Table 1 shows the 39 species of freshwater gastropods we have confirmed for eastern Tennessee River drainages, ranked by their abundances and compared to the inventories of Bickel (1968) and Johnson et al (2013).  Note that two sets of pleurocerid subspecies have been combined for this analysis.   It is interesting to observe that these 39 species include 15 not found in Atlantic drainages immediately east of the Blue Ridge, raising our combined (ten-state) confirmed fauna to 83 species.  By subtraction, it is easy to see that 24 freshwater gastropod species are shared across the Appalachian divide, and that 44 seem unique to the Atlantic drainages.

In our discussions of the freshwater gastropod faunas of the Atlantic drainage states we have focused entirely upon the biogeographic effects of ecoregion and latitude.  This is the first evidence we have seen that drainage divides might affect freshwater gastropod distribution, independent of these two factors.

The 39 species shown in Table 1 have been divided into a set of 21common species (represented in our database by ten records or more) and a set of 18 species apparently more rare (shaded below).  We will focus first on the 21 common species, with an eye toward discerning biogeographic pattern.

Nine species listed in the top half of Table 1 may be considered cosmopolitan – found throughout East Tennessee apparently uncorrelated with region or environment: Campeloma decisum, Physa acuta, Lymnaea humilis, L. columella, Helisoma anceps, Menetus dilatatus, Laevapex fuscus, Ferrissia rivularis and Ferrissia fragilis.  Among the remainder of the common species, it is possible to discern biogeographic patterns by stream size, patterns by ecoregion, and in a few cases, patterns correlated with latitude.

Three species of Pleurocera seem to be characteristic of the Blue Ridge ecoregion: Pleurocera proxima, P. modesta, and P. catenaria, the first two in small streams, the last in the main Hiwassee River.  All three of these are trans-Appalachian in their distributions, being at least as common in other drainages to the east or south as they are in the upper Tennessee.

The last subset of nine common species appears to be characteristic of the Ridge and Valley ecoregion: 1 pulmonate, 7 pleurocerids, and 1 other prosobranch.  This group can be further subdivided into a set that seems to be characteristic of small streams (Pleurocera troostiana, P. simplex, P. gabbiana, and Physa gyrina ) and a set more characteristic of medium to large rivers (Io fluvialis, Leptoxis praerosa, Pleurocera canaliculata, P. clavaeformis, and Pomatiopsis cincinnatiensis).  Distribution maps also seem to suggest that populations of P. cincinnatiensis and P. gabbiana are more common in the northern half of our study region, and that Pleurocera canaliculata is more common in the south, below Knoxville.

The overall impression one takes from the distribution of the 21 most common species of freshwater gastropods in eastern Tennessee River drainages is that, as diverse as the regional fauna most certainly is, large-scale biogeographic patterns such as those identified above combine to make it locally less so.  Some of these patterns may certainly result from historical accident.  But it is difficult to avoid the hypothesis that some long term adaptation to minimize interspecific competition within such a large guild of generalized grazers might also have been at work as well.

We next shift our attention to the 18 species represented by fewer than ten records in our database.  This set includes a subset that of three that seem to have been undercollected, a subset of nine reaching the margins of a larger range in the present study area, and a subset of six that appear to be legitimately rare.

Three small-bodied species shaded in Table 1 were certainly undercollected by our sampling methods: Fontigens nickliniana, Marstonia arga, and Pomatiopsis lapidaria.  Populations of F. nickliniana are adapted to springs and the uppermost reaches of spring runs, M. arga apparently adapted to the impounded reaches of large rivers, and P. lapidaria is so amphibious that it is more often collected by workers sampling for land snails.

Nine species shaded in Table 1 are reaching the margins of broader, more definite ranges in eastern Tennessee River drainages.  Pleurocera laqueata is much more common to the west, in central Tennessee, north Alabama, and Kentucky.  Leptoxis carinata and Somatogyrus virginica are elements of the Atlantic fauna to the east, perhaps introduced to the present study area by stream capture.  The natural range of V. subpurpureus is generally given as central Mississippi River drainages.  The single record of its occurrence in East Tennessee may represent an artificial introduction.

Physa pomilia is widespread in Atlantic drainages to the south and east of the present study area, although often at low population densities, primarily on river margins.  Its appearance in East Tennessee was a surprise to us.  Physa pomilia is difficult to distinguish in the field from P. acuta, with which it sometimes occurs, and thus is easy for workers in the field to overlook.

Helisoma trivolvis, Gyraulus parvus, Amnicola limosa, and Lyogyrus granum range widely thoroughout eastern North America, primarily in slow moving rivers or lentic habitats.  The natural aquatic habitats of East Tennessee are primarily lotic, however, and not particularly suitable for these four. Thus these four species might be described as "naturally  under-represented" in the present study area.

Physa pomilia is widespread in Atlantic drainages to the south and east of the present study area, although often at low population densities, primarily on river margins.  Its appearance in East Tennessee was a surprise to us.  Physa pomilia is difficult to distinguish in the field from P. acuta, with which it sometimes occurs, and thus is easy for workers in the field to overlook.

Six species of freshwater gastropods listed in the shaded region of Table 1 appear to be legitimately rare.  These include Leptoxis crassa (“Athearnia anthonyi”) and Marstonia ogmorhaphe, both currently on the federal endangered species list as well as the lists of the states involved.  No more need be said here.

The obligately cave-dwelling hydrobiid Holsingeria unthanksensis is nearly endemic to Lee County, Virginia, with an odd, outlying population 500 km north in the Shenandoah Valley. Also quite legitimately rare seems to be the river-dwelling hydrobiid Clappia umbilicata, our surveys returning just three records from the upper Powell and Sequatchie Rivers.  The hydrobiid genus Clappia has always been assumed endemic to the Mobile Basin of Alabama, and in recent years feared extinct.  Our discovery of East Tennessee populations thus comes as a welcome surprise.

Our surveys returned surprisingly few records of Lithasia verrucosa.  Historically the species ranged throughout the main Tennessee River and its major tributaries, a large population inhabiting the lower Little Tennessee River until its impoundment in 1979.  Today the only populations of L. verrucosa apparently surviving in East Tennessee inhabit a few shoal areas in the lower reaches of the Nolichucky and French Broad Rivers.

Populations of Somatogyrus also appear to be quite rare in East Tennessee drainages.  Their taxonomic history is complex. Populations from the main Tennessee River were originally described by Tryon (1865) as Somatogyrus aureus, and subsequently confirmed in the "Tennessee River near Knoxville" by Walker (1904:134).  A population of S. aureus was also reported in the "Nolachucky River near Greeneville" by Pilsbry & Rhoads (1896).  The Somatogyrus populations of the Holston River have been referred to three different specific nomina: S. currierianus (by Lewis 1871), S. aureus (by Walker 1904) and S. tryoni (by Bickel 1968).  Populations in the Powell River were originally described by Tryon (1865) as Somatogyrus parvulus, and have not subsequently received mention.  Much more recently a "Somatogyrus sp. 2" has been reported from the Hiwassee River, appearing in the Tennessee Conservation Strategy (TWRA 2005: 50) without formal attribution.

We have confirmed the Powell River S. parvulus population, but have been unable to relocate Somatogyrus populations in the Nolichucky, Holston, or main Tennessee rivers, despite directed searches.  The Hiwassee population appears referable to S. virginicus, much more widespread in Atlantic drainages to the east, but the Powell River Somatogyrus population appears distinct to us, and most certainly does seem quite rare.

In summary, sixteen species of freshwater gastropods from drainages of the Tennessee River above the Alabama line have been listed in the comprehensive wildlife conservation strategies of the four states covered: Virginia (VDGIF 2005), Tennessee (TWRA 2005), North Carolina (NCWRC 2005), and Georgia (GADNR 2005).  These 16 species are ranked in Table 2 by their records in our database, listing synonyms as appropriate. 

The taxonomic uncertainty that has prevailed in the freshwater gastropod fauna of East Tennessee since the nineteenth century, especially with respect to the Pleuroceridae, has led to a great deal of confusion regarding the conservation status of many species.  Table 2 shows that the four state wildlife agencies together have used 34 different nomina to refer to their 16 species of conservation concern.  When the taxonomic situation is clarified, many of these 16 species will not appear especially rare or threatened.   

> References

Bickel, D. (1968)  Checklist of the Mollusca of Tennessee. Sterkiana 31: 15-39.
Georgia Department of Natural Resources (2005)  A Comprehensive Wildlife Conservation Strategy for Georgia.  GADNR-WRD: Social Circle, GA.
Johnson, P. D., and 13 coauthors (2013)  Conservation status of freshwater gastropods of Canada and the United States.  Fisheries 38: 247-282.
Lewis, J. (1871)  On the shells of the Holston River.  American Journal of Conchology 6: 216-226.
Minton, R. L. & Lydeard, C. (2003)  Phylogeny, taxonomy, genetics and global heritage ranks of an imperiled, freshwater snail genus Lithasia (Pleuroceridae).  Molecular Ecology 112: 75-87.
North Carolina Wildlife Resources Commission (2005)  A Wildlife Action Plan for North Carolina: The Comprehensive Wildlife Conservation Strategy.
Pilsbry, H. & Rhoads, S. (1896)  Contributions to the Zoology of Tennessee, Number 4, Mollusca. Proc. Acad. Nat. Sci. Phila. 1896: 487-506.
Tennessee Wildlife Resources Agency (2005)  Tennessee's Comprehensive Wildlife Conservation Strategy.  TWRA: Nashville, TN.
Virginia Department of Game and Inland Fisheries (2005)  Virginia's Comprehensive Wildlife Conservation Strategy. VDGIF: Richmond, VA.
Tryon, G. W. Jr. (1865)   Descriptions of new species of Amnicola, Pomatiopsis, Somatogyrus, Gabbia, Hydrobia and Rissoa.  American Journal of Conchology 1: 219-222, pl 22, figs 5-13.
Walker, B. (1904)  New species of Somatogyrus.  The Nautilus 17: 133 - 142.