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]

The Freshwater Gastropods of Missouri

The FWGNA Project is pleased to announce that a ninth region has been added to our burgeoning portfolio of online resources: The Freshwater Gastropods of Missouri, by R.T. Dillon, Martin Kohl, and Bruce Stephen.  Our coverage now extends across all or part of 23 states, from the Atlantic drainages onward into The Great Plains.

And just for fun, try entering the FWGMO site from our easy-to-use front page, freshly enhanced by our new friend Greg Nemes of Tanager Creative with a fancy clickable map, here:

FWGNA

Our survey of approximately 788 springs, streams, rivers, swamps, ponds and reservoirs across the Show-Me State yielded 1,760 records of 44 species and subspecies, eight of which are new to the FWGNA Project [1]. Fresh species pages for each of these eight brings our total coverage up to 156.  Two of the pleurocerid subspecies are new combinations, more about which in coming months.  Our database, as usual, is freely available for the asking.

The major drainage basins of Missouri, showing 788 sample sites.

We have stood on the shoulders of giants.  The giant of whom we must make special mention here is Dr. Shi-Kuei Wu, who along with Ronald Oesch and Mark Gordon published the wonderful 97-page, staple-bound report, Missouri Aquatic Snails, in 1997 [2].  We spent a very productive week working in Shi-Kui’s carefully-curated collection at the University of Colorado Museum back in 2021, ultimately yielding 1,261 (70%) of the records transmitted here.  Our new FWGMO web resource is dedicated to him.  We also owe special debts of gratitude to Leanne Elder and Hsiu-Ping Liu for hosting us graciously in the collection and in the home, respectively.

And speaking of gracious hosts.  Our colleagues Randy Sarver and Dave Michaelson were most generous with time, talent, and seven-year-old blacktop vials of macrobenthos at the Missouri Department of Natural Resources in Jefferson City [3].  Sorting through those vials, and the meticulous records that came with them, yielded another 325 (18%) of the records in the FWGMO database.

Ste. Genevieve-Modoc Ferry at low water

The remainder of our records were personally-collected by the three of us. The vastly-trapezoidal patch of the world enclosed by the boundaries of Missouri in 1821 is flat-out gorgeous, not all over but in spots, and I for one greatly enjoyed wetting a boot toe in her sparkling waters.  We concentrated our efforts on undersampled regions and habitats, especially the main Mississippi and Missouri Rivers, which we collected at historic low waters in September of 2023.

I have cousins in St. Louis and in Moberly, about an hour north of Jeff City, and had it explained to me that Missouri is not one state but two, “Mi-zoor-ree” in the northern flatlands and “Muh-zoor-rah” in the Ozark highlands of the south.  A biogeographer might point out that the Mississippi lowlands of the bootheel region could be considered a third state, at minimum.  The freshwater gastropod fauna reflects those striking physiographic, cultural, and military [4] distinctions.  See the FWGMO Discussion [link] for an elaboration.

We have also updated our Synthesis of relative abundances across the entire 127 species, 23-state fauna covered by the FWGNA project to the present date.  This new “Version 3.2” [link] has been a long time coming.  Since our previous Synthesis v3.1 (12May22), we have added the Great Plains (1,482 records) and the Gulf drainages of Georgia (650 records) as well as the 1,760 records newly collected from Missouri, for a grand total of 25,468.  The present update has (of course) resulted in an adjustment of the incidence rankings over the entire 127 species set, the less common half of the distribution, in any case.

The Gasconade River, Osage Co.

And finally.  We gratefully acknowledge and sincerely thank Mr. Greg Nemes of Tanager Creative for his skillful enhancement of the entire FWGNA site.  In addition to the keen clickable map on the front page, Greg did a tremendous job re-wiring the site to facilitate routine maintenance and update, freshening up the fonts, adding cool new analytics and hooking up an email platform with subscription boxes at the bottom of every page, to name but a few improvements.  So, remember the little red bird with the great big bag of skills, tanager.org, when clear communication matters to your nature, science, or humanities nonprofit!

Notes:

[1] The eight new species and subspecies freshly added: Amnicola stygia, Antrobia culveri, Fontigens aldrichi, F. antroecetes, F. proserpina, Leptoxis arkansensis, Pleurocera simplex ozarkensis, P. canaliculata lawrencii.

[2] Wu, S-K., Oesch, R. & Gordon, M. (1997) Missouri Aquatic Snails. Missouri Department of Conservation, Natural History Series #5. 97 pp.

[3] We posted a photo of our handsome colleagues from the Missouri DNR late last year:

• Art, science, and public policy: A dialogue in three languages [10Dec25]

[4] Infantry representing the northern half of Mi-zoor-ree shot south with rifled muskets; Muh-zoor-rah State Guard returned fire with obsolete smoothbores.

Water hardness, stream size, and A.E. Boycott: A New River reminiscence

Just across the Blue Ridge, where the high meadows lay,

And the galax spreads through the new mown hay,

There's a rusty iron bridge, 'cross a shady ravine

Where the hard road ends and turns to clay [1].

The New River is the second-oldest river in the world, just a bit younger than the Nile.  I have heard that assertion stated so often and so forcefully that it must be so.  Born on the slopes of North Carolina’s Grandfather Mountain (G, down below), the New River first flows rather improbably to the northeast, through high meadows parallel to the Blue Ridge, into the Commonwealth of Virginia.  Then, quite astoundingly, the river shifts its course northwest, orthogonal to the Appalachian Mountains, near the little city of Blacksburg (B), and cuts a deep notch through the Allegheny Plateau, diagonally across West Virginia to The Ohio.  The earliest explorers of the American interior named its lower half the Kanawha River, never imagining that the New River and the Kanawha River might connect through those hundreds of miles of rugged terrain.

New River at Grandview

My mother was born and raised in the little town of Floyd, Virginia, perched on the New River plateau, looking down over the Blue Ridge (M).  And my father was born and raised in Rock Castle Gorge, deep in the Blue Ridge down below (F).  Many were the sparkling summers I spent rocking on my grandmother’s porch, many were the spring times I fished the chilly creeks for trout, many the falls I hunted the dark forests for squirrels. 

With a suitcase in his hand

There the lonesome boy stands

Gazing at the river sliding by beneath his feet,

But the dark water springs from the black rocks and flows

Out of sight where the twisted laurel grows. 

So, I matriculated at Virginia Tech, the University of Blacksburg, in the fall of 1973.  And I have shared with this readership quite a few anecdotes about my education in those hallowed halls [2], especially highlighting the impact of my mentor, Dr. E. F. (Fred) Benfield.  Longtime readers might remember that, even as I was finding a seat in my freshman biology class, plans were well advanced to construct a double-dam pump-storage facility on the upper New River at the Virginia / North Carolina border, which would have sunk 100 miles of the world’s second-oldest river into inky blackness and inundated thousands of square miles of lovely farmland.  And the Virginia Tech Center for Environmental Studies had been contracted to prepare the Environmental Impact Statement [3].

 

And so it was that I, a mere sophomore of age 19, found myself checking out boots, buckets and nets from the storeroom of the Virginia Tech Center for Environmental Studies, pulling the keys to a pickup truck from the pegboard, and driving off into the high meadows of the upper New River for my first systematic survey of a freshwater molluscan fauna [4].  And in my undergraduate research thesis, defended in May of 1977, I reported a modest 6 species of unionid mussels, 4 species of pisiidid clams, 4 species of prosobranch snails, and 6 species of pulmonate snails from 87 sites sampled across the drainage in ten counties of southern Virginia and northwestern North Carolina [5].

 

It materialized that those 20 species of freshwater mollusks were not all evenly distributed across my ten-county study area.  Even to my young and untrained eye, patterns manifest themselves.  And the most striking pattern was closely correlated with the underlying geology of the upper New River basin.  Which sent me to the library, looking for clues.

The geology of the New River Valley of Virginia

I do not remember the day or the hour I first discovered the wonderful 1936 work of A. E. Boycott [6], but I do remember the impact.  Boycott’s 70-page review was a thing of beauty; so complete, so rigorous, so bubbling over with cheerful facts about the biology of the creatures we both obviously loved that 20 years later, I patterned an entire book after it [7].  And in retrospect, may have produced but a pale imitation.

 

So Boycott, after reviewing the general biology and local habitats of the 62 species of freshwater mollusks inhabiting the Island of Great Britain, turned to “The relevant qualities of the habitats.”  Item (b) in his list of eight relevant qualities was “lime,” and item (c)  was “reaction,” by which he meant pH, which (he hastened to point out) was essentially the same thing as lime [8].

 

Boycott went on to classify (and map) the 62 British species as 25 “soft-water” species (“all those which we can find in soft water without surprise”) and 30 species “calciphile or calcicole in the sense that they are habitually found in hard water [9].”  He could not identify any British species as “calcifuge,” i.e., restricted to soft water.  Thus, overall molluscan diversity increased in the hard waters, as the 30 calciphiles were added to the 25 softwater species in the richer environments.

 

Well, that explains a lot right here in the good old USA, I thought to myself.  From its North Carolina origin through the first (roughly) hundred miles of its journey, the New River runs through ancient metamorphic rocks such as gneiss and schist, remaining quite soft.  But about halfway to its hard left turn at Blacksburg, the New enters a region of limestone and dolomite, and the water hardens up.  The distribution of most of the unionids and a couple of the gastropods (Pleurocera shenandoa, Physa gyrina) seemed to be restricted below that invisible barrier, as though they were, in Boycott’s terminology, “calciphile.”

 

But there was a second obvious factor in the distribution of the upper New River mollusk fauna, and Boycott had that one covered as well.  Item (d) of Boycott’s eight “relevant qualities” was “Size and Volume.  The larger units of water are liable to contain the more Mollusca.”  And indeed, most of the unionid mussel species were confined to the main New River, apparently unable to inhabit the smaller tributaries.  That also seemed true for Campeloma decisum.

Dillon & Benfield [16]

That’s pretty much where I dropped the shovel for my 1977 undergraduate thesis and sat down in front of the typewriter.  Each of my four subheadings under Results and Discussion – Unionidae, Sphaeriidae, Prosobranch Gastropods, and Pulmonate Gastropods – had a section entitled “Effect of Hardness” and a section entitled “Effect of Stream Size.”  All four subheadings also had a section entitled “Effect of perturbation” or “Effect of (artificial) enrichment.”  This was the 1970s, after all, I had to get pollution in there somehow.  I defended in May, got married in June, and moved to Philadelphia in July.

 

Ah, but.  Hidden deep inside my thesis was the germ of an idea.  Under the “Unionidae” subheading was also a brief section entitled “Interaction of factors.”  And there I speculated, at the age of 21, “Perhaps hardness and stream size interact in some manner so that a large stream can support Elliptio dilatata even though its hardness may be low, and a small stream can support mussels if it has high hardness.”  That hardness x stream size interaction was also obvious in the pulmonate snails.  About the origin of the phenomenon, at my young age, I would not hazard a guess. 

Past the coal-tipple towns in the cold December rain

Into Charleston runs the New River train,

Where the hillsides are brown, and the broad valleys stained

By a hundred thousand lives of work and pain. 

Ecology at The University of Pennsylvania and ecology at Virginia Tech are as different as Philadelphia and Blacksburg.  At Penn, I found the intellectual focus entirely upon the interactions among organisms, not the interactions between organisms and their environment.  Density dependence was the key, density-independence a quaint anachronism.  I remember vividly the argument made by the chairman of my graduate committee, Bob Ricklefs.  “A population without density-dependent control will either go extinct, or cover the world ass-deep.”  Since freshwater gastropods exist [10], and we are not ass-deep in them, they must be under density-dependent control.  Food availability or predation might certainly qualify as potential controls for their distribution, perhaps, certainly not the availability of calcium to build their shells.

 

Robert MacArthur’s theory of island biogeography was also still very much en vogue at Penn in the late 1970s, with its focus on island size.  And somewhere during my first year of graduate training, it dawned on me that both the size of a stream and the hardness of its water might affect its productivity.  And that the quantity of food might be controlling freshwater mollusk distributions in the upper New River, neither the calcium nor the stream size directly.

 

And so, at age 22, I hazarded a guess.  In July of 1978 I drove back down I-95/I-81 south from Philadelphia into those high meadows of the Upper New River where I had spent the summers of my youth, with beakers, cylinders, burettes, bottles of indicators and a 0.02 N sulfuric acid titrant carefully stowed behind the hatch of my 1973 Pinto.  I revisited and re-sampled every one of those 87 sites I had collected in the years previous, this time taking a streamside measurement of alkalinity, more reliable than pH, much easier to measure than calcium or overall hardness.  And this time I estimated a rank abundance for each of the five New River pulmonate snails (excluding limpets, which were omnipresent): Physa acuta, Physa gyrina, Lymnaea humilis, Lymnaea columella, and Helisoma anceps [11].

Dillon & Benfield [16]

I found pulmonate snails at 26 of the 87 sites, marked letters A – Z in the map above.  Confirming and deepening my undergraduate results, pulmonates were common and widespread in the main New River and most of its tributaries downstream from where the river entered the limestone/dolomite zone.  Above that zone, however, pulmonates were generally found only in the main river itself.

 

I summed the rank abundances of the five species into an overall measure of pulmonate abundance for each site.  And I calculated the Kendall rank correlation between pulmonate abundance, alkalinity, stream drainage area and (here’s the key!) the alkalinity x drainage area interaction.  The correlation with interaction (0.35**) was greater than either alkalinity or drainage area alone.

 

OK, one last thing.  Notice in the table above that alkalinity and drainage area were negatively correlated.  I guess that’s not too surprising – a little creek running through a limestone valley can get much harder than a big river, buffered as it is by its large catchment.  This gave me the idea of a nonparametric partial correlation coefficient, analogous to (parametric) partial correlation – a correlation between two variables holding a third variable constant.

Dillon & Benfield [16]

So, I invented nonparametric partial correlation [14].  The table above shows that the Kendall rank correlation between abundance and interaction remains high, even if alkalinity is partialled out (0.31) or if drainage area is partialled out (0.27).  The primary phenomenon is the interaction – the increased productivity that both water hardness and stream size promote – not the calcium nor the stream size directly.

 

When the paper by Dillon & Benfield [16] finally reached publication in 1982, I honestly thought I would become famous.  Brilliant young scientist invents novel statistical technique to answer a fundamental question of freshwater biology!  Alas, no. 

In a tar-paper shack out of town across the track

Stands an old used-up man trying to call something back

But his old memories fade like the city in the haze

And his days have flowed together like the rain

 

And the dark water springs from the black rocks and flows

Out of sight where the twisted laurel grows. 

Notes:

 

[1] The lovely and haunting lyrics interleaved with this month’s essay come from a song entitled “Twisted Laurel” by The Red Clay Ramblers, one of the greatest bands of the postmodern era.  Some good music is still being made today, but for mysterious reasons has fallen out of fashion with the popular multitude.  Exactly the same could be said for science.

 

[2] A small sample of previous essays in which I have reminisced about my undergraduate experiences at Virginia Tech and traced their subsequent influence on my scientific career:

• To Identify a Physa, 1975 [6May14]
• Pleurocera shenandoa n.sp. [11Mar19]
• Interpopulation gene flow: King Arthur’s lesson [7Sept21]
• Growing up with periwinkles [6Apr23]

[3] For more about APCO’s Blue Ridge Project and its ultimate fate, see:

• Woodard, R. S., Jr. (2006) The Appalachian Power Company along the New River: The defeat of the Blue Ridge Project in historical perspective.  M.A. Thesis, Virginia Tech, Blacksburg.  139 pp. [pdf]

 [4] We never change.  Fifty years later, I am still doing exactly the same thing.

 

[5] Dillon, R.T., Jr (1977) Factors in the distributional ecology of upper New River mollusks (Va/NC).  Undergraduate research thesis, Virginia Tech. [pdf]

 

[6] Boycott, A.E. (1936) The habitats of freshwater molluscs in Britain.  Journal of Animal Ecology 5: 118 – 186.

 

[7] Dillon, R.T., Jr. (2000) The Ecology of Freshwater Molluscs. Cambridge University Press.  509 pp.

 

[8] Boycott verbatim: “Broadly speaking, for the natural waters of this country the reactions run parallel with the quantities of calcium.”

 

[9] Boycott did not categorize 7 of the 62 British species because he felt that his data were insufficient.

 

[10] OK, I realize that I have touched a controversial point.  The vast majority of my colleagues today, including (quite likely) most of the handful of you who will ever read this footnote, believe, as an article of faith, that freshwater gastropod populations are indeed going extinct.  Possibly that every living creature on this earth, except cockroaches, mosquitos, thee and me, is going extinct?  And I will grant you all that the vast majority of all species that have ever lived have, indeed, gone extinct.  That is evolution.  That is not a crisis; that is not even bad.  That is normal.

 

[11] Back in 1978, I was still using George Te’s [12] identifications for the Physa, so Physa acuta = “hendersoni” and Physa gyrina = “pomilia.”  I identified the Lymnaea humilis as “Fossaria obrussa” [13] and used “Pseudosuccinea” as the genus for columella.  Science advances.

 

[12] For the complete story, see:

• To identify a Physa, 1971 [8Apr14]
• To identify a Physa, 1975 [6May14]
• To identify a Physa, 1978 [12June14]
• To identify a Physa, 1989 [3Oct18]
• To identify a Physa, 2000 [6Dec18]

[13] For the complete story, see:

• The American Galba and The French Connection [7June21]
• The American Galba: Sex, Wrecks, and Multiplex [22June21]
• Exactly 3ish American Galba [6July21]

[14] I was unable, however, to offer any statistical inference on my newly-invented Kendall partial rank correlation coefficients.  I was experimenting with Monte Carlo techniques at the time [15], and (in retrospect) should have done so.   Then I would have become famous.  Surely.

 

[15] Dillon, R.T., Jr. (1981) Patterns in the morphology and distribution of gastropods in Oneida Lake, New York, detected using computer-generated null hypotheses. American Naturalist 118: 83-101.  [pdf]

 

[16] Dillon, R.T. and E. F. Benfield (1982) Distribution of pulmonate snails in the New River of Virginia and North Carolina, U.S.A.: Interaction between alkalinity and stream drainage area. Freshwater Biology 12: 179-186. [pdf]