FWGNA > Freshwater Gastropods of Tennessee
Tenneessee Photobar
Tennessee (eastern)
The state of Tennessee spans eight USEPA Level III Ecoregions, from the Blue Ridge Mountains west across the Cumberland Plateau to the Mississippi Alluvial Plain.  The present survey of freshwater gastropods focuses exclusively on the eastern third of the state, drained by the Tennessee River above its entry into Alabama.  This survey also includes tributaries of the Tennessee River in western Virginia, North Carolina, and Georgia, covering a total catchment of approximately 57,000 square kilometers.

Most of this region was intensively farmed through the 19th and early 20th centuries, but by the 1930s had become economically depressed, at the creation of the Tennessee Valley Authority.  Over the ensuing four decades the TVA constructed 27 dams in rivers of the Tennessee drainage above the Alabama line, entirely impounding the main river and several of its larger tributaries.  Today the Tennessee River is commercially navigable through locks and channels from its mouth to its origin one mile (1.6 km) above Knoxville.  Also commercially navigable are the lower 98 km of the Clinch River, 47 km of the Little Tennessee River, and 35 km of the Hiwassee River.   
Entering the 21st century, 67% of the regional population lived in metropolitan areas:  Knoxville (699,000), Chattanooga (524,000), Asheville (413,000), and Bristol/Kingsport/Johnson City (403,000).  Only 27% of the area was agricultural (primarily pastureland) and a remarkable 64% had returned to forest.

Waters flowing east into the Tennessee River off the Cumberland Plateau are generally soft and poor in nutrients, as are waters flowing west from the Smoky Mountains.  The freshwater gastropod fauna inhabiting these tributaries is not diverse.  Our survey of the Great Smoky Mountains National Park in connection with their "all-taxa biodiversity inventory" returned 12 species from the 2,100 km2 catchment, 9 pulmonates and 3 pleurocerids (Discover Life in America 2011).

Most of the present study area lies in the ecologically rich Ridge and Valley Ecoregion, however, underlain by Paleozoic limestones and dolomites that render the waters hard and productive.  Populations of pleurocerid snails, in particular, have reached great diversity and abundance in this region.

Goodrich (1940) catalogued approximately 140 specific nomina of pleurocerids with ranges that might include tributaries of the Tennessee River above the Alabama line, 31 of which he regarded as valid.  These included 14 species of Goniobasis, 9 species of Pleurocera, two Lithasia, two Eurycaelon, two Nitocris, one Anculosa, and the single species of Io.  Subsequent authorities have differed on the genus-level taxonomy, although Goodrich’s inventory of pleurocerid species has stood unexamined for 70 years.

It is possible that the very abundance and diversity of pleurocerid snails in the rivers of East Tennessee has discouraged workers from conducting more comprehensive surveys to include the other freshwater gastropod taxa.  The earliest notable inventory of Tennessee mollusks was published by Pilsbry & Rhoads (1896).  Their list of 11 freshwater pulmonate and 28 prosobranch nomina was compiled from a single field survey conducted in May and June of the previous year.  Updating the Pilsbry and Rhoads taxonomy would lower their list to 20 prosobranch and 8 freshwater pulmonate species, 25 of which might be expected to range through the eastern third of the state.

Bickel's (1968) review of the large, old, and scattered literature returned a remarkable checklist of 133 bivalve, 224 land snail, and 98 freshwater gastropod nomina from the state of Tennessee.  Of the 98, records for 50 nomina seem to include the eastern third of the state: 28 pleurocerids, 6 other prosobranchs, and 16 pulmonates.

Stewart & Dillon (2004) included the seven southwestern counties of Virginia draining into the Tennessee along with their larger review of the freshwater gastropods of the Old Dominion as a whole.  They tallied 16 prosobranch species (10 pleurocerids and 6 others) from the region covered by the present survey, as well as 8 pulmonates.

Many of the nominal pleurocerid species on these lists are most commonly associated with rocky riffles in larger rivers, a habitat type that almost disappeared from East Tennessee during the middle decades of the 20th century.  Alarms were raised regarding the status of Io fluvialis by Stansbery & Stein (1976) and McLeod & Moore (1978), ultimately leading to re-introduction efforts (Ahlstedt 1991).  Leptoxis crassa (also known as Athearnia anthonyi) was added to the federal endangered species list in 1994, with re-introduction efforts again following (Dillon & Ahlstedt 1997, Garner & Haggerty 2010).

Despite shrinking ranges and taxonomic uncertainty, however, Shoup's (1943) observations regarding the relationship between stream alkalinity and gastropod distribution in Tennessee were among the more influential early contributions to freshwater mollusk ecology.  And 40 years of research on the Pleurocera clavaeformis populations at the Oak Ridge National Laboratory have added significantly to our understanding of stream ecology in general (e.g., Steinman 1991, Hill 1992, Rosemond et al. 1993, Hill et al. 2010).  Grazing by populations of P. clavaeformis can have a significant impact on algal biomass, primary productivity, and periphyton community structure.  For a review see Dillon (2000: 86 - 91).

Recently it has become clear that the pleurocerid diversity of East Tennessee, while undeniably impressive, has been overestimated.  Using a survey of gene frequencies at ten allozyme-encoding loci in 15 pleurocerid populations, Dillon (2011) demonstrated that the shell morphological criteria by which Goodrich and all workers previous and subsequent have distinguished most of the genera and species of pleurocerids in East Tennessee seem to be local responses to stream size, possibly ecophenotypic in origin.  The time would seem ripe for a critical re-examination of the freshwater gastropod fauna of Tennessee drainages above the Alabama line as a whole.

> Methods

The database analyzed here comprises 1,674 records from approximately 767 discrete sites: 335 sites in Tennessee, 252 in Virginia, 165 in North Carolina, and 15 in Georgia, collected 1982 - 2011.  The largest fraction of our records are from personal collections made by RTD (852 records) or MK (222 records).  We also include 276 records transmitted to us by Mr. Brian Watson of the Virginia Department of Game and Inland Fisheries, including collections and observations made by DGIF personnel and their contractors, as well as 51 records from the Virginia Museum of Natural History.  The great majority of our North Carolina data (261 records) were collected by the NC Department of Water Quality 1985-2005 and housed at the North Carolina State Museum, although not catalogued into the museum collection as of 2005.  Our entire database is available (as an excel spreadsheet) from the senior author upon request.

Collecting methods have varied greatly.  The NCDWQ samples are semi-quantitative, taken by EPA standard methods (Barbour et al. 1999) combining kick-nets, timed searches, etc.  Other collections were entirely qualitative, the result of simple untimed searches.

The taxonomy employed by the FWGNA project is painstakingly researched, well-reasoned and insightful.  Needless to say, it often differs strikingly from the gastropod taxonomy in common currency among casual users and most natural resource agencies.  First-time visitors looking for information about particular species or genera might profitably begin their searches with a check for synonyms in our alphabetical index.

> Acknowledgements

Mr. Brian Watson of the VDGIF has been most forthcoming with data and supportive through all phases of this project.  Some of the early surveys and genetic work were funded by the VDGIF through the USFWS State Wildlife Grant Program (contract 2006-9308).  We thank Dr. Art Bogan and Ms. Jamie Smith for hosting us graciously at the North Carolina State Museum, providing technical assistance in a most timely and efficient manner. The success of this project has in large part depended on the great patience of web wizard Steve Bleezarde.  Funded in part by a grant from the USDI-NPS Great Smoky Mountain National Park, Office of Inventory & Monitoring.


> References

Ahlstedt, S. (1991)  Reintroduction of the spiny riversnail, Io fluvialis (Say 1825) (Gastropoda: Pleuroceridae) into the North Fork Holston River, southwest Virginia and northeast Tennessee. American Malacological Bulletin 8: 139-142.
Barbour, M., J. Gerritsen, B. Snyder, & J. Stribling (1999) Rapid bioassessment protocols for use in streams and wadeable rivers: Periphyton, benthic macroinvertebrates, and fish, Second edition. Washington, DC, US EPA 841-B-99-002.
Bickel, D. (1968)  Checklist of the Mollusca of Tennessee. Sterkiana 31: 15-39.
Dillon, R. T., Jr. (2000)  The Ecology of Freshwater Molluscs. Cambridge University Press, Cambridge.
Dillon, R. T., Jr. (2011)  Robust shell phenotype is a local response to stream size in the genus Pleurocera (Rafinesque, 1818). Malacologia, 53: 265-277.
Dillon, R., Jr. & Ahlstedt, S. (1997)  Verification of the specific status of the endangered Anthony's river snail, Athearnia anthonyi, using allozyme electrophoresis. The Nautilus 110: 97-101.
Discover Life in America- All Taxa Biodiversity Inventory (2011) Great Smoky Mountain All Taxa Biodiversity Inventory.  http://www.dlia.org.  Gatlinburg, Tennessee 37738.
Goodrich, C. (1940)  The Pleuroceridae of the Ohio River system. Occas. Pprs. Mus. Zool. Univ. Mich.,417: 1-21.
Garner, J. T. & T. M. Haggerty (2010) Distribution, density, and population dynamics of the Anthony riversnail (Athearnia anthonyi) in Limestone Creek, Limestone County, Alabama.  Amer. Malac. Bull. 28: 121-126.
Hill, W. (1992)  Food limitation and interspecific competition in snail-dominated streams. Can. J. Fish. Aquat. Sci. 49: 1257-1267.
Hill, W., Smith, J. & Stewart, A. (2010)  Light, nutrients, and herbivore growth in oligotrophic streams. Ecology 91: 518-527.
McLeod, M. & Moore, J. (1978)  Change in the gastropod Io spinosa (Pleuroceridae; Mollusca) in 70 years. American Midland Naturalist 99: 198-205.
Pilsbry, H. & Rhoads, S. (1896)  Contributions to the Zoology of Tennessee, Number 4, Mollusca. Proc. Acad. Nat. Sci. Phila. 1896: 487-506.
Rosemond, A., Mulholland, P. & Elwood, J. (1993)  Top-down and bottom-up control of stream periphyton: effects of nutrients and herbivores. Ecology 74: 1264-1280.
Shoup, C. (1943)  Distribution of fresh-water gastropods in relation to total alkalinity in streams. The Nautilus 56: 130-134.
Stansbery, D. & Stein, C. (1976)  Changes in the distribution of Io fluvialis (Say 1825) in the upper Tennessee River system. Bulletin of the American Malacological Society 1976: 28-33.
Steinman, A. (1991)  Effects of herbivore size and hunger level on periphyton communities. Journal  of Phycology 27: 54-59.
Stewart, T.W. & R.T. Dillon, Jr. (2004) Species composition and geographic distribution of Virginia’s freshwater gastropod fauna: a review using historical records. American Malacological Bulletin 19:79-91.