Editor’s notes: 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.
This essay was subsequently published as: Dillon, R.T., Jr. (2023b) CPP Diary: The Many Faces of Professor Troost. Pp 41 – 49 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other Essays. FWGNA Project, Charleston, SC.
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 |
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.
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 in May of last 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 |
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.
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) |
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!
In our next installment... Huntsville hunt!
Notes:
[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]
- The heritability of shell morphology in Physa h^2 = O.819! [15Apr15]
[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: