Editor's Note - The research results telegraphed below were ultimately published by Dillon (2011) in Malacologia 53: 265 - 277.
Last month (1) we made passing reference to the Awakening of Calvin Goodrich, that 1934-41 period during which he published “Studies of the Gastropod Family Pleuroceridae” I – VIII (2). During this stage of his career, our hero developed his thesis that the characters upon which most pleurocerid taxonomy had been based - shell dimensions, coloration, ornamentation and so forth - are highly plastic and subject to environmental influence (3). In 2007 I coined the term “Goodrichian taxon shift” to describe situations where a single population of pleurocerids might vary down an environmental gradient to such an extent that taxonomists have recognized two different species (4). Or perhaps even two different genera?
I realize that the heading at the top of this essay is “Mobile Basin III.” But I beg your indulgence for the next three paragraphs to stray one drainage north, to the headwaters of the Tennessee River.
In my "Goodrichian" essay of 20Feb07 I reported unpublished observations on variation at two allozyme loci in a population of Goniobasis from Indian Creek, a tributary of the Clinch/Powell in southwest Virginia (5). In the headwaters, this single population of pleurocerids bore shells that were strongly carinate (historically referred to "G. acutocarinata" CA1), in middle reaches their shell morphology was smooth and typical of G. clavaeformis (C1), and upon joining the main Powell River, their shells became chunky and angulate, showing the morphology generally assigned the nomen “Pleurocera unciale” (P1).
In the last couple years I have extended this research program down the width of East Tennessee, to include populations from the Little River drainage near Maryville (figs CA2, C2, P2 at left), the Conasauga/Hiwassee near Etowah (figs CA3, C6, P3), and the Coahulla/Oostanaula/Coosa in North Georgia (figs CA4, P4). I have also sampled populations of G. simplex from each of these four regions to calibrate expected levels of interpopulation divergence at the 10 allozyme loci examined (fig S7).
The results of my 2007 study in Indian Creek are confirmed (6). Each of the four samples of “Pleurocera” was more genetically similar to the Goniobasis population just upstream than to any other sample of Pleurocera. Apparently taxonomists have been misled by ecophenotypic variation to identify populations of a single, widespread species under two different genera.
This result extends from the rivers of Tennessee into the Coosa drainage of North Georgia. Tradition has always held that elements of the Mobile Basin fauna are treated as endemic, even when indistinguishable from the fauna of neighboring drainages. Thus Goniobasis populations from small creeks in the Alabama/Coosa drainage showing the strongly carinate shell morphology are typically identified as “Goniobasis carinifera,” and those with heavier, smoother shells downstream as “Pleurocera vestitum.” Interestingly, if carinifera/vestitum is indeed conspecific with clavaeformis/unciale, the oldest name for the entire, sprawling species, from Virginia to Alabama, would be the Coosa name Goniobasis carinifera (Lamarck 1822).
In August we kicked this series off with news that the US Fish & Wildlife Service has proposed Alabama populations of pleurocerids identified as “Pleurocera foremani” for listing under the Endangered Species Act (7). According to the notice posted in the Federal Register, P. foremani is found today at only two locations, the lower Coosa River below Wetumpka Shoals and lower Yellowleaf Creek, a tributary of the Coosa (8). I understand from colleagues with field experience in this region that, sampling upstream, P. foremani is replaced by Pleurocera prasinatum in the smaller rivers, and that P. prasinatum is replaced by Pleurocera vestitum, and that vestitum is replaced by Goniobasis carinifera in the creeks.
In 1944, the mature Calvin Goodrich wrote regarding the Coosa species of Pleurocera, "On close study and comparison, they resolve themselves into four forms, and even these are not very distinctive. The specific names, in short, are to be considered conveniences in sorting rather than clean-cut differentiations" (9). He then went on to list P. vestitum "especially common in head streams," P. prasinatum "in the middle and lower Coosa," and P. foremani, which he distinguished by shell sculpture that "in one locality of the Cahaba is plainly a reversion, the same thing may be true of the shells of the Coosa.
"I have not seen the dissertation of Jeffrey Sides. But again referring to the 29Jun09 Federal Register (8), his sequence data suggested that P. foremani "was genetically more closely allied to a co-occurring species in the genus Elimia (Goniobasis)" than to any other species in the genus Pleurocera. Really? Even though double-digit sequence divergence is not uncommon within even conspecific populations of pleurocerids (10), the divergence between P. foremani and the local upstream Goniobasis appears to be of minor consequence?
Is the foremani/prasinatum/vestitum/carinifera taxon continuum in Alabama nothing but a unciale/clavaeformis/acutocarinata Goodrichian taxon shift, one state south? Has the US Fish and Wildlife service proposed federal protection for a local ecophenotypic variant of the most widespread pleurocerid in the American southeast? Pleurocera puzzles, indeed!
Stay tuned for more ...
(1) "Mobile Basin II: Leptoxis Lessons." Post of September '09 .
(2) A nice Goodrich bibliography is available from Kevin Cummings' website at the INHS
(3) "The Legacy of Calvin Goodrich." Post of January '07.
(4) "Goodrichian Taxon Shift." Post of February '07.
(5) Dillon, R.T. & J. D. Robinson (2007) 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]
(6) I presented this research at a NABS symposium this May in Grand Rapids. A manuscript is currently in preparation. Dillon, R. T. (in prep) Genetic and morphological divergence among populations of pleurocerid snails inhabiting rivers of the Southern Appalachians: Evidence of a two-stage process.
(7) "Mobile Basin I: Two Pleurocerids Proposed for Listing." Post of August '09.
(8) Follow the link from the FWS Press Release: Service Proposes Endangered Species Status and Critical Habitat Designations for the Georgia Pigtoe Mussel, Interrupted Rocksnail, and Rough Hornsnail.
(9) Goodrich, C. 1944. Pleuroceridae of the Coosa River basin. Nautilus 58(2):40-48.
(10) "The Snails the Dinosaurs Saw." Post of March '09.