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?

The peculiar pleurocerids of the Interior Highlands I: Pleurocera potosiensis

You all like maps, am I right?  Who among my elite and erudite readership does not, at least occasionally, revel in an old-time paper map, and lament their impending extinction?

 So last summer I was taking a walk down memory lane in Derring Hall, home to both the Biology Department and the Geology Department at my alma mater [1], Virginia Tech (1973 – 1977).  And I stopped to admire a gigantic (1996) Tectonic Map of North America [2], preserved behind floor-to-ceiling Plexiglas sheeting outside the Geology Department office.  And snapped the photo below.

I was stricken by the obvious inference that at some point in the ancient past, a chunk broke off my old familiar East Tennessee stomping grounds and floated west beyond the Mississippi into the wilds of Arkansas and Oklahoma.  That chunk is the Ouachita Mountains.  Indeed, the tectonic theory of North America suggests that the Ouachita Mountains and the Appalachians are “sisters,” created together by the collision of the Gondwanan plates around 300 mya, subsequently separated by the Cretaceous Embayment.

 

Like most of us here in The East, I tend to lump the Ouachita Mountains together in my mind with the Ozark Highlands, but the Ozarks were uplifted as a dome and subsequently dissected.  That rugged region, extending north from NW Arkansas well into Missouri, is not visible in the 1996 map above.  I cannot find any consensus on the date or cause of the Ozark uplift; it may also have been a function of the Paleozoic orogeny that formed Ouachita Mountains, or may have been post-Paleozoic.  That doesn’t matter for the yarn I’m fixin’ to spin.

 

Melania potosiensis [6]

The most important thing is the snails.  Today, the characteristic freshwater gastropod of the USGS Interior Highlands Physiographic Division, which includes the Ozark Highlands, the Ouachita Mountains, and the Arkansas Valley between them, is Pleurocera potosiensis.  That snail is a regional endemic – widespread and locally quite common in rivers and streams throughout the physiographic division, unknown elsewhere.  Shi-Kuei Wu and colleagues [3] documented populations across the entire southern half of Missouri, an observation we have now thoroughly confirmed [FWGMO map].  Christian and Hayes [4] reported P. potosensis populations equally widespread across the northwestern quarter of Arkansas.  The species range also extends westward into Oklahoma, and touches the SE corner of Kansas [FWGGP map].  Further south, populations of P. potosiensis become less common in tributaries of the Arkansas River, and spotty in tributaries of the Ouachita.

Isaac Lea’s brief, Latinate description of Melania potosiensis appeared in 1841 [5], as #46 in that same “litter of 57 pleurocerid puppies” I catalogued in my essay of [20Aug25], with a more complete English description and figure following in 1843 [6].  Tryon [7] transferred the nomen to Goniobasis, reprinting Lea’s 1843 description verbatim, while adding, perceptively, 

“Were it not for the wide differences of locality I should suspect this to be identical with simplex.  I have not seen specimens, but the figure and description are certainly very close to that species.”

Goodrich [8] recognized four subspecies.  The most widespread he identified as Goniobasis potosiensis plebius (Anthony 1850), which he considered common in rivers and creeks throughout the Ozarkian area of Missouri, Arkansas, and Eastern Oklahoma.  The typical subspecies Goniobasis potosiensis (ss) he considered “a shell of the upland streams of a few Missouri counties” only.  Goodrich also recognized a Goniobasis potosiensis crandalli (Pilsbry 1890) “known only from Mammoth Springs, Fulton County, Arkansas” and a Goniobasis potosiensis ozarkensis (Call 1886) which he considered a “depauperate” form [9, 10], “only from springs of Shannon, Carter, Washington, Dent, and Camden counties, Missouri.”  Burch [11] transmitted Goodrich’s entire four-subspecies system along with their ranges verbatim, pausing only to swap out the well-established genus Goniobasis for the zombie taxon [12] Elimia.

 

As a laboratory for the study of evolution, the widely-dispersed and genetically diverse Ozark/Ouachita populations of P. potosiensis may rival the Pleurocera proxima populations of the southern Appalachians [14].  By the Grace of God, they seem to have slipped through 200 years of malacological malpractice to arrive in the 21st century almost unsplit by taxonomic exuberance.  Our colleague Russ Minton and his coworkers took advantage of this happy situation in both a shell morphological study [15] published in 2011, and in a 2017 study of intraspecific genetic divergence [16].

 

The 2011 paper, a landmark-based study of 500 individual shells sampled at 25 m intervals from a spring run and adjacent tributary of the Ouachita River in Garland County, Arkansas, was most memorable for its peculiar Figure 1.  True to the school of landmark-based morphometrics, there was no scale on Minton’s photo, reproduced below.  The caption simply read, “Morphological variation in Elimia potosiensis from Arkansas.”

Minton et al. [15] Figure 1

The first thing that struck me when I read Minton's 2011 paper some years ago was that the second shell (B) was clearly that of Leptoxis arkansensis, not P. potosiensis at all.  I’m sure that’s a common mistake, to naïve eyes.  I myself had no field experience in The Interior Highlands until 2024.  All I knew about the malacofauna of that biogeographically fascinating part of the world until quite recently was what I had learned from a week studying the Wu-Oesch-Gordon Missouri collections at the University of Colorado Museum in 2021, and even there I found some not-insubstantial Leptoxis/Pleurocera confusion.

But what really struck me upon my first reading of Minton’s 2011 morphometric study was that third shell (C).  It was clearly out of scale with the other two – probably 30% magnified, by my eye.  And recalling the words of Tryon, I found that shell completely indistinguishable from my old friend from the East, Pleurocera simplex. 

 

I picked up my first Pleurocera simplex when I was a student at Virginia Tech back in 1975, and since then have sampled hundreds of populations from SW Virginia all across Tennessee, Kentucky, and north Alabama.  I have published five papers and notes on P. simplex thus far [17], supplemented by at least eight blog posts.  Rob Dillon knows Pleurocera simplex.  Were populations of P. simplex on that plate-tectonic raft when it broke loose from its Tennessee moorings and washed up on the banks of the Wide Missouri way back in the Paleozoic?

 

Russ Minton’s 2017 paper was an even more interesting read.  He and his colleagues reported the results of two separate studies, a fine-scale study using ISSR markers very similar to the allozyme study I published on P. proxima way back in 1988 [14], and a study of 16S sequence divergence at the scale of many of the allozyme studies I published on P. proxima and others in the early-2000s [18].  The fine-scale study was poorly designed, with only 10 snails sampled for each of 12 sites down approximately 500 meters of stream, such that the ISSR markers (110 unique genotypes among the 120 individuals) returned no results.  But the study of mtDNA sequence divergence was fascinating.

Adapted from Figure 1 of Minton et al [16]

Minton and colleagues sequenced the 16S gene from 61 individual snails identified as “Elimia potosiensis” from 16 sites in southern Missouri, 14 sites in northern Arkansas, and 1 site in eastern Oklahoma.  Their map of sample sites, color-coded by drainage system, is reproduced above. This is the second-best data set [19] on interpopulation mtDNA sequence divergence ever published for any nominal species of pleurocerid snail.

 

Minton discovered four strikingly different sets of sequences, each about 10% different from the other three, none of which demonstrated any correlation to geography whatsoever.  Minton’s Figure 4 is reproduced below.  I have labelled those four sets of haplotypes X, Y, Z, and L.

 

Who among my loyal readership finds this result surprising, in the least?  How many blog posts have I dedicated to the phenomenon of mitochondrial superheterogeneity in freshwater gastropods [20], since Bob Frankis and I first stumbled upon the phenomenon [21] back in 2004?  Speaking now to any new visitors we might be entertaining in the columns of the FWGNA Blog this month, and to any other readers who might otherwise imagine that double-digit mitochondrial sequence diversity is unusual within pleurocerid populations, you are earnestly invited to footnote [22] for approximately 30 minutes of remedial study. 

Adapted from
Minton [16] Fig. 4

Ah, but.  Some of the details in Minton’s Figure 4 did not match the expectation I have developed over years of familiarity with mtSH.  Yes, Minton identified a majority haplotype, which I have labelled X, just as Whelan and Strong found in the best study of mtSH to date, their 2016 paper on Alabama Leptoxis [19].  Whelan & Strong also discovered five other haplotypes, all demonstrating double-digit sequence divergence from the majority haplotype, four of which were quite rare.  My jetlagged wildebison model would suggest that those five rare Leptoxis haplotypes had evolved somewhere far away (in snail-space or snail-time) to be scattered into the present-day study area by dirty birds.  And in the case of the Whelan & Strong dataset, we cannot identify four of those other five places.  Fine.

And yes in fact, the cluster I have labelled Y in Minton’s Figure 4 does fit our expectation for mtSH, under the jetlagged wildebison model.  That haplotype is 10.1% different from haplotype X, it is rare, and there is no divergence among the five individuals (found in three populations) carrying it.

 

But the clusters labelled Z and L in Minton’s Figure 4 do not look like mitochondrial superheterogeneity to me.  They are not rare.  Moreover, both show evolutionary structure – a branching within cluster.  Within cluster Z, for example, Population #2 branches first – the only two individuals sampled for the study, together.  Then population OK (from Oklahoma) branches off – all three of the individuals sampled, together.

 

And as I sat at my desk late one evening several years ago, examining the population OK data published in that paper, a bell tinkled way in the back of my addled brain.

 

Russ Minton only figured one shell in his 2017 paper, pasted into the corner of his Figure 2, showing a map of the sample sites for his ISSR study.  Quoting his Figure 2 caption: “Shell of E. potosiensis from population OK is shown.”  I have clipped that shell from Russ’ 2017 Figure 2 and pasted it in the lower left corner of my adaptation of his Figure 1 map above.  That is very clearly the same scaleless individual shell he labelled “Elimia potosiensis from Arkansas” in his 2011 paper.  And that shell looked as much like Pleurocera simplex in 2017 as it did in 2011.

 

All three of the individual snails that Minton sequenced from Oklahoma carried haplotype Z.  And the other 13 individuals carrying haplotype Z were scattered all across Minton’s three-state study area, as I have marked in red above.  In what direction could all those clues be leading?  Tune in next time.

 

Notes:

 

[1] For sweet, gauzy memories from my halcyon days at dear old Virginia Tech, see:

• Water hardness, stream size, and A.E. Boycott: A New River Reminiscence. [8July25]

[2] Muehlberger, W.R. (1996) Tectonic Map of North America.  American Association of Petroleum Geologists, Tulsa, OK.

 

[3] Wu, S-K., Oesch, R. & Gordon, M. (1997) Missouri Aquatic Snails. Jefferson City: Missouri Department of Conservation. 97 pp.

 

[4] Christian, A. D. and D. M. Hayes (2007) Diversity and distribution of freshwater gastropods from the Ozark Region of Arkansas.  Arkansas Game & Fish Commission, unpublished report. 34 pp.

 

[5] Lea, Isaac (1841) Continuation of Mr. Lea's paper on New Fresh Water and Land Shells.  Proceedings of the American Philosophical Society 2: 11 – 15.

 

[6] Lea, Isaac (1843) Description of New Fresh Water and Land Shells.  Transactions of the American Philosophical Society (New Series) 8: 163 – 250.

 

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

 

[8] 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.

 

[9] We first mentioned “depauperization” in our essay of [20Aug25] on Melania acutocarinata.  Goodrich [10] defined “depauperization” as “the outward manifestation of disease, accident or malnutrition or a reaction to inimical environment.”

 

[10] Goodrich, Calvin (1939) Aspects of depauperization.  The Nautilus 52: 124 – 128.

 

[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] We reviewed the taxonomic controversy here:

• Goniobasis and Elimia [28Sept04]

It ultimately didn’t matter, because both Goniobasis and Elimia were synonymized under Pleurocera by Dillon [13] in 2011.

 

[13] 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]  For a review, see:

• Goodbye Goniobasis, Farewell Elimia [23Mar11]

[14] General references on the population genetics of P. proxima:

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]

Dillon, R.T. (1984) Geographic distance, environmental difference, and divergence between isolated populations. Systematic Zoology 33:69-82. [pdf]

Dillon, R.T. (1988) Evolution from transplants between genetically distinct populations of freshwater snails. Genetica 76: 111-119. [pdf]

Dillon, R.T. (1988) The influence of minor human disturbance on biochemical variation in a population of freshwater snails. Biological Conservation 43: 137-144. [pdf] For a review, see:

• Intrapopulation gene flow: The polymorphic Pleurocera of Naked Creek [12Oct21]

[15] Minton R.L., Lewis E.M., Netherland B, Hayes D.M. (2011) Large differences over small distances: plasticity in the shells of Elimia potosiensis (Gastropoda: Pleuroceridae). International Journal of Biology 3(1): 23 - 32.

 

[16] 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.

 

[17] General references on the population genetics of P. simplex in the Southern Appalachians:

Dillon, R. T., Jr., & G. M. Davis (1980) The Goniobasis of southern Virginia and northwestern North Carolina: Genetic and shell morphometric relationships. Malacologia 20: 83-98. [pdf]

Dillon, R. T., Jr., & J. D. Robinson (2007) The Goniobasis ("Elimia") of southwest Virginia, I. Population genetic survey. Report to the Virginia Division of Game & Inland Fisheries, 25 pp. [pdf]

Dillon, R. T., Jr. (2016a) Two reproductively isolated populations cryptic under Pleurocera simplex (Say, 1825) inhabiting Pistol Creek in Maryville, Tennessee. Ellipsaria 18(2): 15-16. [pdf]

Dillon, R. T., Jr. & J. D. Robinson (2016) The identity of the "fat simplex" population inhabiting Pistol Creek in Maryville, Tennessee. Ellipsaria 18(2): 16-18. [pdf]

Dillon, R. T., Jr. (2016) Match of Pleurocera gabbiana (Lea, 1862) to populations cryptic under P. simplex (Say, 1825) Ellipsaria 18(3): 10 - 12. [pdf]

 

[18] Regional surveys of pleurocerid population genetics:

Dillon, R. T. and A. J. Reed (2002) A survey of genetic variation at allozyme loci among Goniobasis populations inhabiting Atlantic drainages of the Carolinas.  Malacologia 44: 23-31. [pdf]

Dillon, R T. and J. D. Robinson (2009) The snails the dinosaurs saw: Are the pleurocerid populations of the Older Appalachians a relict of the Paleozoic Era?  Journal of the North American Benthological Society 28: 1 - 11.  (Rosemary Mackay Award)  [pdf]

Dillon, R. T. and J. D. Robinson (2011) The opposite of speciation: Population genetics of Pleurocera (Gastropoda: Pleuroceridae) in central Georgia.  American Malacological Bulletin  29: 159-168.  [pdf]

 

[19] The blue ribbon goes to the data set of Whelan, N.V. & E. E. Strong (2016) Morphology, molecules and taxonomy: extreme incongruence in pleurocerids (Gastropoda, Cerithiodea, Pleuroceridae). Zoologica Scripta 45: 62 – 87.

 

[20] A search on the word “superheterogeneity” using the box in the upper right of your screen will return hits in an impressive 24 essays.  And that doesn’t even include the essays I posted on the subject before I coined the term “mitochondrial superheterogeneity” in 2016.

 

[21] Dillon, R. T., and R. C. Frankis. (2004) High levels of DNA sequence divergence in isolated populations of the freshwater snail, Goniobasis.  American Malacological Bulletin 19: 69 - 77. [pdf]

 

[22] Mitochondrial superheterogeneity (mtSH), where two or more of the members of a single population demonstrate greater than 10% divergence in any single-copy mtDNA gene, not sex linked, is remarkably common in freshwater gastropods.  In pulmonate populations, I wouldn’t be surprised if most or all mtSH is ultimately traceable to cytoplasmic male sterility [23].  In prosobranch populations, however, I think mtSH is a signature of great age, plus low-frequency long distance dispersal, the “Jetlagged Wildebison Model.”  Here is a sample of my previous posts on mtSH:

• The Snails the Dinosaurs Saw [16Mar09]
• Mitochondrial superheterogeneity: What we know [15Mar16]
• Mitochondrial superheterogeneity: What it means [6Apr16]
• Mitochondrial superheterogeneity and speciation [3May16]
• Mitochondrial heterogeneity in Marstonia lustrica [3Aug20]
• Testing the periwinkle hypothesis [9May23]

[23] David, Patrice, Cyril Degletagne, Nathanaëlle Saclier, Aurel Jennan, Philippe Jarne, Sandrine Plénet, Lara Konecny, Clémentine François, Laurent Guéguen, Noéline Garcia, Tristan Lefébure, Emilien Luquet (2022) Extreme mitochondrial DNA divergence underlies genetic conflict over sex determination.  Current Biology 32: 2325 - 2333.  https://doi.org/10.1016/j.cub.2022.04.014.  For a review, see:

• Cytoplasmic Male Sterility in Physa! [9June22]