D.M. Hayes, C. Paight, and K. Inoue (2017) Genetic structuring in the pyramid Elimia, Elimia potosiensis (Gastropoda, Pleuroceridae), with implications for pleurocerid conservation. Zoosystematics and Evolution 93(2) 437-449.

 

[3] Minton uploaded these three identical haplotypes into GenBank as MO6A, MO6B, and MO6C, but they were labeled 6-A, 6-C, and 6-E on his figure 4.  I have retained the A-C-E system here.

 

[4] If the rationale for this statement is not immediately obvious to you, please see:

• What is character phase disequilibrium? [4Jan22]

[5] The biographical details on the life of R. Ellsworth Call are extracted from:

• Johnson, Richard I. (1975) R. Ellsworth Call with a bibliography of his works on mollusks and a catalogue of his taxa.  Occasional Papers on Mollusks (MCZ Harvard) 4 (54): 133 – 144.

[6] There’s a paragraph reviewing Call’s surveys of Kansas in the introduction to the Freshwater Gastropods of the Great Plains [FWGGP].

 

[7] Call R. E. & Pilsbry H. A. (1886). On Pyrgulopsis, a new genus of rissoid mollusk, with description of two new forms. Proceedings Davenport Academy of Natural Sciences 5: 9-14.  For more see:

• The SNHTHICACWB Marstonia 5: scalariformis [4Oct22]

[8] Call, R.E. (1886) Description of a new strepomatid mollusk of the genus Goniobasis.  Bulletin of the Washburn Laboratory of Natural History (Topeka) 1(7): 189 – 190.

 

[9] There is no scale bar on the plate, which almost certainly means that the reproduction was 1:1 in the original.  Call stated that “The average dimensions of the seven largest specimens are, for length 10.77 mm, breadth 6.21 mm.”  You do the math.

 

[10] Goodrich, C. (1939) Pleuroceridae of the Mississippi River basin exclusive of the Ohio River system.  Occasional Papers of the Museum of Zoology, University of Michigan 406: 1 – 4.

 

[11] 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).

 

[12] The population in Crystal Springs, AR, where Minton did his morphometric work [13], is almost certainly P simplex ozarkensis.

 

[13] Minton RL, Lewis EM, Netherland B, Hayes DM (2011) Large differences over small distances: plasticity in the shells of Elimia potosiensis (Gastropoda: Pleuroceridae). International Journal of Biology 3(1): 23 - 32.

 

[14] We used a photo of “Boiling Spring” (Texas County, MO) in the photobar at the top of our brand new “Freshwater Gastropods of Missouri” web resource.  Look at the top of the [FWGMO] front page, if you’re curious.

 

[15] For a review of hybridization in the North American Pleuroceridae, see:

• Widespread hybridization between Pleurocera laqueata and P. troostiana in streams of the Tennessee/Cumberland [15Oct24]
• Reticulate evolution in the North American Pleuroceridae [12Nov24]

[16] And, rats!  Dick Johnson [5] picked shell #2 as the lectotype.  Let’s keep that a secret, just between me and all two of you who read my irritatingly discursive footnotes, okay?