The birth, death, and resurrection of Melania acutocarinata

It was from the pen of our old buddy Isaac Lea [5Nov19], early on a frosty morning in 1841, that Melania acuto-carinata was born, in brief Latinate form, number 46 in a litter of 57 pleurocerid puppies [1].  Among its more famous siblings were #2 castanea [12Nov24], #7 ebenum [3Oct19], #10 clavaeformis [20Feb07, 12Oct09], #25 virgata [9May23], and #37 edgariana [5June20].  Lea followed with more complete English descriptions and figures of the entire litter in 1843 [2].

The type locality of Melania acuto-carinata was vague, “Tenn., Dr Currey.”  We first met Dr. Richard Owen Currey (1816 – 1865), impeccably credentialed with Presbyterian heritage and doctorate from the University of Pennsylvania, back in [3Oct19].  Currey was born and raised in Nashville but moved to Knoxville in 1846 to become “the first person with an earned doctorate to teach science” at East Tennessee University, later the University of Tennessee [3].

Melania acuto-carinata [2]

Lea’s description of the acuto-carinata shell was almost as vague as its type locality: “It seems to be distinct in its large carina which extends over all the whorls, but it is scarcely distinct on the last.”  Nevertheless, the nomen was launched into malacological stardom in 1858 by the English naturalists Henry and Arthur Adams, in their justifiably maligned “Genera of Recent Mollusca” [4].

It seems highly unlikely to me that either of the Adams brothers ever laid eyes on a North American pleurocerid snail in their entire, justifiably-maligned lives.  Tryon [5] reprinted their classification verbatim in 1873, “in order that the insufficiency of their genera may become more apparent from the incongruous assemblage of shells of which they have composed them.” 

 

Among the 16 genera Henry and Arthur recognized in the Melanidae was Io (of Lea) “remarkable for the peculiar elongation of the axis anteriorly, and for the spinose nature of the last whorl.”  Fine.  Then, as a subgenus of Io (good grief!) the Adams brothers proposed, “Elimia” for pleurocerids bearing shells fusiformly ovate, “whorls reticulate or nodulose, carinate in the middle.”  And under Elimia they listed 16 apparently random pleurocerid specific nomina in alphabetical order, the first of which was “acuticarinata, Lea,” without the dash, misspelled, bearing a shell that is neither reticulate nor nodulose.

 

Tryon dismissed the Adams classification from further consideration and assigned acutocarinata to the genus Goniobasis, newly proposed by his mentor Isaac Lea in 1862 [6].  And pretty much the entire malacological community fell in line with Tryon and Lea, most notably Calvin Goodrich [23Jan07], toward whom our attention now turns.

 

In a 1939 paper that should be better known than it is, Goodrich [7] used Goniobasis acutocarinata as an example of “depauperization,” by which he meant “the outward manifestation of disease, accident or malnutrition or a reaction to inimical environment.”  He wrote: 

“In Goniobasis, very loose coiling appears to be a sign of depauperization.  Goniobasis acutocarinata, although described as a species, has not been found in pure colonies. Lea, its author, had only one specimen.  The shell occurs as a rare variant among G. clavaeformis in springs and spring branches of East Tennessee.” 

For that reason, in his influential review of 1940, Goodrich [8] synonymized Melania acutocarinata Lea 1841 under Goniobasis clavaeformis (Lea 1841).  And that should have been the end of it.  Pleurocerid nomina that did not survive Calvin Goodrich’s 1930 – 1950 piecemeal crusade to monograph the family were not, as a rule, listed or figured in the Burch Bible [9].  They have died, been buried, and forgotten, RIP.

 

P. clavaeformis from Burch [9]

And that is indeed what happened to 12 of the 13 specific nomina [10] synonymized under clavaeformis by Goodrich in 1940.  They disappeared, never to be seen again.  But not acutocarinata.

Although generally following Goodrich quite closely, in the 1980s Jack Burch [9] gave Goodrich’s system one great big kick and a bunch of little tiny anonymous tweaks [11].  Out of a sense obligation to the letter of the ICZN law [12], Burch elected to resurrect the Adams brothers’ obscure Elimia in place of the widely used Goniobasis, confounding the work of Lea, Tryon, Goodrich and everybody else over the previous hundred years.  Following Henry Pilsbry [26Jan21] and Pilsbry’s local collaborator Samuel Rhoads [13, 14], Burch selected acutocarinata as the type of the Adams polyglot genus, by virtue of its first place in the alphabetized list of 16 odd lot species included.

 

And among his tweaks, Burch saved Isaac Lea’s acutocarinata.  To be precise, what he did was exhume two acutocarinata specimens from the UMMZ collection (Figs 404 and 405 above) and figure them alongside a singleton specimen of typical clavaeformis (Fig 403), labeling both of the former “E. acutocarinata = ? E. clavaeformis.”  Why did Burch dump 12 of Goodrich’s clavaeformis synonyms and double-figure the thirteenth?  I don’t know.  If it was a coincidence that acutocarinata was the type of his resurrected Elimia, it was certainly a convenient one.  But for whatever reason, Isaac Lea’s 1841 acutocarinata was spared into a kind of taxonomic purgatory, a place of shadows, neither alive nor dead.

 

In retrospect, I myself should have rescued the nomen from the netherworld for use at the subspecies level in my 2011 paper on Goodrichian Taxon Shift [15], later renamed cryptic phenotypic plasticity (CPP) [16].  Longtime readers will remember that I selected three sites in East Tennessee (and one in North Georgia) inhabited upstream by what I was calling, at that time, Goniobasis acutocarinata, in the mid-reaches by what I was calling, at that time, Goniobasis clavaeformis, and downstream by what I was calling, at that time, Pleurocera unciale.  Those sites included the famous [17] Indian Creek tributary of the Powell on the VA/TN border (IC, map way down below) and the even more famous [19] Pistol Creek of Maryville (PC), as well as the (rather obscure) Lick Creek tributary (LC) of the Hiwassee.

 

My allozyme analysis showed that each acutocarinata population was more genetically similar to its downstream clavaeformis population than to any other acutocarinata, and that each unciale population was genetically more similar to its upstream clavaeformis population than to any other unciale.  On that basis I folded both acutocarinata (no surprise) and unciale (big surprise) under clavaeformis, and both Goniobasis and Elimia under Pleurocera.

 

P. clavaeformis in Indian Creek, Va - Tn

I elected, however, to save uncialis/unciale at the subspecific level, as Pleurocera clavaeformis unciale (Hald 1841), both because of its utility to describe a familiar shell form, and its indexing function to the older scientific literature.  Note that subspecific designation does not carry with it any assumption regarding the heritability of the distinguishing characteristics, much less genetic relationships [20].  It’s just useful.

By the same reasoning, I should also have elevated acutocarinata back up from taxonomic purgatory to the subspecific level, as Pleurocera clavaeformis acutocarinata (Lea 1841).  That should have happened back in 2011.  In my own defense, all I can offer is that unciale was recognized at the species level by Goodrich in 1940, and acutocarinata was not, and resurrecting old pleurocerid names from the nether regions is not the direction anybody wants to go.  But I’ve done it subsequently [21].  The indexing utility of Lea’s nomen acutocarinata is just as significant as that of Haldeman’s unciale/uncialis, probably greater, and the shell morphology just as distinctive.

 

Then let it be so, better late than never.  This month I have added a new Pleurocera clavaeformis acutocarinata page to the FWGNA website, and a new photo to the FWGTN gallery, and a new entry in the FWGTN dichotomous key – the second time I have found myself doing this in recent memory [22].  The number of freshwater gastropod species now covered by the FWGNA web resource is 145 + 1 = 146. 

 

And I have dug back through my old collections and re-identified 22 populations of P. clavaeformis from the typical subspecies to acutocarinata.  Almost all of these inhabit small streams south of Knoxville, primarily direct tributaries of the main river or of the Hiwassee.  The Indian Creek population, way up on the Virginia/Tennessee Line, appears to be an outlier.

 

In all my earlier writings on Goodrichian Taxon Shift and CPP, I have tended to favor explanations that are not heritably genetic.  Although I have always left room for natural selection, most of the column inches in the discussion sections of the papers I have published on this subject [23] focus on the (many and striking) laboratory demonstrations of ecophenotypic plasticity in freshwater gastropod shell morphology.

 

Reidentified acutocarinata in blue

Quite recently a third possible explanation has dawned on me, in the specific case of the apparent transition between P. clavaeformis populations bearing shells of the typical morphology, and those bearing shells of the acutocarinata morphology.  Hybridization [24].

Populations of P. clavaeformis reach maximum abundance in streams of moderate size – marginally wadeable, where you’d bait up for redeye bass and bream.  Populations certainly extend upstream into colder waters suitable for trout, however, where in East Tennessee they bump into big populations of P. troostiana [25].

 

It is conceivable to me that Pleurocera clavaeformis acutocarinata may be a clavaeformis x troostiana hybrid.  I don’t have any genetic evidence of that [26].  This is just a speculation, based on the acutocarinata shell, which appears morphologically intermediate between the two species, both in its carination and in its overall length-to-width, body-whorl-to-apex slenderness.

 

But to conclude, I must emphasize.  Whether the shell morphological distinction between the (now three) subspecies of P. clavaeformis results from adaptation, hybridization, or ecophenotypic plasticity is irrelevant to their subspecific identity.  Here we resurrect Lea’s 1841 nomen acutocarinata because it is a useful descriptor of shell form, and because it has appeared frequently in the published literature.  And will appear again.  Stay tuned.

Notes:

[1] 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.

 

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

 

[3] Quoting the Richard Owen Currey entry in the University of Tennessee “Volopedia” archives: “In 1861, Currey entered Confederate service as a chaplain-surgeon. In 1865, he started caring for Union prisoners in North Carolina. He died while working in a disease-infested hospital on February 17, 1865.” He must have been a wonderful man.  It would have been an honor to shake his hand.

 

[4] Adams, Henry and Arthur (1854 - 1858) The Genera of Recent Mollusca Arranged According to Their Organization. In Three Volumes.  London: J. Van Voorst.  The Melanidae is covered in Volume 1, pp 293 – 311.

 

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

 

[6] Lea, Isaac (1862) Description of a new genus (Goniobasis) of the Family Melanidae and eighty-two new species. Proceedings of the Academy of Natural Science of Philadelphia 19: 262 – 272.

 

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

 

[8] Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system.  Occasional Papers of the Museum of Zoology, University of Michigan 417: 1-21.

 

[9] 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).

 

[10] Which I will not list, not even in the footnote of an obscure blog post.

 

[11] For the record, here is the complete text of Burch’s Supplemental Note #23: “Elimia H. and A. Adams (type species Melania acutocarinata Lea 1841 = Melania clavaeformis Lea 1841) is used in place of its better-known synonym Goniobasis Lea 1862 (type species Goniobasis osculata Lea 1862).  The classification of the genus Elimia presented here, and the distribution of the various recognized species and subspecies, is that of Goodrich (1930a, 1936, 1939d, e, 1940d, 1941a, b, c, 1942b, 1944d, 1945, 1950).  No attempt has been made to assess the taxonomic validity of the species and subspecies.”

 

[12] Burch, J. B. 2001. On the genus name Goniobasis (Elimia - Gastropoda: Pleuroceridae) and other recent nomenclatural inconsistencies. Walkerana 12:97-105.

 

[13] Pilsbry, H. and S. Rhoads. 1896. Contributions to the Zoology of Tennessee, Number 4, Mollusca. Proc. Acad. Nat. Sci. Phila. 1896:487-506.

 

[14] Note, however, that Pilsbry never used “Elimia” himself.  His imperial majesty continued to prefer Goniobasis in every paper he ever wrote or edited for The Nautilus.  And all the labels in the ANSP collection read “Goniobasis” to this day.  For example:

• Pilsbry, H. 1916. Goniobasis in western Pennsylvania. Nautilus 30:4-5.

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

 

[16] For an entry into the rather extensive literature of CPP in North American pleurocerid snails, see:

• Goodrichian taxon shift [20Feb07]
• Mobile Basin III: Pleurocera puzzles [12Oct09]
• Pleurocera acuta is Pleurocera canaliculata [3June13]
• Pleurocera canaliculata and the process of scientific discovery [18June13]
• Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]

[17] I was wading in waters of Indian Creek, just like John, when the scales fell from my eyes.  John Robinson and I published a gray-literature report on those findings to the VDGIF in 2007 [18], and I coined the term “Goodrichian Taxon Shift” on this blog.  See:

• Goodrichian Taxon Shift [20Feb07]

[18] Dillon, R. T. & J. D. Robinson (2007b) The Goniobasis ("Elimia") of southwest Virginia, II. Shell morphological variation in Goniobasis clavaeformis. Report to the Virginia Division of Game and Inland Fisheries, contract 2006-9308. 12 pp. [pdf]

 

[19] It was at Pistol Creek that I ultimately discovered a forgotten pleurocerid species cryptic under P. simplex.  See:

• The cryptic Pleurocera of Maryville [13Sept16]
• The fat simplex of Maryville matches type [14Oct16]
• One Goodrich missed: The skinny simplex of Maryville is Pleurocera gabbiana [14Nov16]

[20] Subspecies are populations of the same species in different geographic locations, with one or more distinguishing traits.  For an elaboration, see:

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

[21] I resurrected Isaac Lea’s (1862) lyonii, previously subsumed by Goodrich (1940) under laqueata, as a subspecies of P. troostiana in:

• The return of Captain Lyon [6July20]

[22]  I added a new Pleurocera laqueata castanea page to the FWGNA website last fall, with corresponding entries in the FWGTN species gallery and dichotomous key.  See:

• Reticulate evolution in the North American Pleuroceridae [12Nov24]

[23] Published papers on CPP:

• Dillon, R. T. (2011) Robust shell phenotype is a local response to stream size in the genus Pleurocera (Rafinesque 1818). Malacologia 53: 265-277. [pdf]
• Dillon, R. T., S. J. Jacquemin & M. Pyron (2013) Cryptic phenotypic plasticity in populations of the freshwater prosobranch snail, Pleurocera canaliculata.  Hydrobiologia 709: 117-127.  [pdf]
• Dillon, R. T. (2014) Cryptic phenotypic plasticity in populations of the North American freshwater gastropod, Pleurocera semicarinata.  Zoological Studies 53:31. [pdf]

[24] For more on hybridization in pleurocerids, see:

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

[25] And simplex. And gabbiana.  But I don’t think that is relevant to my argument here.

 

[26] Just the opposite, actually.  In our VDGIF allozyme study [27], John Robinson and I did run gels on N = 31 G. clavaeformis (C1) and N = 30 G. “arachnoidea” (A1 = troostiana) from Indian Creek, finding a little bit of evidence of mixing at some loci, but fixed differences at three others.

 

[27] Dillon, R. T. & J. D. Robinson (2007a) The Goniobasis ("Elimia") of southwest Virginia, I. Population genetic survey. Report to the Virginia Division of Game and Inland Fisheries, contract 2006-9308. 25 pp. [pdf]

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.