Dr. Rob Dillon, Coordinator

Friday, December 6, 2019

On the Trail of Professor Troost

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023b)  On the Trail of Professor Troost.  Pp 35 – 40 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other EssaysFWGNA Project, Charleston, SC.

The Dutch-American geologist Gerard Troost [1] had already led a full life when he stepped off the boat in Philadelphia in 1810.  He had earned a doctorate in medicine from the University of Leyden and done graduate work in crystallography at Paris.  He had been wounded in the Napoleonic Wars, served as chief scientist on an expedition to Java, and was captured and ransomed by privateers, twice, both French and English.  Sometimes we imagine that our lives today are interesting.

Gerard Troost (1776-1850)
In 1812 Troost was elected the first president of the Academy of Natural Sciences, a post he held for five years, tutoring Isaac Lea and Lardner Vanuxem.  Then in 1825 he sailed down The Ohio to “happiness, enlightenment and prosperity” at New Harmony, Indiana, with Thomas Say at his side.  Gerard Troost was not a malacologist, but he certainly had the names of a couple good ones in his Rolodex.

Troost was called away from New Harmony after just two years singing in the choir utopian by the offer of a professorship at the University of Nashville, from whence he was appointed the Tennessee State Geologist in 1831.  These duties sent him on lengthy explorations throughout the Volunteer State, bringing him to the verge of many rivers and streams blessed with rich faunas of freshwater mollusks.  He was often accompanied in the field by Dr. Richard Owen Currey (1816 – 1865), whom we mentioned in October [2], who assumed Troost’s duties as Professor of Geology in Nashville when Troost died in 1850.

Troost apparently began sending shells to his good friend Isaac Lea in Philadelphia very shortly after his 1831 appointment and continued for quite a few years.  Leafing through the Lea bibliography in Scudder [3] I see that at least 25 – 30 new species of unionids Lea described in the 1830s and 1840s were “sent to me by Professor Troost.”  And it was in honor of Professor Troost that Lea described Melania troostiana around 1838ish, approximately.

It will be remembered from last month’s post that through most of his career Isaac Lea was locked in a torrid race for the naming of species, and that the precise dates of his publications, calculated down to the afternoon, mattered a great deal, at least to him.  To establish what any of those dates might actually have been, however, demands scholarship of a higher caliber than the popgun your humble essayist comes packing.

Lea read his initial description of Melania troostiana in brief, Latinate form before the American Philosophical Society on November 4, 1836.  His (more complete, English) description appeared in the Transactions Volume 6 (New Series), Article 1, which (according to Scudder) was “printed and ready for publication” June 15, 1838 [4].  If you download a copy of Transactions Volume 6 from the Biodiversity Heritage Library, however, it very clearly states 1839 on the title page.  And (apparently in error) both Tryon [5] and Goodrich [6] give Lea’s date for the publication of troostiana as 1841.  Burch [7] and Graf [8] harken back to 1838.

But here is the thing that matters to us today.  Melania troostiana was early.  Regardless of its actual publication date, Lea’s description of Gerard Troost’s eponymous pleurocerid certainly preceded the torrent of gastropod nomina that spilled from his pen beginning in 1841 with his “New Fresh Water and Land Shells” series.  If you run your finger down the list of 199 canonical pleurocerid nomina forwarded to us by Goodrich/Burch, troostiana falls out #39 [9]. It was the second species of pleurocerid snail that Lea ever described, that stuck [10].

Lea gave the habitat of M. troostiana as “Mossy Creek, Jefferson County, Ten,” which is quite atypically precise, by 19th century standards.  His description led with “shell elevated” and followed with “thickly striated.”  But in his remarks, he focused primarily on the “sharp carina” demonstrated by the shell, and secondarily on the “numerous striae,” which reminded him of what we call today Pleurocera virginica. 

If you’re not entirely sure about the distinction between a carination and a striation, click the image below for a primer on shell morphology.

I speculate that the nomen “Melania troostiana” receded into obscurity in the 20th century for two reasons.  First, Lea’s 1838 figure doesn’t match any pleurocerid that currently lives or ever has lived in the state of Tennessee especially well, particularly with respect to that dramatic carination that extends from the juvenile into the adult whorls.  Hence all subsequent authors have restricted its range to Mossy Creek.

Pleurocera troostiana (Click for shell terminology)

And second, Mossy Creek is a scummy mess.  I first had the occasion to visit that unfortunate little body of water in the summer of 2011, when I was surveying the Tennessee drainage above Chattanooga for the FWGTN web resource.  The Mossy Creek catchment is a mixture of overly-grazed pastureland and dusty rock quarries, draining directly through Jefferson City into Cherokee Reservoir.  The creek has obviously suffered decades of erosion, sedimentation, and enrichment.  And at none of the (rather few) points of access did I find any pleurocerids whatsoever.

My resolve was reinforced, however, by the troostiana figure in Burch [7], which looked both modern and plausible.  The UMMZ does hold three lots identified as P. troostiana collected from Mossy Creek by somebody named “Andrews.”  I don’t see any collection dates in their online database, but it seems possible to me that Andrews might have been a Goodrich contemporary, and hence his samples might be relatively recent.  And most interestingly, Burch’s figure, almost certainly from one of those UMMZ lots, does not show that weirdly strong carination.

So I returned to Mossy Creek this past spring, and by dint of persistence was able to find exactly N = 6 topotypic specimens of P. troostiana at the Old Andrew Johnson Highway bridge (36.1272, -83.4862).  The shells borne by all six of those specimens were striate, although in one case (T1, above) the striae became obsolete in spots.  Three of the shells matched the figure in Burch quite closely.  And shells of two individuals (T2, above) were so strongly striate that a carination developed reminiscent of Lea’s original figure. figure.

Seven paragraphs ago I wrote that “the” thing that matters to us today is that the nomen, “Melania troostiana” was early.  Let me modify that slightly.  The earliness of the taxonomic act is certainly important.  But just as important is the tremendous variability in shell striation depicted in the figure above.  All four of those shells were borne by a single population of snails inhabiting a single little ten km creek in East Tennessee.  Let that sink in a minute.

Goodrich devoted the entirety of paper number V in his “Studies of the Gastropod Family Pleuroceridae” series [12] to documenting the “transient, sporadic” character of shell “spirals,” or striations.  Selecting “Goniobasis porrecta Lea of the big hillside spring at Cumberland Gap” as one of his many examples, Goodrich observed: 
“No multistriate specimens have been seen as from the type locality, but such individuals amount to 32.4 per cent of seventy-seven shells taken from Gap Spring Creek about four miles below the spring.”
Hey kids, test your memory!  We devoted our entire essay back in August to cryptic phenotypic plasticity in the pleurocerid populations of Gap Spring Creek.  With what name did I identify those populations that Isaac Lea described in 1863 as Goniobasis porrecta?  Answer at footnote [13] below!  Next month, we’ll find out why.


[1] The biographical details for this month’s essay, as well as the striking figure, were extracted from a “Sketch of Gerard Troost,” published anonymously in the June, 1894 issue of The Popular Science Monthly, pp 258 – 264.

[2] It was Currey who sent Isaac Lea the sample of pleurocerids from “Robinson County, Tenn” he described as Melania ebenum in 1843.  See:
  • CPP Diary: What is Pleurocera (aka Melania, aka Goniobasis, aka Elimia) ebenum?  [3Oct19]
[3] Scudder, N. P. (1885)  Bibliographies of American naturalists – II. The published writings of Isaac Lea, LL.D.  Bull. US National Museum 23: 1 – 278.

[4] Lea, Isaac (1838-39) Description of New Freshwater and Land Shells.  Transactions of the American Philosophical Society (New Series) 6: 1 – 154.

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

[6] Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system.  Occas. Pprs. Mus. Zool. Univ. Mich., 417: 1-21.

[7] 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).

[8] Graf, D. L. (2001)  The cleansing of the Augean stables.  Walkerana 12(27): 1 - 124.

[9] The first valid species of pleurocerid snail described by Isaac Lea, by the Goodrich/Burch canon, was Melania acuta (Lea 1830).  This is not the Pleurocera acuta of Say (1821).  This is a “longitudinally undulated and transversely lineated” North Alabama species attributed to Goniobasis by Goodrich, or Elimia by Burch.  It may be a synonym of laqueata (Say 1829).  Or it is possible that Lea's (1830) acuta is a senior synonym of what we are calling troostiana (Lea 1838), in which case we'd have to salt it and butter it as a double-predestinarian hominy under acuta (Say 1821).  Crap, I hate this sort of stuff, and am clean out of patience with it.

[10] Nineteen of the 38 canonical pleurocerid taxa older than 1838 were described by Lea’s nemesis, Timothy Abbott Conrad, which must have ticked him off royally.  Another 12 were from Thomas Say, whom Lea didn’t much care for either, apparently.  See last month's post:
  • Isaac Lea drives me nuts [5Nov19]
[11] I’ll post a dot-map showing the site of my Mossy Creek collection next month.

[12] Goodrich, C. (1935)  Studies of the gastropod family Pleuroceridae – V.  Occasional Papers of the Museum of Zoology, University of Michigan 318: 1 – 12.

[13] I identified the Gap Creek population of G. porrecta as Pleurocera troostiana, of course!  That’s where I’ve been going with this entire essay.  Shame on you for reading this footnote.  Go back and read:
  • CPP Diary: Yankees at The Gap [4Aug19]

Tuesday, November 5, 2019

Isaac Lea Drives Me Nuts

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023b)  Isaac Lea Drives Me Nuts.  Pp 25 – 33 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other EssaysFWGNA Project, Charleston, SC.

He was born the fifth son of an affluent Quaker merchant in the bustling port of Wilmington, DE, and sent to Philadelphia at age 15 to work in the importing and wholesaling business with his eldest brother.  In Philadelphia he struck up a friendship with Lardner Vanuxem (1792 - 1848) also the son of a prominent Quaker merchant, and the pair became interested in natural history together, focusing especially, at this early date, on rocks, minerals, and fossils.  In 1814 both Lea and Vanuxem joined a volunteer rifle company, and were expelled from the Religious Society of Friends wholesale, which was the way their community preferred to do business, in those days.

Isaac Lea (1792 - 1886) [1]
In 1815 Lea and Vanuxem were elected to membership in the Academy of Natural Sciences of Philadelphia, then in existence for three years.  Prominent among the founders of the Academy was, of course, Thomas Say (1787 - 1834), the Father of American Malacology.  The biographical similarities between Say and Lea are striking: both the sons of prominent Philadelphia-area businessmen, both kicked out of the Quakers for volunteering during the War of 1812, both autodidacts.

For this essay I am relying primarily on the scholarly 1885 biography and bibliography compiled by N. P. Scudder [1], with occasional reference to W. H. Dall [2].  And one of the more peculiar aspects of both references is the absence of any exploration of the relationship between Isaac Lea and Thomas Say. On page VIII Scudder recorded: “Mr. Lea remembers that Mr. Say founded his genus Alasmodonta on a single valve which he himself had picked up on the river shore at Chilicothe, Ohio, and which he carried from that place to Philadelphia in his saddle bags.”  And that is (almost) all we know about the relationship between these two giants of American malacology today.

Vanuxem went on to the Paris School of Mines, and from thence to a university professorship.  Lea published his first paper in the Journal of the ANSP in 1817, “An account of the minerals at present known to exist in the vicinity of Philadelphia.”  But he did not pursue an academic degree of any sort [3], marrying the daughter of a prominent publisher and shifting his business to that of his father-in-law.

Scudder attributes the origin of Lea’s malacological interests to the arrival in Philadelphia of two shipments of unionid shells - one from a brother in Cincinnati, the second from the 1825 expedition of Major Long.  And in 1827 Lea published his first malacological paper, “Description of six new species of the genus Unio.”

One cannot help but notice that two years previous, Thomas Say had quit Philadelphia and sailed down The Ohio to help found the utopian community at New Harmony.  Was the malacological awakening of Isaac Lea a coincidence?  Or was there room for just one malacologist in Philadelphia?  Thomas Say had served as the chief zoologist on Major Long’s expedition to the headwaters of the Mississippi, which means, ironically, that Say may have collected the unionid shells that Lea first described, just as Lea claims to have collected unionid shells first described by Say.  The relationships among the 19th-century American malacologists seem to have been a complex thing.

For whatever reason, the explosion of malacology that erupted from the pen of Isaac Lea would continue almost unbated for 50 years.  Ultimately, he published 279 articles, papers, monographs and books, describing as he did 1,842 species.  To wit:

Species described by Isaac Lea [1]. Click for larger.
Through the 1830s, Lea focused his efforts almost exclusively on this first love, the Unionid mussels.  His first big swing at our favorite organisms came in 1841, with the publication of his “New Fresh Water and Land Shells,” in which he described 57 species of Melania, including such notables as M. clavaeformis and M. ebenum [4].  This series (using variants of the same title) continued until 1848.  In the 1850s he seems to have been distracted once again by other taxa, but he came roaring back to the freshwater gastropods in the 1860s, describing scores of additional species and four new pleurocerid genera, including Goniobasis.

I have modified the adjective “unabated” with the adverb “almost” in the sentence two paragraphs above for two reasons: Lea’s extensive European tours of 1832 and 1853.  On his return from the 1832 trip: 
“to his great astonishment he found that advantage had been taken of his absence, which had prevented him from securing his share of the collection of Tertiary fossils of Alabama, made by Dr. Gates … and that the whole of the Philadelphia quota had been placed in the hands of Mr. Conrad, who was not one of the subscribers. Mr. Lea was not made acquainted with the fact until he saw the first numbers of Mr. Conrad's published descriptions.”
This, I infer, was the beginning of a lifelong feud between Lea and Timothy Abbott Conrad (1803 – 1877), the prominent Trenton-area paleontologist and malacologist, also an excommunicated Quaker [5], elected to membership of the ANSP in 1831.  The feud also seems to have involved Thomas Say.  Because when Lea returned from his second European tour in 1853 [6], again quoting Scudder: 
“He found that Mr. Conrad had published “A Synopsis of the family of Naiades of North America” in the early part of the year, which was full of errors both of date and facts.  These were all stated in Dr. Lea's "Rectification," published immediately on his return, in order to correct any false impressions they might have given. He likewise found that Mr. Say had also published a short list of the species, in which he differed much from Mr. Conrad, but he did not give Dr. Lea a single species.” 
By his death at the age of 94 Isaac Lea had become “the Nestor of American Naturalists [7].”  He was president of the ANSP from 1858 – 1863, and (indeed!) elected president of the AAAS in 1860, and apparently lionized in his day.  He cannot be judged otherwise in ours [8].  Had the noun “species” been defined by the community of systematic biologists active in the 19th century, which I don’t think it was, it would have been something like, “a type considered distinct by a competent taxonomist,” which Isaac Lea most certainly was.  I cannot find any evidence that he ever heard of Charles Darwin [9], much less integrated evolutionary thinking into his work, but I honestly don’t think the discipline of malacology met Darwin until well into the 20th century, and it would be unfair to expect otherwise.

Would it be fair to judge Lea through the eyes of a contemporary?  George W. Tryon Jr. (1838 - 1888) was prominent in the generation of malacologists who followed in Lea’s footsteps.  In 1866 Tryon founded the “Conchological Section” of the ANSP, which he directed until his untimely death [11].  And in 1873 Tryon published the first comprehensive monograph of the North American Pleuroceridae (“Strepomatidae”) [12], in the preface of which he thanked his “kind friend [13]” Dr. Isaac Lea “who not only gave me constant access to his noble collection, but on many occasions aided me by comparing specimens and elucidating knotty questions in synonymy.”

G.W. Tryon (1838 - 1888) [11]
So as of 1873, Tryon recognized 464 valid species of pleurocerids [15] in ten genera.  His monograph is a marvelous work of scholarship, to which I refer often, but I just do not have the patience this morning to go through all 435 pages and count all the synonyms under all those 464 species.  Let me select the typical genus Pleurocera, in which Tryon recognized 99 valid species [16].
Thumbing through the 89 monograph pages in which Tryon reviewed the 99 species [17] of Pleurocera that he considered valid as of 1873, I count 49 real synonyms, setting aside alternate spellings.  Those synonyms comprise 29 of Isaac Lea and 20 from eight other pleurocerid researchers combined [18].  So even Tryon, who was hosted by Lea in his collection and instructed by him personally in the finer points of synonymy, considered that his kind friend, seated in bearded eminence at the high table over his right shoulder [19], had screwed up 29 times in the genus Pleurocera alone.

And the problem is even worse than it looks.  Of the 99 Pleurocera species considered valid by George Tryon, 71 were described by Isaac Lea.  If Lea had 29 more rings than he had pegs to throw them on, one would expect that he would toss them on his own pegs 71/99 = 72% of the time, and on anybody else’s pegs the remaining 28% of the time.  But in fact, only 14/29 = 48% of Lea’s synonyms were self-synonyms according to Tryon, while 15 of Lea’s synonyms were cast on the (just 28) species previously described by somebody else.

Did Isaac Lea willfully ignore the work of his colleagues?  Or was he just too busy talking to listen to anybody else?  Either way, this drives me nuts.

Well, although Calvin Goodrich (1874 - 1954) did not monograph the Pleuroceridae in any formal sense [20], through the 1930s and 1940s he effectively pared the American list of pleurocerid species and subspecies down to 199, of which a mere 93 were Isaac Lea’s.  Those 199 were sanctified by Burch [21] into holy scripture.  How many of the 93/199 = 47% of all the canonical species of North American pleurocerids described by Isaac Lea might be valid, by modern biological criteria?  We’ve nibbled around that question on several occasions in recent years [22].  We’ll take another nip at it in the next couple essays.


[1] Scudder, N. P. (1885)  Bibliographies of American naturalists – II. The published writings of Isaac Lea, LL.D.  Bull. US National Museum 23: 1 – 278.

[2] Dall, W. H. (1888)  Some American Conchologists.  Proc. Biol. Soc. Washington 4: 95 – 134.

[3] One usually sees him referred to as “Dr.” Isaac Lea.  Harvard College bestowed an honorary LL.D. on Lea in 1852.

[4] Lea. I. (1841)  New fresh water and land shells.  Proc. Amer. Philos. Soc. 2: 11 – 15.

[5] Conrad was stricken from the roll of The Religious Society of Friends in the same year he was elected to the ANSP, “because of a preference for walking afield to attending religious services.”  The Quakers founded the middle colonies and dominated them, in every way imaginable, for over a hundred years.  But today, even in the Philadelphia metro area, only approximately 1% of adults identify with the Religious Society of Friends, according to the recent Pew survey.  That’s approximately the same proportion as the Hindus.

[6]  I think the biographer Scudder may be confused here.  Conrad’s “Monography of the Family Unionidae” was published between 1835 and 1847.  And Say died in 1834.  So I think all this unionid sturm und drang kicked off after Lea’s first (1832) European tour, rather than after his second tour of 1853. Ironically, Scudder seems to be fussing about Lea fussing about Conrad screwing up dates, as Scudder is screwing up dates.  But at this point, to quote Bill Murray, “It just doesn’t matter.”

[7] This sobriquet comes from Dall [2].  And I confess that I had to google it, too.  In both The Iliad and The Odyssey, Nestor was the wise and ancient king to whom younger warriors, such as Agamemnon and Achilles, turned for advice.  The effect of Nestor’s wisdom was diluted, however, by his boastfulness.  His advice was always prefaced by lengthy accounts of his own heroic actions under similar circumstances in the distant past.

[8] One of the most unpleasant aspects of human nature is that we all set ourselves at the high bench to judge our fellow man.  And historic figures we judge by our own entirely personal, inevitably modern criteria.  So for example, in recent years we seem to have cast Thomas Jefferson into the outer darkness, convicted of the high crime of slavery, his merits as a patriot forgotten.  In the days of my youth, Jefferson was a demigod.  But turning back two pages, if you’d asked contemporaries like George Washington or John Adams, they couldn’t stand the jerk.

[9] Scudder chronicles Lea’s European tours of 1832 and 1853 in breathless detail [10], including everybody Lea met, and didn’t meet, and what everybody said to him, and what he said back.  In England he did meet Faraday, Dalton, and Lyell, among many others, but not Darwin.  William Hooker yes, his son Joseph Dalton Hooker no.  Sedgwick yes, Huxley no.

[10] Here’s an example: “Calling at the Museum of Economic Geology, Dr. Lea found Sir Henry De la Beche busily engaged. He expressed much pleasure at seeing him again. While conversing with him a very old gentleman came in whom he introduced as Mr. Weaver. He was entirely deaf, and Dr. Lea had to write his part of the dialogue.  He was ninety-one years old, was perfectly erect, and had a remarkably fine face. Dr. Lea was very much pleased to see him, for he was of the old school of science, and was a fellow-student with Humboldt. He was kind enough to say that he had heard in the hall that Dr. Lea was in the private room with Sir Henry, and he could not refrain from coming to shake hands with one who had done so much for American science.”  Don’t you wish that Scudder had chronicled anything like that detail for the relationship between Isaac Lea and Thomas Say?

[11] Ruschenberger, W.S.W. (1888) A biographical notice of George W. Tryon, Jr. Proceedings of the Academy of Natural Sciences of Philadelphia 40: 399 – 418.

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

[13] Tryon was of Lutheran heritage, converted [14] to the Quakers in 1853, and then to the Unitarians in 1876, according to Ruschenberger [11].  So, the noun “friend” may have carried special meaning between Tryon and Lea in 1873.

[14] Tryon would have been 23 years old when war broke out between the states.  So, his transitory affiliation with the Quaker faith is not difficult to understand.

[15]  The number 464 was given by Tryon in his introduction.  The actual number of pleurocerid species Tryon ultimately recognized in his monograph seems to have been more, but I don’t have patience to hand-count.

[16] Tryon quoted 84 species of Pleurocera in his introduction, but by actual count there are 99.

[17] The 99 species included Pleurocera leaii Tryon, 1861.

[18] Anthony = 9, Menke = 3, Say = 2, Reeve = 2, one each for Brot, Hald, Adams, and Wood/Ward.

[19]  Yes, Isaac Lea, 46 years Tryon’s senior, predeceased him by just two years.  Dall [2] termed the first period of American malacology the “Sayian” and the second period the “Gouldian,” with only Isaac Lea surviving to link the two.

[20]  To learn more about Calvin Goodrich, I would be most gratified if you would purchase FWGNA Volume 3 [html] and read the first ten pages.  Or you could take the easy way out:
  • The Legacy of Calvin Goodrich [23Jan07]
[21] 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).

[22]  Lea hit clavaeformis but whiffed on acutocarinata and ebenum in 1841:
  • Goodrichian taxon shift [20Feb07]
  • CPP Diary: What is Pleurocera ebenum? [3Oct19]
He got a solid hit in 1862:
  • One Goodrich Missed: the skinny simplex of Maryville is Pleurocera gabbiana [14Nov16]
But went 0-for-4 in 1863:
  • CPP Diary: Yankees at The Gap [4Aug19]

Thursday, October 3, 2019

CPP Diary: What is Pleurocera (aka Melania, aka Goniobasis, aka Elimia) ebenum?

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023b)  CPP Diary: What is Pleurocera (AKA Melania, AKA Goniobasis, AKA Elimia) ebenum?  Pp 17 – 24 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other EssaysFWGNA Project, Charleston, SC.

Looking back on my long and checkered career as a malacologist, I think I first developed my mental image of Pleurocera ebenum on a brief side trip to the Falls of the Cumberland in the summer of 1988.  There in the Kentucky State Park, above the falls only – not below – I found the rocks covered with pale, large-bodied pleurocerids bearing heavy, triangular shells that I assumed must be P. ebenum [1].  See the example at far right below.

Well, they certainly matched Figure 370 [2] in Burch [3], second from left below.  And Burch must have Goodrich’s collections at his fingertips for reference, yes?  And Goodrich [4] specifically listed “Cumberland River above the Falls” in the range of Goniobasis (aka Elimia, aka Pleurocera) ebenum, yes?

From left: Lea's original figure of M. ebenum [9], Tryon from Burch [2], Branson [6], fresh from the Falls of The Cumberland
So the FWGNA Project began our initial surveys of the Cumberland drainages about ten years ago, and in the last couple years, as we have sewed up the Green River and the Kentucky River further north, The Cumberland has increasingly come into focus.  Branson [5] promised us that P. ebenum would be “common” in Kentucky, publishing the (rather shabby) figure 27 third from left in the figure above.  And this is the description Branson provided in his key to identify it [6]
“Upper whorls without carinae on periphery of whorls; shell obtusely conical, smooth, spire relatively short; aperture often with purplish tinge within; Cumberland River system”
So indeed, as our survey has progressed, we have found populations of pale, large-bodied pleurocerids bearing heavy shells “obtusely conical, smooth, spire relatively short” in mid-sized rivers and streams all over the Bluegrass State, and Middle Tennessee as well.

And almost everywhere we found populations of P. ebenum, we discovered smaller, darker-bodied populations of Pleurocera simplex in small creeks upstream, bearing gracile shells with higher, more slender spires, convex even to the point of being teardrop-shaped in their outline.  Scroll down to Figure J at the bottom of last month’s post to see a typical example.  Upon careful study, we simply have not been able to draw a line between simplex populations upstream and ebenum populations downstream, anywhere.  It has become apparent that the simplex/ebenum relationship must be yet another case of cryptic phenotypic plasticity (CPP), and that ebenum (Lea 1841) is best considered a subspecies [7] of simplex (Say 1825), following the model laid down for us in the Duck River Lithasia by Goodrich [8].

It was only very recently, quite late in the entire process, that I actually read Isaac Lea’s 1843 description [9] of Melania [10] ebenum
“Shell smooth, obtusely conical, thick, black; spire obtuse; sutures small; whorls somewhat convex; aperture rather large, ovate, subangular at base, within purplish”
The “habitat” Lea gave surprised me, a bit: “Robinson County, Tenn. Dr. Currey.”  Tennessee has no constituent “Robinson” County, but a Robertson County was organized in 1796 about ten miles north of Nashville.  The county lies on a plateau draining west to the Red River, entering the Cumberland at Clarksville, about 65 miles downstream from Nashville.  There isn’t much mid-sized riverine habitat especially suitable for P. ebenum in Robertson County.

I was also surprised by Lea’s remarks that his M. ebenum were “very dark-coloured” with “convex” whorls and “usually purplish on the whole of the inside of the aperture.”  Those characters sound much, much more like typical Pleurocera simplex to me.

M = Springfield, N = Tyree Springs, O = Shackle Island
Did Lea have pale, triangular, heavily-shelled specimens on his desk when he described his Melania ebenum in 1841 [9]?  That would make the taxon a subspecies of P. simplex today.  Or were Lea’s specimens indeed dark, gracile, and teardrop shaped?  Is it possible that Lea’s ebenum is just a simple synonym of P. simplex today, rightly consigned to the dustbin?

So the evening of 15May19 found me checking into a cheap motel in Goodlettsville, north of Nashville, with big plans to find Isaac Lea’s type locality on the morrow.  The “Dr. Currey” referred to by Lea must have been Dr. Richard Owen Currey (1816 – 1865), the prominent Nashville physician impeccably credentialed with a Presbyterian heritage and University of Pennsylvania degree.  So, my plan was to survey the rivers and streams of southern and eastern Robertson County first, closest to Nashville, then expand my search area north and west as necessary.

The morning of 16May19 dawned crisp and clear.  And I enjoyed lovely field conditions surveying the pretty little springs and streams of southern Robertson County, taking three good swings, striking out.  So I headed west, and on my second at-bat, made solid contact in Springfield, the county seat (M, above).   I found three species of pleurocerids inhabiting Sulfur Fork at the US 431 bridge in Springfield, although none common: P. laqueata, P. troostiana, and (sure enough) P. simplex of triangular, heavily-shelled ebenum morphology [11].  Could this be Isaac Lea’s type locality?  If so, the subspecies hypothesis would seem justified.

The only misgivings I continued to harbor about the population at Springfield, however, were that the animals, in both shell and body, were pale brown, with white apertures, as (I have always assumed) typical for ebenum.  They were not the “very dark-colour” specified by Lea, nor did they sport “purplish” apertures nor “convex” whorls.  And Springfield is 30 miles north of Nashville.  Is that too far afield in 1841?

In any case, I turned my pickup back toward Nashville, with other field objectives now rising higher on my agenda.  And my route happened to take me south down TN 258, by an historic marker for Tyree Springs.  And there, at Site N, I stumbled upon a very attractive nominee for the type locality of Melania ebenum.

All I know about the history of the place you can read for yourself on the sign (click for larger).  There doesn’t seem to be any evidence of a resort at the site today – just second-growth forest and the little stream that runs under the road at far left in the photo above.  And the rocks of that little stream are covered with pleurocerids bearing thick, black shells, with obtuse spires, small sutures, whorls somewhat convex, and apertures rather large, ovate, and within purplish [12].  In other words, absolutely typical P. simplex.  If Tyree Springs is (indeed) the type locality of Lea’s ebenum, the simple-synonym hypothesis must prevail.

Tyree Springs is a comfortable day trip via horse-and-buggy from Nashville.  It is easy to imagine that a prominent physician such as Dr. Currey would be familiar with such a place in 1841.  There is just one problem.  Tyree Springs are not located in “Robinson” County nor even in Robertson County.  The springs are located in Sumner County.  The Robertson County line is another couple miles north up the ridge.
Topotype nominees: (M) Springfield, (N) Tyree Springs, (O) Shackle Island
Tyree Springs drain south, through Drakes Creek into the Cumberland River above Nashville.  So more out of intellectual curiosity than anything else, I drove downstream to sample Drakes Creek at the little town of Shackle Island [13], marked O on the map above.  And there I found a sparse population of pleurocerids bearing the paler, heavier shells with flat whorls and white apertures I have always considered typical of the ebenum form.  The Drakes Creek population of P. simplex, from headwaters to mouth, demonstrates a textbook case of cryptic phenotypic plasticity.

As I pointed my pickup back toward home on the evening of 16May19, I admit to a bit of frustration.  I wasn’t any closer to answering my questions about Pleurocera ebenum than I had been at the dawn of that long, lovely day.  In fact, the picture was murkier.

And then it occurred to me that the taxonomic situation mirrored rather beautifully the biological situation.  The taxonomic validity of Lea’s (1841) nomen ebenum is precisely as clear as the morphological distinction of those pleurocerid populations to which the name has subsequently been applied.

In the end, I resolved to follow the rule of Sunday afternoon.  The subspecies hypothesis as forwarded to us by Tryon, Goodrich/Burch, and Branson is the call on the field.  Against which there is not enough evidence to overturn.


[1] Actually, the Pleurocera population at the Falls of the Cumberland is a mixture of P. ebenum and P. semicarinata.  And the two species are difficult to distinguish here.

[2]  The great majority of the figures in Burch [3] are obviously from the pen of Mr. John Tottenham, an accomplished scientific illustrator specifically engaged for the project.  But one of the mysteries of Burch’s work is why many of his pleurocerid figures, in particular, were borrowed from elsewhere.  Burch reproduced his figure of “Elimiaebenum from Tryon’s (1865-66) Monograph of the Family Strepomatidae.

[3] This is a difficult work to cite.  J. B. Burch's North American Freshwater Snails was published in three different ways.  It was initially commissioned as an identification manual by the US EPA and published by the agency in 1982.  It was also serially published in the journal Walkerana (1980, 1982, 1988) and finally as stand-alone volume in 1989 (Malacological Publications, Hamburg, MI).

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

[5] Branson, B.A. and D.L. Batch (1987)  Distribution of aquatic snails (Mollusca: Gastropoda) in Kentucky with notes on fingernail clams (Mollusca: Sphaeriidae: Corbiculidae)  Trans. Ky. Acad. Sci. 48: 62 – 70.

[6] Branson, B. A. (1987)  Keys to the aquatic Gastropoda known from Kentucky.  Trans. Ky. Acad. Sci. 48: 11 – 19.

[7]  I would be most gratified if you purchased my FWGNA Volume III (Prosobranchs) [html] and read pp 77 – 91.  Or you might certainly take the easy route by clicking:
  • What is a subspecies? [4Feb14]
  • What subspecies are not [5Mar14]
[8] Goodrich, C. (1934) Studies of the gastropod family Pleuroceridae - I. Occasional Papers of the Museum of Zoology, University of Michigan 286:1 - 17. For more, see:
  • CPP Diary: The spurious Lithasia of Caney Fork [4Sept19
[9] Isaac Lea published his brief, Latinate description of Melania ebenum in 1841: “New Fresh Water and Land Shells,” Proceedings of the American Philosophical Society 2: 11 – 15.  That paper he followed with a larger paper, offering more complete English descriptions, in 1843: “Description of New Fresh Water and Land Shells” Transactions of the American Philosophical Society 8: 163 – 250.

[10] Isaac Lea originally described ebenum in the genus Melania.  He created his genus Goniobasis in 1862, formally transferring ebenum and scores of additional pleurocerid species into it.  I subsumed Goniobasis under Pleurocera in 2011.  For more, see:
  • Goodbye Goniobasis, Farewell Elimia [23Mar11]
[11] Perceptive readers might note that in Springfield, a pale, robust, heavily-shelled population of the ebenum form seems to inhabit a mid-sized stream without a corresponding dark, lightly-shelled simplex population upstream.  Maybe there is, or was, and I missed it?  Or maybe the Springfield ebenum population has colonized Sulfur Fork from below?

[12]  In the interest of full disclosure, it looks to me as though at least 85% of the pleurocerid population at Tyree Springs are smallish, dark Pleurocera laqueata, and another 10% or so are smallish, dark P. troostiana.  And the apical sculpture on the shells of those two populations is not strong, in many cases.  And even I had a hard time distinguishing the Tyree simplex from the laqueata.  But I refuse to entertain the possibility that Lea’s ebenum might be a junior synonym of Say’s (1829) laqueata or anything else.  Not gonna do it.  Wouldn’t be prudent.

[13] To be only slightly more precise.  My site (O) was located in Drakes Creek at the perfectly-manicured campus of the most grotesque megachurch I have ever seen.  I counted a staff of 18 sweetly-smiling pastors and 3 directors on their website, not counting the office support, building and grounds staff, rock musicians and stagehands.

Wednesday, September 4, 2019

CPP Diary: The spurious Lithasia of Caney Fork

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023b)  CPP Diary: The Spurious Lithasia of Caney Fork.  Pp 9 - 15 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other EssaysFWGNA Project, Charleston, SC.

The Duck River is our Galapagos, and Calvin Goodrich our Darwin.  Or possibly our inverse-Galapagos, and our anti-Darwin, I’m not sure.  For the brilliant evolutionary insight that Goodrich glimpsed in 1934 through the lens of this rich, fresh waterway coursing through the heart of middle Tennessee was not more species, but less.

It’s an old, old story [1], but let’s tell it again.  Prior to the dawn of the modern synthesis, North American freshwater malacology recognized at least 12 – 15 species of pleurocerids in the Duck River, probably more.  The lower reaches of the Duck (e.g., Wright Bend site E) were inhabited by heavily-shelled, “obese” populations identified as Lithasia geniculata, distinguished by their shells with prominently-shouldered whorls.  The slightly-less-obese, smooth-shouldered populations of the middle reaches (e.g., US41A, at site C) were identified as Lithasia fuliginosa.  And the headwaters of the Duck River (e.g., Old Fort site B) were inhabited by Anculosa pinguis, lightly-shelled populations without any shoulders on their whorls at all.

CPP in the Lithasia geniculata of the Duck River

In 1934, Goodrich published his “Studies of the Gastropod Family Pleuroceridae I,” in which he synonymized all these forms as subspecies under L. geniculata [2].  He just did it, at the top of the section, without making any sort of declaration, or using any form of the noun “synonym,” as though his unique insight were already an article of established malacological doctrine [3].  He then meticulously documented, town to town and bridge to bridge down the length of the Duck, the gradual transition of the three subspecies from one form to the next.

Even to me, his latter-day apostle, Goodrich’s 1934 intuition about the plasticity of shell phenotype in freshwater gastropods was startlingly profound.  His “Studies in the Gastropod Family Pleuroceridae” series inspired me to coin the term “Goodrichian taxon shift” in his honor in 2007, subsequently generalized to cryptic phenotypic plasticity (CPP) by Dillon, Jacquemin and Pyron in 2013 [4].

So, six years later, Goodrich published a paper reviewing the taxonomy and distribution of the pleurocerid fauna of the Ohio River drainage in its entirety, not just the Lithasia of the Duck River but all species in all seven genera in a vast region touching 14 states [5].  And the only other population of Lithasia geniculata pinguis of which he was aware inhabited the Caney/Collins drainage of the Cumberland River, the headwaters of which interdigitate with the Duck immediately to the east, in the vicinity of McMinnville.

But alas, even in Goodrich’s day the diverse pleurocerid fauna of the Tennessee/Cumberland was rapidly disappearing in the face of impoundment, canalization, and widespread development for navigation and hydropower across the southern interior.  The Caney/Collins system was terribly impacted by the impoundment of Center Hill Lake in the late 1940s, and the same almost happened in the Duck River in the 1970s, themes to which we shall return.  Motivated by longstanding conservation concerns, in 2003 our colleagues Russ Minton and Chuck Lydeard undertook to construct a gene tree for the North American genus Lithasia [7].

Russ and Chuck a good job rounding up samples from 11 of the species and subspecies of Lithasia listed by Goodrich/Burch, 25 populations in all, sequencing in some cases as many as 6 individuals per population.  From the Duck River Russ and Chuck sequenced one population of L geniculata geniculata (1 individual), three populations of L. geniculata fuliginosa (1, 3, and 3 individuals), and one population of L. geniculata pinguis (6 individuals).  And they also included 2 individuals of nominal Lithasia geniculata pinguis from a Caney/Collins population.  And here is their gene tree:

Minton & Lydeard [7] Figure 3, modified.

By now my readership will understand gene trees are dependent variables, not independent variables [8].  You cannot work out the evolution of a set of organisms from a gene tree.  But if you have developed an evolutionary hypothesis from good solid data of some broader sort, you may be able to understand what a gene tree is telling you.

To completely unpack the message being telegraphed to us by the enigmatic arboreal specimen figured above would require at least 6 – 8 blog posts of standard length [9].  But for the present let us focus on just the two little branches labelled “geniculata pinguis” that I have circled in red.  The two sequences obtained from the 6 individuals sampled from the Duck River, D1 and D2, cluster with all the other Lithasia.  And the two sequences obtained from the Caney/Collins system, C1 and C2, are way off with pleurocerids of other genera.  To quote Minton & Lydeard verbatim: “Further work needs to be undertaken to determine the identity and placement of the Collins River taxa.”

Thanks, Captain Obvious! If Calvin Goodrich had enjoyed access to collections from the Caney/Collins system of the same quality and detail that he enjoyed for the Duck, he might well have recognized a gradual progression in the shell phenotype of Pleurocera simplex quite analogous to that he documented for Lithasia geniculata in 1934.

Most of the headwaters of Caney Fork and its tributaries (e.g., site J) are inhabited by rather typical-looking populations of the widespread Pleurocera simplex simplex, no different from those one might find in tributaries of the Holston River around Saltville, VA, from whence the species was described by Thomas Say in 1825 [11].  We featured the P. simplex population inhabiting Pistol Creek at Maryville, TN in a series of essays published in 2016 [12], and the P. simplex population inhabiting Gap Creek, TN, last month [13].  All of these populations are darkly pigmented, and bear gracile, teardrop-shaped shells such as shown in figure J below. 

And all those populations inhabit small creeks and streams primarily of groundwater.  In East Tennessee, populations of Pleurocera simplex do not typically extend into larger rivers [14].

CPP in the P. simplex of the Caney/Collins

But in tributaries of the Cumberland, Kentucky, and Green Rivers, Pleurocera simplex populations often do extend into rivers of substantial size – as long as the currents are good and the rocky substrate they require does not entirely give way to mud.  Here their shells become heavier, chunkier, and more lightly-pigmented.  Goodrich [5] identified paler, heavier-shelled populations such as are found in the Collins River at site K as “Goniobasis ebenum (Lea 1841).”

And in the largest rivers of the Caney/Collins system (e.g., site L), populations of P. simplex are so robustly shelled that they can easily be confused for Lithasia geniculata pinguis.  I speculate it may have been a sample of P. simplex that Minton & Lydeard collected from the Collins River back in 2003, demonstrating intrapopulation morphological variance so extreme as to prompt an (erroneous) identification of Lithasia.

To be clear.  Bona fide populations of Lithasia geniculata pinguis do indeed inhabit the Caney/Collins system, as Goodrich stated.  But they are typically sparse, and often swamped by dense populations of P. simplexPleurocera simplex populations do not range west into the Duck River drainage, however, and so confusions of this sort did not complicate the story Goodrich told us in 1934.

So broadening the subject out through the rest of Middle Tennessee and into Kentucky.  What is this enigmatic taxon described by Isaac Lea in 1841, "Melania" (aka Goniobasis, aka Elimia, aka Pleurocera) ebenum?  Goodrich [5] identified ebenum populations through most of the Cumberland River drainage, from “Cumberland River above the falls” through “Smith’s Shoals, Pulaski County, Kentucky” west beyond Nashville to “springs and small streams” in Dickson County, Tennessee.  Could all these populations that Goodrich called "Goniobasis ebenum" be pale, triangular, robustly-shelled P. simplex?  Stay tuned.


[1] The best entry into this literature would be to purchase FWGNA Volume 3 [html] and read pages 1 – 10 and 93 – 99.  Or you could click through it piecemeal:
  • The Legacy of Calvin Goodrich [23Jan07]
  • Goodrichian Taxon Shift [20Feb07]
  • Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]
Both of those latter two essays feature scans of Goodrich’s Plate 1, showing the Duck River Lithasia.

[2] Goodrich, C. (1934) Studies of the gastropod family Pleuroceridae - I. Occasional Papers of the Museum of Zoology, University of Michigan 286:1 - 17.

[3] This is actually a bit frustrating.  Looking back on Goodrich’s body of work, there is almost never anything quotable – some “Aha moment” where the fullness of his vision is revealed.  Like Charles Darwin.

[4] Dillon, R. T., S. J. Jacquemin & M. Pyron (2013) Cryptic phenotypic plasticity in populations of the freshwater prosobranch snail, Pleurocera canaliculata.  Hydrobiologia 709: 117-127. [pdf]  For more, see:
  • Pleurocera acuta is Pleurocera canaliculata [2June13]
  • Pleurocera canaliculata and the process of scientific discovery [18June13]
[5] Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system. Occasional Papers of the Museum of Zoology, University of Michigan 417: 1 - 21.

[6] 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).

[7] Minton, R.L. & C. Lydeard (2003)  Phylogeny, taxonomy, genetics, and global heritage ranks of an imperiled, freshwater snail genus Lithasia (Pleuroceridae).  Molecular Ecology 12: 75 – 87.

[8] The best entry to this complex and long-running theme would be to read FWGNA Volume 2 [html] in its entirety.  Or for a quick lick at the problem, see my essays:
[9] Actually, I’ve already dedicated one blog post [10] to exegesis of the obovata1/obovata2 branch way down below the Caney Fork sequences.  And maybe in a few months we’ll come back to the geniculata/fuliginosa/duttoniana problem in the Duck River and look at that in more detail.

[10] Dillon, R. T. (2014) Cryptic phenotypic plasticity in populations of the North American freshwater gastropod, Pleurocera semicarinata.  Zoological Studies 53:31. [pdf] For more, see:
  • Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]
[11] Say, Thomas (1825)  Descriptions of some new species of freshwater and land snails of the United States.  Journal of the Academy of Natural Sciences of Philadelphia 5: 119 – 131.

[12] I explored the complex relationship between Pleurocera simplex and P. gabbiana in East Tennessee in a series of three blog posts in 2016:
  • 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]
[13] We opened our series on CPP in Pleurocera simplex last month with:
  • CPP Diary: Yankees at The Gap [4Aug19]
[14] Rocks and riffles in the mid-sized rivers in East Tennessee are often covered bank-to-bank by dense populations of Pleurocera clavaeformis and Leptoxis praerosa, neither of which ranges into drainages of the Cumberland.  I wonder if East Tennessee populations of P. simplex are restricted to smaller creeks and streams by grazing competition?

Sunday, August 4, 2019

CPP Diary: Yankees at The Gap

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023b)  CPP Diary: Yankees at The Gap.  Pp 1 – 7 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other EssaysFWGNA Project, Charleston, SC.

I like Cumberland Gap.  Daniel Boone discovered this hidden doorway through the Cumberland scarp in 1775, and decided to build a road through it, and found a new state on the other side, which he named Kentucky, in honor of his favorite recipe for fried chicken.

The Gap assumed tremendous strategic importance during the War Between the States, changing hands four times.  Confederate forces under the command of Gen. Felix Zollicoffer abandoned it to Union forces under Gen. George W. Morgan in June of 1862, who was himself forced out by Gen. Edmund Kirby Smith’s confederates three months later.  Elements of Smith’s army held the gap 12 months, surrendering it a second time in September of 1863.

Detail from Capt. Lyon's Map [1]
It was during the first northern incursion of 1862 that Captain Sidney S. Lyon of the US Topographical Engineers arrived at Cumberland Gap, uninvited.  Captain Lyon was immediately ordered by General Morgan to draft a map “showing the location of the works constructed by the enemy and those erected by the forces of the United States.”  And at some point during the discharge of those duties Capt. Lyon happened to pass along Gap Creek, a lovely little stream of cold, clear water emerging from a spring above the town of Cumberland Gap, TN, and coursing freshly through its precincts at about point A.  And there he alertly stooped to capture an entire squadron of pleurocerid snails, without firing a shot.

These rebel pleurocerids he dutifully posted back behind the lines to Dr. Isaac Lea at the Academy of Natural Sciences in Philadelphia.  And in May of 1863, a scant nine months later, Lea [2] described four new species of Goniobasis “sent to me from Gap Creek and Spring by Capt. S. S. Lyon, U.S. Army,” as follows: Goniobasis aterina, G. cumberlandensis, G. porrecta, and G. vittatella.

The municipality of Cumberland Gap, Tennessee, certainly must have been a busy and exciting place during those years.  But in 1889 the first of several railroad tunnels was blasted out of the mountains above the town, and an automobile tunnel added alongside in 1996, difficult though these engineering feats certainly are to envision, for those who have not seen them.  So the pretty little town is today located down in a deep hole about a half mile below all modern arteries of commerce, as thousands of vehicles pass through tunnels high above, and nobody stops to consider the possibility that anybody might be living way down in there.

P. "aterina" at Gap Creek
I first visited the town of Cumberland Gap in 2006, in connection with a small grant from the Virginia Department of Game and Inland Fisheries to study several potentially endangered pleurocerids in Southwest Virginia.  I found the rocks of Gap Creek covered with a strikingly high density of small, dark, eroded pleurocerids matching Lea’s figures of aterina, mixed with a smattering of small, dark, eroded pleurocerids matching Lea’s figures of porrecta and vittatella [3]. 

The allozyme data collected by John Robinson and myself [6] strongly suggested that Lea’s aterina was a (chubby, dwarfed) local population of the widespread Goniobasis (now Pleurocera) simplex, and that Lea’s nomina porrecta and vittatella were attached to a (not quite as chubby, but still dwarfed) local population of the widespread Goniobasis arachnoidea (now Pleurocera troostiana).

And in fact, had I sampled Gap Creek further downstream into Tennessee in 2006, the elaborate population genetic analysis undertaken by John Robinson and myself might well have been unnecessary.  The figure below compares typical shells collected at Cumberland Gap (site A) to a couple shells I collected in 2019 at site (B), approximately 5 km south at the state route 63 bridge.  Here Gap Creek has slowed, and warmed, and taken on a richer character more typical of the East Tennessee Ridge and Valley Province.  The populations of both P. simplex and P. troostiana under that bridge bear larger, more gracile shells of completely typical shell morphology.

The pleurocerid populations of Gap Creek display the phenomenon for which the term “cryptic phenotypic plasticity” (“CPP”) was coined in 2013.  They demonstrate intrapopulation morphological variance so extreme as to prompt a (erroneous) hypothesis of speciation. Isaac Lea (and George Tryon right behind him, and Goodrich, and Burch) all thought that the eroded, dwarfed pleurocerid populations in the cold, clear, high-velocity headwaters of Gap Creek were different species than the populations in the richer waters downstream.

CPP in P. simplex and P. troostiana of Gap Ck.
In recent years the phenomenon of cryptic phenotypic plasticity has been shown very-nearly universal in the pleurocerid populations of the Eastern United States [7].  Here in the columns of this blog I have documented CPP in Pleurocera clavaeformis, Pleurocera canaliculata, Pleurocera semicarinata, and Pleurocera laqueata [8].  In the next several essays, I will extend such studies to include two of the most widespread pleurocerids in the southeast, P. simplex and P. troostiana.  And perhaps lighten the burden with a few stories along the way?  Stay tuned.


[1] Map of Cumberland-Gap and Vicinity laid down from Surveys, made by Capt. Sidney S. Lyon, acting Topographical Engineer, under Order of Genl. G. W. Morgan, commd'g. 7th Div., Army of the Ohio. Showing the location of the works constructed by the enemy and those erected by the forces of the United States.  I myself have highlighted Gap Creek in blue.

[2] Lea, Isaac (1863) Descriptions of fourteen new species of Melanidae and one Paludina.  Proceedings of the Academy of Natural Sciences of Philadelphia 15: 154 – 156.

[3]  There are no pleurocerids matching Lea’s figure of cumberlandensis inhabiting Gap Creek as far upstream as Cumberland Gap today.  Tryon [4] synonymized cumberlandensis under Goniobasis adusta.  Goodrich [5] synonymized both adusta and cumberlandensis under the widespread Goniobasis (now Pleurocera) clavaeformis.  And indeed, Gap Creek downstream at site B is inhabited by a P. clavaeformis population of typical shell morphology, as well as the P. simplex and P. troostiana populations that are the subject of the present essay.

[4] Tryon, G. W., Jr. 1873. Land and Freshwater Shells of North America. Part IV, Strepomatidae.  Smithsonian Miscellaneous Collections 253, 435 pp. Washington, D.C

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

[6] 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.  25 pp.  [pdf]

[7] 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]

[8] The most convenient entrance into this rather extensive literature would be to read essays 4, 12, 13, 16, 18 and 19 in:  Dillon, R.T., Jr. (2019c) Essays on The Prosobranchs.  Freshwater Gastropods of North America, Volume 3.  FWGNA Press [html].  Or, if you’d prefer to click your way through it piecemeal:
  • Goodrichian taxon shift [20Feb07]
  • Mobile Basin III: Pleurocera puzzles [12Oct09]
  • Pleurocera acuta is Pleurocera canaliculata [3June13]
  • Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]
  • Pleurocera alveare: Another case of CPP? [7Aug18]
  • Is Gyrotoma Extinct? [5Sept18]