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]

The freshwater gastropods of Georgia Gulf drainages

We here in South Carolina think of Georgia as our younger brother, now outgrown us.  The colony to our immediate south was founded in 1732 to protect us from Spaniards, and in that function, at least, has been largely successful.  Our port cities, Charleston and Savannah, have grown up as sisters.  Our capital cities, Atlanta and Columbia, were both burned by Sherman.  We take turns beating Alabama for national championships.  Georgia grows more onions; we grow more peaches.

But if there is any honor of which our younger brother boasts that we here in South Carolina might envy, it is topographic diversity.  From North Georgia arise real mountains, the southern terminus of the ancient Appalachians.  And from those mountains are born rivers flowing in four directions through six USEPA Level III Ecoregions: north to the Tennessee/Ohio, west to the Alabama/Mobile Basin, south to the Gulf and east to the Atlantic.  The luxuriant aquatic biodiversity of Georgia more than compensates for any shortfall it might suffer in production of cannable fruit.

FWGGA v1.0, release 26Mar07

The Freshwater Gastropods of Georgia (FWGGA) web resource debuted online (at our old cofc.edu address) way back in 2007 with a survey of the Atlantic drainages of Georgia only, approximately 45% of the state.  Our database at that time comprised 845 records of 37 species.  It was migrated (as v2.0) to its present address in 2010, and saw minor upgrades (with additional data and fresh maps) in 2013 (v2.1) and 2024 (v2.2).  So as of last month, the FWGGA website v2.2 reported 960 records of 41 species and subspecies.  But that was for the Atlantic drainages alone.

Speaking now for myself and my coauthors Martin Kohl, Will Reeves, and Tim Stewart, today we are pleased to announce Version 3.0 of the FWGGA web resource, now expanded to include the Gulf drainages of Georgia, extending through 11 counties of the Florida panhandle between the Apalachicola and Suwanee Rivers.  Our database has grown to 1,608 records, documenting 56 species and subspecies of freshwater gastropods in the 85% of Georgia now covered.  Check it out today!

[FWGGA v3.0]

 

Most of our new records were gleaned from the extensive and well-curated mollusk collection held by the University of Florida Museum of Natural History in Gainesville.  Our initial search of the FLMNH online database returned 577 freshwater gastropod records from the Gulf drainages of Georgia and 495 from the 11 Florida counties downstream, for a total of 1,074 records in our enlarged study area.  Simple duplicates – records differing only by date or method of preservation, for example – were removed.  The records were then screened by three primary criteria: (1) dated since 1955, (2) locality data of sufficient quality to be plottable, and (3) habitat not brackish, as far as could be determined.

 

That last requirement turned out to be surprisingly restrictive.  Dr. Fred G. Thompson, who was the senior curator of mollusks at the FLMNH for 50 years, specialized in the hydrobioid snails.  And the museum cabinets that survive him in Gainesville to this day are packed with teensy-little vials full of teensy-weensier little gastropods, collected across every square foot of the Sunshine State, curated with great care.

 

FWGGA v3.0, release 19May25

The hydrobioid snails – especially the cochliopids – are famous for their adaptation to varying salinity.  Ultimately, however, we found it necessary to eliminate Heleobops, Onobops, and Littoridinops monroensis/palustris from our survey, because we could not find any records of any of these inhabiting entirely fresh waters in the area under study here.  That last omission was especially surprising, because Littoridinops monroensis was described from the (entirely fresh) upper St. Johns River system on the Atlantic side of Florida [1].  But this is the Freshwater Gastropods of North America Project, not the brackish, and we must draw a line somewhere, even if our study organisms do not.

So.  This past February I printed the resulting list of 454 panhandle Florida + 255 Gulf Georgia = 709 FLMNH records on a clipboard and made an appointment with our good friend and colleague John Slapcinsky, the collection manager way down south in Gainesville.  And we must take a moment to thank, from the bottom of our hearts, our buddy John for his infinite patience, great good humor, and skillful manipulation of the local parking authority hosting us during our most recent sojourn on his sprawling campus.  All identifications were verified, or (if the corresponding museum lot could not be located), deleted.

All qualified, verified lots of freshwater gastropods from our GA/FL Gulf drainage study area were then georeferenced and plotted.  As a rule of thumb, the FWGNA requires that no pair of records for a single species be collected from the same body of water any closer than 5 km.  Removal of the older record from all such near-duplicate pairs yielded a total of 257 Florida + 230 Georgia = 487 unique, modern, verified, georeferenced FLMNH freshwater gastropod records from the Gulf drainages of our two-state study area.  These were added to the 392 older Atlantic-drainage records, to yield the total of 879 FLMNH records analyzed in FWGGA version 3.0.

 

The additional 161 records newly reported in the FWGGA expansion were almost entirely collected by RTD using simple untimed searches 2003 - 2025, specifically targeting freshwater gastropod habitat [2].  Ultimately our survey covered approximately 645 discrete sample sites, located across the Atlantic and Gulf drainages of Georgia, extending through the 11 counties of the Florida panhandle between the Apalachicola and the Suwanee.  See the map above.  No “absence stations” are shown.  If freshwater gastropods were not collected at a site, then no record resulted. 

Marstonia castor UF22178

Our entire 1,608 record database is available (as an excel spreadsheet) from yours truly upon request.

The list of 56 species and subspecies of freshwater snails we have documented from our study area omits Marstonia castor, described by Thompson in 1977 as endemic to Cedar Creek in Crisp County, Ga [3].  Although the FLMNH collection also includes more recently-collected lots of M. castor from Swift Creek (Crisp Co) and Mercer Mill Ck (Worth Co), our 2023 efforts to locate a viable population anywhere in the region were unsuccessful.  The FWS listed M. castor as extinct in 2017, and we concur.

Combining subspecies for analysis, the freshwater gastropod fauna of the region under study here reduces to 53 species: 35 prosobranchs and 18 pulmonates.  Of the pulmonates, three are extralimital or introduced: Biomphalaria, Promenetus, and Physa gyrina. Helisoma scalare is Floridian.  The remaining 14 pulmonate species are all common throughout the southeastern United States, in some cases stratified by ecoregion: Physa carolinae and Helisoma trivolvis (for example) restricted to the coastal plain, Ferrissia rivularis in the piedmont and mountains.

 

A Gradual Transition

More biogeographic signal is apparent in the prosobranchs.  Of the 35 prosobranch species we identified in our study area, 10 are unique to the Gulf drainages, 10 are unique to the Atlantic drainages, and 15 are shared across the state of Georgia broadly.  This observation does not support the hypothesis advanced by Thompson & Hershler [4] that “with the exception of Lyogyrus and two species of Viviparus,” the prosobranch faunas of the Atlantic and Gulf drainages of Georgia “have no species in common.”  Rather, the distributions of the freshwater gastropods of Georgia apparently reflect a gradual transition or blending between the faunas of Atlantic drainages to the east, Alabama/Coosa drainages to the west, and Florida to the south.

 

The DFS Zone

Our modern survey has, however, corroborated the 1991 observations of Thompson & Hershler [4] that the drainage basins of the Satilla and the St. Marys Rivers of the Atlantic drainage, plus the upper portions of the Suwannee and Ochlockonee River systems of the Gulf drainage, are virtually “devoid of freshwater snails.”  The striking absence of sample sites in that region clearly evident on the map above is not due to a lack of effort on our part.  We travelled that area extensively, donning boots and searching keenly, ultimately returning with no freshwater gastropod observations to report.  We here refer to that region as the “DFS Zone,” for “devoid of freshwater snails.”

 

Citing evidence from the paleontological results of Aldrich [5], Thompson and Hershler suggested that the DFS Zone had a rich freshwater gastropod fauna in the Pleistocene, similar to that of surrounding regions today, and attributed the depauperization of the modern fauna to “water chemistry factors” recent in their origin.  We ourselves are hesitant to generalize the fresh/brackish Pleistocene malacofauna catalogued by Aldrich from the lower Satilla across the entire DFS Zone.  But the hypotheses that Thompson & Hershler advanced regarding the influence of bedrock and soil type on water chemistry, and the influence of water chemistry upon freshwater gastropod distributions, are well supported [6].

To live and die in Dixie...

The Satilla, the St Marys, the upper Suwanee and the upper Ochlockonee drainages in South Georgia are underlain by Cretaceous gravels and sands, yielding soft, acidic, low-carbonate surface waters to which freshwater gastropod populations are often poorly adapted.  And to the inhospitable water chemistry of this region, we would hasten to add the inhospitable water physics.  Silt.

 

Clench & Turner [7] suggested that “the greatest source of damage” to the freshwater mollusk fauna of the Georgia Gulf drainages “seems to be land erosion and consequent silting of the rivers.”  For over a century, most of the state was intensively farmed for cotton, stream bank to stream bank.  Harding and colleagues [8] reported that the best predictor of current macroinvertebrate diversity in East Tennessee river systems is not present land use, but rather land use prior to 1950.  We suggest that the intensive burdens of silt that have been carried, and that continue to be carried, by the rivers of South Georgia, together with the softness, acidity, and poor buffering capacity of the regional surface waters, account for the phenomenon we here describe as the DFS Zone.

Although relatively minor in areal extent, the expansion of FWGNA coverage to include this diverse little drip of North American freshwater nevertheless resulted in the addition of 9 new gastropod species and subspecies to the 136 previously included in our coverage, bringing our continental total to 145.  Come visit us again, for the first time!

 

Notes:

 

[1] Von Frauenfeld, G. R. (1863) Verhandel. Kais. Konig. Zool. Botan. Ges. Wein 13: 1023.

 

[2] Dillon, R.T., Jr. 2006. Freshwater Gastropoda. pp 251 - 259 In The Mollusks, A Guide to their Study, Collection, and Preservation. Sturm, Pearce, & Valdes (eds.) American Malacological Society, Los Angeles & Pittsburgh.

 

[3] Thompson, F.G. (1977) The hydrobiid snail genus Marstonia.  Bulletin of the Florida State Museum, Biological Sciences 21: 113 – 158.

 

[4] Thompson, F.G. & R.H. Hershler. 1991. Two new hydrobiid snails (Amnicolinae) from Florida and Georgia, with a discussion of the biogeography of freshwater gastropods of South Georgia streams. Malac. Rev. 24:55-72.

 

[5] Aldrich, T.H. (1911) Notes on some Pliocene fossils from Georgia with descriptions of new species.  Nautilus 24: 131 – 132, 138 – 140.

 

[6] For a review of the effects of calcium concentration and related water chemical variables on the distribution of freshwater gastropods, see pages 326 – 338 in:

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

[7] Clench, W.J. & R.D. Turner. 1956. Freshwater mollusks of Alabama, Georgia, and Florida from the Escambia to the Suwannee River. Bull. Fl. State Mus., Biol. Sci. 1:95-239.

 

[8] Harding, J.S., E.F. Benfield, P.V. Bolstad, G.S. Helfman and E.B.D. Jones (1998) Stream biodiversity: The ghost of land use past. Proceedings of the National Academy of Sciences 95: 14843 - 14847.

The Freshwater Gastropods of The Great Plains

We are pleased to announce a major expansion of the FWGNA Project, now extending our coverage westward to include the prairie states of Kansas, Nebraska, South Dakota, and North Dakota.  The Freshwater Gastropods of the Great Plains, by Bruce J. Stephen, Robert T. Dillon, Jr, and Martin Kohl is now online and available for reference!  Check it out:

Visit the FWGGP

In this important new web resource, we report the results of an original survey of 795 rivers, streams, lakes, and ponds across a big slice of the American heartland, documenting 33 gastropod species.  For each species we provide range maps and ecological notes, with a photo gallery and a dichotomous key for easy identification.

Although in areal extent our 308,000 square mile Great Plains study area is the largest of the eight regions thus far covered by the FWGNA Project, by freshwater gastropod species richness it is the smallest.  We suggest two historical factors to account for the relative poverty of the Great Plains malacofauna: the absence of landform diversity, and the absence of time sufficient for a regionally adapted fauna to evolve.  The effects of Pleistocene glaciation, if any, seem to have been to increase species richness.  State subtotals were 16 species in Kansas, 18 in Nebraska, 19 in South Dakota, and 23 in North Dakota.

We also document reductions in species richness for three of the four Great Plains states when compared to expectation from the published literature.  Kansas seems to have lost 4 species, South Dakota 6 and Nebraska 14.  The freshwater gastropod species apparently missing from each state typically become more common further north.  Although some of this phenomenon is certainly due to sampling error, we think it likely that climate change may have been a factor in the decreased species richness of The Great Plains.

Atlantic Drainages Update

Our hunger to advance the cause of freshwater gastropod science is insatiable here at the general headquarters of the FWGNA Project.  I’m always scanning the literature for the latest research and looking to add new records to the database, even for those regions we covered and published many years ago, from which we seem to have long moved on.  We haven’t “moved on” from anywhere.  Our coverage extends over all or part of 17 states, expanding south and west, active to the present day.

But it has been ten years – if you can believe it – since we last updated the five web resources that cover the freshwater gastropod fauna of U.S. Atlantic drainages:  Georgia (FWGGA), South Carolina (FWGSC), North Carolina (FWGNC), Virginia (FWGVA) and the Mid-Atlantic (FWGMA).

Fresh 2023 Format

So a couple months ago we were able to twist the arm of our good friend Martin Kohl to help us with a fresh set of maps, which is the biggest piece of the chore.  And today we are pleased to announce that the results of Martin’s considerable GIS skills are now available for download from the pages of the 72 species and subspecies of gastropods inhabiting rivers, lakes, ponds and streams of the vast (ten-state) Atlantic-drainage area.

The maps newly available for 2023 are built on a database of 12,138 records.  That number represents a 4.2% reduction from the 12,674 Atlantic-drainage records upon which we based our (most recent) Synthesis v3.1 and Biogeography v2.0 back on 12May22.  The new total reflects a pruning of our FWGNC database from 4,425 down to 3,809 records to remove a big batch of near-duplicate samples, collected by NCWRC teams upstream and downstream from bridges, for example.

Other FWGNA Atlantic-drainage databases have been slightly augmented by routine collecting, however, up from 895 to 960 in Georgia, from 1,938 to 1,989 in South Carolina, from 2,333 to 2,396 in Virginia, and from 3,150 to 3,159 in the Mid-Atlantic states.  Note that the sum of those five figures totals slightly more than 12,138 due to double counting where rivers comprise state lines.

Our 2013 maps emphasized rivers, streams, and vegetative cover.  Our new 2023 maps have been significantly reformatted to show the major USGS/EPA Ecoregions, with counties and cities (very lightly) in the background.  Close comparison of the two examples (above and below) will reveal a slight reduction in data density for North Carolina, and some fresh data mapped, especially in Georgia.

Old 2013 Format

The contents of all 128 species pages on the FWGNA site have also been refreshed in recent months – not just the 72 species and subspecies of Atlantic drainages.  I am always on the lookout for new research to add to the bibliographies – ecology, life history, systematics, evolution – anything and everything, really.

Whenever any of you publish anything new, please send me a link or a reprint.  Indeed, if you happen to read a new paper with especially interesting or important results on any aspect of the biology of North American freshwater gastropods, written by anybody else, I always appreciate a heads-up.

For many years, my customary sign-off has been, “Keep in touch.”  I mean it, I’m serious!

Freshwater Gastropods of the Tennessee/Cumberland

Today we are pleased to announce the expansion of our FWGTN coverage from its East Tennessee origins though the entirety of the Tennessee and Cumberland River drainage basins, increasing our sampling area from approximately 22,000 square miles to over 58,000.  We document 54 species of freshwater gastropods with 16 additional subspecies in this malacologically rich region, offering ecological and systematic notes for each, as well as detailed distribution maps, a dichotomous key and a photo gallery.  This expanded web resource, coauthored by R.T. Dillon, M. Kohl and R. Winters, is available here:

The Tennessee/Cumberland

The previous version of our FWGTN website, brought online in 2011 by Dillon & Kohl, covered only the Tennessee River drainage system from SW Virginia and western North Carolina through East Tennessee to skim the top of North Georgia and stop at the Alabama border.  Our 2011 database included 1,674 records from approximately 767 discrete sites, documenting 39 species and 2 subspecies.  The expanded database we release today includes 4,003 records from approximately 1,700 discrete sites, ranging though North Alabama and Middle Tennessee to clip the corner of NE Mississippi, plus a big slice across southern Kentucky as well.

Click for larger

Among many interesting findings, we report here that three pleurocerid species previously thought restricted to East Tennessee range significantly further west: Pleurocera simplex (with its subspecies ebenum), Pleurocera troostiana (with subspecies perstriata, edgariana, and lyonii) and Pleurocera clavaeformis (subspecies unciale).  We have also discovered that Pleurocera semicarinata, previously unknown further south than Kentucky, ranges through Cumberland drainages well into Tennessee.  The distributions of several hydrobioid species are also clarified and expanded – more about this in coming months.

Our complete FWGNA database, covering the drainages of The Ohio as well as Atlantic drainages from Georgia to the New York line, now contains 22,044 records documenting 107 species of freshwater gastropods, with 21 subspecies.  We have updated our overall website with a new continent-scale biogeographic analysis, dividing records into North Atlantic, South Atlantic, Ohio, and Tennessee/Cumberland subsets.  Our analysis suggests that natural selection has been more important in the evolution of freshwater pulmonates than gene flow restriction, but that gene flow restriction has been more important in the evolution of freshwater prosobranchs than natural selection.

We also announce today the publication of an updated “Synthesis v3.1,” ordering our 107 species by their incidence in our continental database and assigning fresh FWGNA incidence ranks to all.

So, visit the FWGNA web resource again, for the first time!

Intrapopulation gene flow: King Arthur's lesson

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023b)  Intrapopulation gene flow: King Arthur's lesson.  Pp 111 – 120 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other Essays.  FWGNA Project, Charleston, SC.

A frequent visitor to the Malacology Department at Academy of Natural Sciences, during the sweet gauzy years of my graduate education in Philadelphia, was a charming scholar of aristocratic bearing named Prof. Arthur J. Cain [1].  Prof. Cain made his reputation on the study of color polymorphism in the European land snail Cepaea, publishing several very influential papers on the subject in the 1950s and early 1960s [2].  He was a wellspring of stories about E.B. Ford and the good old days of “Ecological Genetics,” stomping about the English countryside, collecting data on the frequencies of the myriad color morphs of Cepaea and their correlation with temperature, ground cover, predation, and so forth [3]. 

Arthur J. Cain (1921 - 1999)

King Arthur once remarked, “Since the term was coined by the founding fathers of the modern synthesis – I don’t know by whom, and it does not matter – nobody has ever imagined that the concept of panmixia might apply to a natural population of gastropods.”

So I had arrived in graduate school knowing that I wanted to focus my research on the phenomenon of speciation in freshwater snails.  And it was crystal clear to me that in order to understand speciation, I had to understand population divergence.  And before I could understand population divergence, I had to understand genetic variation within populations.  And in order to understand genetic variation within populations, I had to understand all those things that Arthur Cain loved to talk about – barriers to dispersal, isolation-by-distance, gene flow and the lack thereof.  In short, why freshwater gastropod populations are not panmictic.  That was the first thing [4].

Like a good student, I started in the library.  The literature on intrapopulation gastropod dispersal was mind-bendingly huge, even when I first dug into it in the late 1970s.  So I focused on the movement of freshwater gastropods in lotic environments, which was the most directly relevant to my research interest in pleurocerid populations in the Southern Appalachians.  And the first impression I got was the universal tendency for freshwater snails to actively disperse upstream, into currents.  And the second impression I got was the equally-universal likelihood of passive dispersal downstream, by wash-down.  Both of these phenomena are functions of current speed.

The oldest paper in the yellowing folder I still have filed under “Intrapopulation Dispersal” in the cabinet to the right of my desk also turned out to be among the most relevant to my subsequent dissertation research with Pleurocera (“Goniobasis”) proxima – a 1952 study of the Campeloma population inhabiting a small creek in Michigan.  Bovbjerg [5] observed striking aggregations of his large-bodied, burrowing study organism downstream from rocky riffles, which they seemed to have difficulty traversing.  His mark-release study, conducted over six days in a long, sandy region without such obstructions, returned a (surprisingly high) mean upstream dispersal rate of 350 cm per day OTSIWATFA [6], with no downstream dispersal whatsoever.

So the increased population densities Bovbjerg observed downstream from riffles would seem to be consistent with the opposite of dispersal… a barrier, right?  Exactly analogous to the grassy fields and gravel roads that Arthur Cain and the ecological geneticists focused so much of their attention upon in the English countryside, yes?  Back in 1978, I marked Bovbjerg’s paper with a little yellow sticky-tab.

On your mark!  Get set...

Marked with an orange sticky-tab in my intrapopulation dispersal file was a section from the 1978 dissertation of Mancini, involving a Pleurocera (“Goniobasis”) semicarinata population inhabiting a small Indiana stream [7].  Between September 1974 and January 1976, Mancini performed 10 release experiments, each two months in duration, involving 100 marked snails.  The fourth and fifth columns of the author’s Table 7 (reproduced below) show net migration, combining both upstream and downstream movement.  The mean over all five summer values was a modest 4.0 cm per day net upstream movement OTSIWATFA, and the winter mean a very modest +1.9 cm per day.

At the top of my intrapopulation dispersal stack, marked with a red sticky-tab, was the 1966 study of Crutchfield [9] on the Pleurocera proxima population of a North Carolina stream – the same study organism I had targeted for my own dissertation research, in exactly the same environment.  His was a single-release study of 15 week’s duration, between late December and early April of 1958.  Of the 53 snails Crutchfield marked in December, only 5 could be relocated in April, a result the author correlated to rains and high water.  All five snails had moved upstream, at a median distance of 55 feet (16.0 cm/d OTSIWATFA), ranging from 20 feet (5.8 cm/d) to “>100” feet (>29 cm/d)

Perhaps unsurprisingly, however, the best studies of freshwater gastropod dispersal published (as of my years in graduate school) focused on the medically-important planorbid Biomphalaria [10].  Adult Biomphalaria are adapted for lentic waters, not lotic, bearing clunky, bulbous shells typically enfolding an air bubble.  But the mark-release experiments of Paulini (for example), conducted in a ditch with a gentle current of 10 cm/sec, yielded a mean upstream dispersal rate of 130 cm/day OTSIWATFA.  After 6 days, the population mode was 15 meters upstream from Paulini’s release point.

In higher currents, the likelihood of dislodgement seems to become quite significant in Biomphalaria.  Pimentel’s mark-release experiment in a rapid Puerto Rican stream returned a net downstream transport that initially averaged 55 cm/day OTSIWATFA, as against only 36 cm/day upstream, for his first week of observation.  The same downstream-bias continued until observations ended at day 42, although at about half the initial rate.

The most refined study of freshwater gastropod dislodgement of which I am aware is Dussart’s [11] comparison of Biomphalaria with six other pulmonates, conducted in a transparent pipe.  He introduced each snail into a gentle current, allowing them to crawl directly on the PVC walls, and (of course) they all oriented upstream.  Gradually increasing the flow rate, he recorded the time at which each snail became dislodged and calculated a peculiar statistic (cubic centimeter minutes) measuring the total flow to which each snail had been exposed.  The most important variable predicting mean detachment flow (in cc.m) turned out to be profile area of the shell, rather than overall mass or foot size.

From Mancini [7]

One of the more memorable field experiences of my undergraduate education at Virginia Tech took place over the 24 hours I spent with a graduate student named Jim Kennedy sampling macroinvertebrate drift from the New River bridge in Fries, Virginia.  Jim had rigged a battery of plankton nets to suspend in the water column under the bridge, and we checked them every hour through one very cold winter afternoon in 1977, the night, and the morning that followed. 

Freshwater gastropods are recovered from such nets at a surprisingly high frequency.  The most spectacular case of which I am aware was documented by P. C. Marsh [12], working in a small Minnesota Creek draining 17.6 square kilometers of extensively-ditched agricultural lands.  Marsh reported collecting at least a few Physa [13] in his drift nets at each of 10 sampling periods over the course of 20 months.  Heavy rains and high discharge seem to have been responsible for a peak mean of 326 snails/net/day on one sample Marsh collected in early April. 

But in late May a second peak in snail count was observed, this unaccompanied by rain.  Marsh’s average catch of 1,398 snails/net/day translated to an eye-popping 533,000 snails/cubic meter discharge/second, averaged over the entire day of observation.  This figure represented at that time (and may still be) the highest macroinvertebrate biomass ever documented from stream drift sampling.  Marsh speculated that this drift even may have been triggered by crowding and competition in the rapidly-growing Physa population upstream.

Marsh’s literature review found three studies of macroinvertebrate drift published between 1944 – 1980 containing quantitative data on gastropods [14].  Sitting in my carrel at the University of Pennsylvania Library, I was able to add a 1981 paper by Waters [15] and an excellent study by McKillop and Harrison [16] on the Caribbean Island of St Lucia, published in 1982.

New River Bridge at Fries, VA [17]

McKillop and Harrison positioned standard dip nets at the outfalls of several small, enclosed dasheen (or taro) marshes, recording total captures at 06:00 and 18:00 hours for 11 days during the month of April [19].  Summing all 11 samples taken at 18:00 hours, the authors recorded 29 individual Biomphalaria glabrata and 40 individuals of the little hydrobioid Pyrgophorus parvulus.  Totaled across all samples drawn at 06:00, McKillop and Harrison tallied 59 B. glabrata and 114 P. parvulus.  The authors compared the size frequency distributions of drifting Biomphalaria and Pyrgophorus to those of the resident populations and found that drift contained significantly increased frequencies of the smallest size classes.

The data of McKillop and Harrison demonstrate one of the best-documented characteristics of macroinvertebrate drift, diel periodicity.  At night, especially on the new moon, stream organisms actively enter the water column at an increased frequency.  So as was the case with the Physa population in the Minnesota ditches, the data of McKillop & Harrison suggest that at least some of the Biomphalaria and Pyrgophorus inhabiting St. Lucia marshes intentionally release into the water currents.

For eleven years, ecology students at the University of Cape Town, South Africa [20] sampled five sites along the 11 km Liesbeek River nearby.  The first two sites were near the mountainous source of the river, showing average March current velocities of 61 cm/sec and 47 cm/sec.  Sites 3, 4, and 5 were approaching the coast, with average March current speeds of 41, 30, and 13 cm/s, respectively.  The students first collected invasive Physa acuta at site 5 in 1979.  By 1980 the population had spread 3.1 km upstream to site 4, and in 1981 it first appeared at site 3, another 1.8 km upstream.  Thus, over two years, P. acuta displayed a minimum net upstream movement of 4.9 km, or 671 cm/d.  Active dispersal cannot account for such a pace.

The Physa population became established at sites 3, 4, and 5 over the next seven years, in some places reaching densities in the hundreds per square meter, but as of 1988 had not spread upstream to site 2.  These observations are all consistent with an hypothesis that the Liesbeek River Physa population has relied on avian transport, bait bucket hitchhiking, or some similarly passive agency for upstream dispersal.  My readership is referred back to my 2016-17 series of essays on the arial dispersal of freshwater gastropods [21] for further development of this interesting theme.

The bottom line for this month is that the gastropod populations inhabiting lotic environments move.  Their movement upstream is slow but measurable, while their movement downstream is rapid and episodic.  What are the evolutionary consequences?  Might these two processes be sufficient to maintain panmixia, contrary to King Arthur’s lesson?  Tune in next time.

Notes

[1] For more about this “polymath, and one of Britain’s leading evolutionary biologists,” see:

• Clarke, B.C. (2008) Arthur James Cain, 25 July 1921 – 20 August 1999.  Biographical Memoirs of Fellows of the Royal Society 54: 47 – 57.

 [2] My favorites from Arthur Cain’s bibliography:

• Cain, A.J. and P.M. Sheppard (1954) Natural selection in Cepaea.  Genetics 39: 89 – 116.
• Cain, A.J. and P.M. Sheppard (1954) The theory of adaptive polymorphism.  American Naturalist 88: 321 – 326.
• Cain, A.J. and P.M. Sheppard (1956) Adaptive and selective value.  American Naturalist 90: 202-203.
• Cain, A.J. and J.D. Currey (1963) Area effects in Cepaea.  Philosophical Transactions of the Royal Society Series B 246: 1 – 81.

[3] The masterful review of J.S. Jones and colleagues might fairly be said to sum up the entire research corpus of the British school of ecological genetics:

• Jones, J.S., B.H. Leith, and P. Rawlings (1977) Polymorphism in Cepaea: A problem with too many solutions?  Annual Review of Ecology and Systematics 8: 109 – 143.

[4] No, I did not try to work out the phylogeny of the Pleuroceridae in my dissertation!  That is the LAST thing, not the first thing.  I haven't gotten there yet, and it doesn't look like I ever will.

[5] Bovbjerg, R.V. (1952)  Ecological aspects of dispersal of the snail Campeloma decisum.  Ecology 33: 169 – 176.

[6] OTSIWATFA = Of The Snails I Was Able To Find Again.

[7] Eugene Mancini’s 1978 dissertation was an old-school gem packed with great observations on pleurocerid biology.  I first heard about it from Steve Chambers [7], and in March of 1981 wrote a long letter to Dr. Mancini, then working at Woodward-Clyde Associates in California, to request a copy.  He cordially complied in April.  I was so impressed by the 93 page work that I showed it to my mentor George Davis, at that time editor of Malacologia.  George agreed with me that it should be published, and so I wrote a second letter to Mancini, thanking him for his kindness, complimenting him on his work, and offering a prescription of modest edits by which we felt it could be published in Malacologia.  I never heard from him again.

• Mancini, E.R. (1978)  The biology of Goniobasis semicarinata (Say) in the Mosquito Creek drainage system, southern Indiana.  Ph.D. Dissertation, University of Louisville.  93 pp.

[8] Steve was an important influence on my young career, and a good friend.   See

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

[9] Crutchfield, P.J. (1966)  Positive rheotaxis in Goniobasis proxima.  Nautilus 79:80 -86.

[10] My favorite references on intrapopulation dispersal in Biomphalaria:

• Pimentel, D., P.C. White & V. Idelfonso (1957) Vagility of Australorbis glabratus intermediate host of Schistosoma mansoni in Puerto Rico.  Am J Trop Med Hyg 12: 191 – 196.
• Scorza, J.V., J. Silva, L. Gonzalez, & R. Machado (1961) Stream velocity as a gradient in Australorbis glabratus (Say, 1818).  Zeitschrift fur Tropenmedizin und Parasitologie 12: 191-196.
• Paulini, E. (1963) Field observation on the upstream migration of Australorbis glabratus.  Bull WHO 29: 838 – 841.
• Etges, F.J. & L.P. Frick (1966) An experimental field study of chemoreception and response in Australorbis glabratus (Say) under rheotactic conditions.  Am J Trop Med Hyg 15(3):434-438.

[11] Dussart. G.B.J. (1987) Effects of water flow on the detachment of some aquatic pulmonate gastropods.  American Malacological Bulletin 5: 65 – 72.

[12] Marsh. P.C. (1980) An occurrence of high behavioral drift for a stream gastropod.  American Midland Naturalist 104: 410 – 411.

[13] Marsh referred to his study organism as “Physa gyrina,” but I’m sure my readership will agree with me that Physa acuta is a much more likely identification.

[14] Papers documenting freshwater gastropod drift, from Marsh [11]:

• Dendy, J.S. (1944) The fate of stream animals in stream drift when carried into lakes.  Ecological Monographs 14: 333-357.
• Logan, S.M. (1963)  Winter observations on bottom organisms and trout in Bridger Creek, Montana.  Trans Am Fish Soc 92: 140 -145.
• Clifford, H.F. (1972) Drift of invertebrates in an intermittent stream draining marshy terrain of west-central Alberta.  Can J Zool 50: 985 – 991.

[15] Waters, T.F. (1981) Drift of stream invertebrates below a cave source.  Hydrobiologia 78: 169 – 175.

[16] McKillop. W. & A. Harrison (1982)  Hydrobiological studies of eastern Lesser Antillean Islands.  VII. St. Luca: Behavioral drift and other movements of freshwater marsh mollusks.  Archiv fur Hydrobiologie 94: 53 – 69.

[17] You are looking at the only straight patch of road in all of Grayson County, Virginia.  Jim and I spent 24 hours living in a van, parked down by the river at left.  About 3-4:00 PM a good old boy, driving a jacked-up Plymouth, stopped by for a chin-wag.  As he departed, he boasted that he could “get rubber in four gears” across that bridge, in the roughly 300 yards from the store at left to the mountain wall at right.  I’m not sure how he knew he could do it [17], but he did it.

[18] But I’ve got a pretty good hunch.

[19] McKillop & Harrison sampled other months, but the diel periodicity is not as dramatic, primarily because of the confounding effects of rain.  Refer to their paper directly for the entire data set.

[20] Appleton, C.C. & G.M. Branch (1989)  Upstream migration by the invasive snail, Physa acuta, in Cape Town, South Africa.  South African Journal of Science 85: 189 – 190.

[21] The phenomenon of aerial dispersal in freshwater gastropods has been reviewed on four occasions in the long history of this blog:

• Freshwater gastropods take to the air, 1991 [15Dec16]
• A previously unrecognized symbiosis? [11Jan17]
• Accelerating the snail’s pace, 2012 [24Apr17]
• Freshwater snails and passerine birds [26May17]