There is no substitute for field experience. You can relentlessly track the modern scientific literature of malacology, pore through 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 are just scattered pieces in a puzzle that will not come 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 plan was to zigzag south from Jefferson City through the Missouri Ozarks, west across the top of Arkansas into eastern Oklahoma, then 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.
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| 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 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 to be a 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!
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| Detail from Minton et al. [2] Fig. 4 |
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.
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| 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 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.
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| 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].
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?
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