Dr. Rob Dillon, Coordinator

Tuesday, January 7, 2020

CPP Diary: The Many Faces of Professor Troost

Editor’s note: If you have not read last month’s post, read it.  And if you did read last month’s post, go back and read it again. The essay that follows was written under the assumption that the themes developed on 6Dec19 remain fresh in the minds of our readership.

Back in August we introduced the present “CPP Diary” series with an essay focusing on the Gap Creek populations of two freshwater gastropods widespread throughout the Tennessee/Cumberland [1].  In September and October, we explored the phenomenon of cryptic phenotypic plasticity (“CPP”) in one of those, Pleurocera simplex [2].  This month let’s back up and get a fresh start at that other species, shall we?

As I mentioned in August, the mission that sent me on my first visit to Gap Creek, way back in the summer of 2006, was a comprehensive VDGIF-funded survey of the pleurocerid fauna of SW Virginia.  John Robinson and I ultimately identified 83 populations of three species in our five-county study area, including 13 populations of what we were calling, at that time, “Goniobasis arachnoidea.”  We analyzed genetic polymorphism at 11 allozyme-encoding loci in 12 of our 83 populations, including three of the “arachnoidea,” reporting our results to the VDGIF in 2007 [3].

Courtesy of Chris Lukhaup
Almost all our Virginia populations of “arachnoidea” bore slender shells with small body whorls, usually striate, at least near the apex.  Shell (B) depicted second from left below, collected from Gap Creek at the TN 63 bridge, is typical.  But while genetically matching our other arachnoidea populations, the sample we analyzed from the headwaters of Gap Creek bore entirely smooth and strikingly stunted shells as shown at far left (A).  That population is so morphologically distinctive that it was described by Isaac Lea as not one but two unique species: Melania porrecta and M. vittatella [4].  This is an obvious example of CPP.

So to what population of snails, precisely, was the nomen “arachnoidea” originally intended to apply?  John G. Anthony [5] described his Melania arachnoidea in 1854 as “rather thin, spire slender and much elevated, very strongly striated and ribbed,” giving its type locality as “a small stream emptying into the Tennessee River near London, Tennessee.”  This must be a misspelling of Loudon, the town south of Knoxville, where the Tellico Dam [6] backs up the Little Tennessee River from its mouth to Chilhowee.  

I surveyed the precincts of Loudon in the summer of 2007 and happened to sample Steekee Creek at the bridge where it enters the corporate limits (35.7242, -84.3473) on its way to The Tennessee.  There I found, in addition to P. simplex and P. gabbiana, a population of pleurocerids bearing thin shells of slender spire and strong striation such as depicted in figure (C) below. 

Goodrich [7] considered Lea’s (1862) Goniobasis spinella a subspecies of arachnoidea (Anthony 1854).  Lea [8] gave the type locality of spinella as “Sycamore, Claiborne County, Tennessee.  On maps of Claiborne county today one can find a wide place in the road marked “Sycamore Hall,” with a Little Sycamore Creek flowing freshly by just down the hill.  I collected specimen (D) depicted at far right below on May 14 of this year, from Little Sycamore Ck at the Estes Road bridge (36.4534, -83.5076).

(A) Gap Ck upstream, (B) Gap Ck downstream, (C) Steekee Ck, (D) Sycamore Ck
But wait.  Before we plunge any further into the roaring 60’s, we really ought to paddle back about 20 years and pick up Isaac Lea’s (1841) Melania teres and M. strigosa [9].  The descriptions and figures of the two 1841 species were nearly identical to each other, as well as to arachnoidea of 1854 and spinella of 1862.  The shell of teres Lea observed was “remarkably elevated” with “spire drawn out” and “last whorl very small.”  The strigosa shell he also described as “spire drawn out,” remarking as he did that “This species is somewhat like teres herein described.”  Lea did mention “striate above” for strigosa, while not mentioning any shell ornamentation for teres.

The habitat Lea gave for Melania teres (Fig 27 way down below) was just “Tennessee Dr. Troost,” too vague to send us on a hunt today, although Goodrich [7] suggested “Small streams of Walden Ridge, Tennessee, flowing eastward.”  Figure 356 is scanned from Burch [10], presumably collected from one of those Walden Ridge teres populations illustrating Goodrich’s concept of the taxon.

For strigosa Lea did a bit better, “Tennessee Dr. Troost, Holston River Dr. Warder.”  This is probably John Aston Warder (1812 – 1883), who was born in Philadelphia and lived in Cincinnati, but traveled broadly [11].

The individual shell depicted in figure (E) down below was collected from Little Flat Creek at the Emory Road bridge in August of 2010 (36.1411, -83.7961). I am offering that particular population of pleurocerids bearing shells with spires drawn out, striate above, as topotypic for M. strigosa for four reasons.  First, apparently at least some of Lea’s sample(s) came from the Holston River drainage of Tennessee, somewhere.  And second, that entire drainage is a mess.

In the last 10 – 12 years I have surveyed the pleurocerid fauna of the entire seven-county region drained by the Holston in East Tennessee, and have only discovered populations of pleurocerids bearing striate shells with spires-drawn-out in two streams: Mossy Creek in Jefferson County, about which you just read in my essay of 6Dec19 (Don’t tell me that you didn’t) and Little Flat Creek in Knox County.  Since Mossy Creek is already the type locality of M. troostiana, it seems an unlikely nominee for the type locality of M. strigosa.  That leaves Flat Creek as the sole remaining candidate.
A=Gap up, B=Gap down, C=arachnoidea, D=spinella, E=strigosa, F=striatula, T=troostiana

The third reason I am offering Flat Creek as the type locality for M. strigosa is that it is in Knox County, and Goodrich [7] subsequently suggested the range of Goniobasis strigosa as “Small streams near Knoxville, Knox County, Tennessee.”  I’m not sure that amounts to a subsequent restriction of type locality, but I do very much value Goodrich’s opinions on matters of this sort.  And the fourth reason I am offering Flat Creek as the type locality for M. strigosa is simply that the shells of the population of pleurocerids living in that little brook match Lea’s figure.  They are not as strongly striate as the Mossy Creek sample that must have been in Isaac Lea’s collection since 1836.  They are merely “striate above.”  OK, good enough.

So what strikes me most about all these pleurocerid populations – much more than the shell striae – is the character that Lea called “spire drawn out” and that Anthony called “spire slender and much elevated.”  In more modern literature, this character is sometimes measured as shell length-to-width ratio, or the ratio of body whorl length to total shell length, although the statistical analysis of ratios is problematic.  In my own research I have preferred the regression of shell width on length [12], or the regression of apex height on body whorl height [13].  Regardless of what that variable is called, or how it is measured, the heritable component can be significant [14].

I am also stricken by the ecological similarities of all these populations.  All of these nominal species – arachnoidea, spinella, strigosa, and the Gap Creek populations that Lea described as porrecta/vittatella – reach maximum abundances in small, rich creeks.  They essentially disappear from the larger rivers of East Tennessee, not unlike Pleurocera simplex, at least in this part of the world.

In exactly that same environment south of Knoxville Goodrich [7] identified populations of pleurocerids bearing striate shells with small body whorls as Goniobasis striatula (Lea 1841/43).  Lea [15] gave the habitat as just “Tennessee,” no help.  Most interestingly, however, Goodrich [16] also identified pleurocerid populations collected from Coahulla Creek in Whitfield County, Georgia, as G. striatula, shown in figure (F) at far right below.  This is one of the very few elements of the pleurocerid fauna that Goodrich admitted might be shared between Tennessee River drainages to the north and drainages of the Alabama/Coosa river system draining south toward the Gulf.

Lea teres [9], Burch teres [10], Lea strigosa [9], Flat Ck (E), Coahulla Ck (F)
We have met the Coahulla Creek pleurocerid fauna before.  It was from Coahulla Creek in the NW corner of Georgia (34.9731, -84.9505) that I collected the sample of P. carinifera I analyzed in Dillon 2011, together with its control population of P. simplex [17].  The rivers and streams at the top of the Alabama/Coosa drainage are separated from drainages of the Tennessee by a couple kilometers at most.  And the genetic differences I found between Coahulla carinifera and simplex were not dramatically different from clavaeformis and simplex populations I sampled all the way through East Tennessee up into SW Virginia.

I did not gather any genetic data on P. striatula when I was sampling pleurocerids for my 2011 study.  But their similarities with Tennessee populations of arachnoidea, spinella, and strigosa in both ecology and shell morphology are striking, are they not?

All of the names given to all of the populations we have reviewed this month: porrecta (Lea 1863), vittatella (Lea 1863), arachnoidea (Anthony 1854), spinella (Lea 1862),  teres (Lea 1841), strigosa (Lea 1841), and striatula (Lea 1841) were proposed more recently than troostiana (Lea 1838).  And all are the same thing.  Populations of one single, highly variable species of pleurocerid snail, best identified as Pleurocera troostiana, extend down the length of the Tennessee River drainage, from SW Virginia through East Tennessee, and even hop the low hills to the upper Coosa drainage in NW Georgia.

Well, we’re not anywhere near done with the subject yet, but I sense that I’m about to lose my audience, all two of you, so let’s bookmark it here.

But our story will continue onward in future episodes, as does the river, downstream into North Alabama.  By the mid-nineteenth century, the fame of Isaac Lea seems to have spread throughout our entire, muscular young country.  And prominent citizens from Huntsville, Tuscumbia, and Florence, Alabama, were scooping up samples of the local gastropod fauna, drying them on their back porches, and packing them for Philadelphia, no different from the citizens of Knoxville and Nashville.

In our next installment... Huntsville hunt!


[1] CPP Diary: Yankees at the Gap [4Aug19]

[2] Cryptic phenotypic plasticity in Pleurocera simplex:
  • CPP Diary: The spurious Lithasia of Caney Fork [4Sept19]
  • CPP Diary: What is Pleurocera ebenum? [3Oct19]
[3] 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]

[4] 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.

[5] Anthony, J.G. (1854) Descriptions of new fluviatile shells of the genus Melania Lam., from the western states of North America.  Annals of the Lyceum of Natural History of New York 6: 80 -132.

[6] This was the infamous “snail darter” dam, that led (perhaps more than any other public works project) to the crystallization of public antipathy for the impoundment of the free-flowing waters of the USA.  For more, see:
Wheeler, W.B. & M.J. McDonald (1986)  TVA and the Tellico Dam 1936 – 1979.  University of Tennessee Press, Knoxville.

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

[8] Lea, Isaac (1862) Description of a new genus (Goniobasis) of the Family Melanidae and eighty-two new species. Proc. Acad. Nat. Sci., Phila., xiv, pp. 262-272.
Lea, Isaac (1863) New Melanidae of the United States.  Journal of the Academy of Natural Sciences of Philadelphia 5: 217 – 356.

[9] Lea, Isaac (1841) Proceedings o the American Philosophical Society 2: 11 – 15.
Lea, Isaac (1843)  Description of New Fresh Water and Land Shells.  Transactions of the American Philosophical Society 8: 163 – 250.

[10] 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).

[11] Wilson, JG & Fiske, J (1889) Appleton’s Cyclopaedia of American Biography Volume VI. Appleton, NY.

[12] Wethington, A.R., J. Wise, and R. T. Dillon (2009) Genetic and morphological characterization of the Physidae of South Carolina (Pulmonata: Basommatophora), with description of a new species.  The Nautilus 123: 282-292.  [PDF]

[13] Dillon, R. T. & J. D. Robinson (2016) The identity of the "fat simplex" population inhabiting Pistol Creek in Maryville, Tennessee.  Ellipsaria 18(2): 16-18. [PDF]
Dillon, R. T. (2016)  Match of Pleurocera gabbiana (Lea, 1862) to populations cryptic under P. simplex (Say, 1825).  Ellipsaria 18(3): 10 - 12.  [PDF]  For more, see:
  • The Fat simplex of Maryville matches type [14Oct16]
  • One Goodrich Missed: The skinny simplex of Maryville is Pleurocera gabbiana [14Nov16]
[14] Dillon, R. T. & S. J. Jacquemin (2015)  The heritability of shell morphometrics in the freshwater pulmonate gastropod Physa.  PLoS ONE 10(4) e0121962. [html] [PDF]  For more, see:
  • The heritability of shell morphology in Physa h^2 = O.819! [15Apr15]
[15] This name is another cold mess.  Lea originally described it as “Melania striata” in that same pair of 1841/43 publications cited at [9] above.  He then discovered that the specific nomen striata was preoccupied, amending it to striatula elsewhere in 1843 (Proceedings of the American Philosophical Society 2:237).  Goodness gracious it will be nice to be done with stuff like this.

[16] Goodrich, C. (1941) Pleuroceridae of the small streams of the Alabama River system. Occas. Pprs. Mus. Zool. Univ. Mich., 427, 1-10.

[17] 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]. For more, see:
  • Mobile Basin III: Pleurocera puzzles [12Oct09]
  • Goodbye Goniobasis, Farewell Elimia [23Mar11]