It is perhaps not surprising that the freshwater gastropod fauna of The Peach State is so poorly known. Boasting only a single navigable port (Savannah), most of Georgia was remote and inaccessible for the first hundred years of European settlement 1732 - 1830. Railroad connections were established with South Carolina and the east in the 1830s, and by 1851 between rapidly-growing Atlanta and the west. All was lost save honor 1864 - 65, however, gone with the wind. The section did not recover from economic ruin until the 20th century.

The only significant survey of Georgia's freshwater gastropod fauna prior to the present inventory was that of Clench & Turner (1956), working the Gulf drainages in the southern quarter of the state. Sampling the entire freshwater molluscan fauna from the Suwanee drainage west to the Escambia/Conecuh of Alabama and south through Florida, Clench & Turner documented a modest 18 species of prosobranch gastropods (describing two as new) and just four species of pulmonates, failing to record any planorbids whatsoever. They considered the malacofauna of the Georgia Gulf drainages westerly derived, bearing closer affinity to the Alabama-Coosa and Tennessee River systems than to the Atlantic drainages of the east.

Goodrich (1939) reported the results of a local survey conducted by Henry van der Schalie on the Ogeechee River, listing 7 prosobranchs and 5 pulmonates. More recently, Sukkestad et al. (2006) documented 4 prosobranchs and 4 pulmonates inhabiting the Fort Stewart Army Installation in the coastal plain near Savannah.

Most of the freshwater gastropod surveys conducted in The Peach State have, however, been directed toward selected prosobranch taxa: the Viviparidae (Call 1894, Goodrich 1942a, Clench 1962), the Pleuroceridae (Goodrich 1942b, Mihalcik & Thompson 2002), and the hydrobioids (Thompson 1969, 1977, Watson 2000).

Thompson & Hershler (1991), although focusing primarily on the hydrobioids, offered some general observations on the biogeography of the entire freshwater gastropod fauna of Georgia. They described an area virtually "devoid of freshwater snails" in the Suwannee, St. Marys, and Satilla Rivers of the state's southern quarter. Thompson & Hershler also hypothesized that, with the exception of Lyogyrus latus and two species of Viviparus, the Atlantic drainages of Georgia share no prosobranch species in common with the Gulf drainages of Georgia.

Dillon & Robinson (2011) confirmed, however, the occurrence of Pleurocera floridensis in Atlantic drainages around Hawkinsville, a species widespread in the Chattahoochee/Flint and other Gulf drainages south into Florida. They also offered evidence that many of the Pleurocera populations formerly believed endemic to Chattahoochee/Flint drainages (e.g. P. albanyensis, P. viennaensis) are conspecific with P. catenaria, widespread in Atlantic drainages north to Virginia. Many additional faunal similarities are documented in the present work.

In the ecological literature, populations of both Pleurocera catenaria and "Ferrissia sp" (probably F. fragilis) were involved in a study published by Nelson & Scott (1962) on food web dynamics in the Middle Oconee River. Krieger and Burbanck (1976) described microhabitat distribution and movement patterns of Pleurocera catenaria (under the alias "Goniobasis suturalis") inhabiting the Yellow River, a tributary of the Ocmulgee. They related environmental features to P. catenaria distribution for purposes of identifying factors limiting local variation in abundance.

> Version History

The original v1.0 of the FWGGA web resource first went online (at cofc.edu) in 2007, migrating to its current address (as v2.0) in 2010. It saw minor upgrades with fresh (regional) maps in 2013 (v2.1) and 2023 (v2.2). In all of these earlier versions, from 3/07 to 4/25, the data analyzed were for the freshwater gastropods of Atlantic Drainages only.

On 19May25 we released the present FWGGA v3.0, which adds the Gulf drainages of Georgia, extending through 11 counties in the panhandle of Florida, together with an updated dichotomous key, updated gallery, and new (Georgia specific) maps. See my essay of DATE.

> Methods

The database here analyzed comprises 1,608 records from two primary sources: museum holdings and personal collections. Of the former, the largest fraction came from the Florida Museum of Natural History in Gainesville (879 records), with smaller contributions from the Georgia Museum of Natural History in Athens, the North Carolina State Museum in Raleigh, the US National Museum, and the Academy of Natural Sciences of Philadelphia (134 records combined).

We queried online databases (as available) to extract all freshwater gastropod records from the Atlantic and Gulf drainages of Georgia, plus the 11 counties of the Florida panhandle between the Apalachicola and the Suwannee Rivers. These records were screened by three primary criteria: (1) dated 1955 or later, (2) locality data of sufficient quality to be plottable, and (3) habitat not brackish. Simple duplicates records differing only by date or method of preservation, for example, were removed. We then visited each museum and personally verified all qualified records, correcting identifications as necessary.

All qualified, verified lots of freshwater gastropods from our GA/FL study area were then georeferenced and plotted. As a rule of thumb, the FWGNA requires that no pair of records for a single species be collected from the same body of water any closer than 5 km. Removal of the older record from all such near-duplicate pairs yielded the total 879 + 134 = 1,013 unique, modern, verified, georeferenced museum records analyzed in the present study.

A total of 595 records were added to our FWGGA database by personal collections: 339 (RTD) + 230 (WKR) + 26 (others). These samples were collected using simple untimed searches 2003 - 2025, specifically targeting freshwater gastropod habitat (Dillon 2006).

Ultimately our survey included approximately 645 discrete sample sites, located across the state of Georgia and into the panhandle of Florida, in all ecoregions, all subdrainages, and all counties. A map showing the distribution of these sites is figured above. No absence stations are shown. If freshwater gastropods were not collected at a site, then no record resulted.

Our entire 1,608 record database is available (as an excel spreadsheet) from the senior author upon request.

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 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.

> Results and Discussion

The 56 species and subspecies of freshwater gastropods we have confirmed from the Atlantic and the Gulf drainages of Georgia (plus 1 unconfirmed) are figured in the FWGGA gallery and distinguished on the FWGGA dichotomous key.  Ecological and systematic notes for each species and subspecies are provided on dedicated pages, together with state distribution maps. The distributions and abundances of the Atlantic-drainage subset (only) is analyzed on a continental scale in our sections on Biogeography and Synthesis.

Combining subspecies for analysis, the freshwater gastropod fauna of the region under study here reduces to 53 species: 35 prosobranchs and 18 pulmonates. Of the pulmonates, three are extralimital or introduced: Biomphalaria, Promenetus, and Physa gyrina. Helisoma scalare is Floridian. The remaining 14 pulmonate species are all common throughout the southeastern United States, in some cases stratified by ecoregion: Physa carolinae and Helisoma trivolvis (for example) restricted to the coastal plain, Ferrissia rivularis in the piedmont and mountains.

A Gradual Transition
More biogeographic signal is apparent in the prosobranchs. Of the 35 prosobranch species we identified in our study area, 10 are unique to the Gulf drainages, 10 are unique to the Atlantic drainages, and 15 are shared across the state of Georgia broadly. This observation does not support the hypothesis advanced by Thompson & Hershler (1991) that with the exception of Lyogyrus and two species of Viviparus, the prosobranch faunas of the Atlantic and Gulf drainages of Georgia have no species in common. Rather, the distributions of the freshwater gastropods of Georgia apparently reflect a gradual transition or blending between the faunas of Atlantic drainages to the east, Alabama/Coosa drainages to the west, and Florida to the south.

The DFS Zone
Our modern survey has, however, corroborated the 1991 observation of Thompson & Hershler that the drainage basins of the Satilla and the St. Marys Rivers of the Atlantic drainage, plus the upper portions of the Suwannee and Ochlockonee River systems of the Gulf drainage, are virtually devoid of freshwater snails. The striking absence of sample sites in that region clearly evident on the map above is not due to a lack of effort on our part. We traveled that area extensively, donning boots and searching keenly, ultimately returning with no freshwater gastropod observations to report. We here refer to that region as the DFS Zone, for "devoid of freshwater snails."

Citing evidence from the paleontological results of Aldrich (1911), Thompson and Hershler suggested that the DFS Zone had a rich freshwater gastropod fauna in the Pleistocene, similar to that of surrounding regions today, and attributed the depauperization of the modern fauna to water chemistry factors recent in their origin. We ourselves are hesitant to generalize the fresh/brackish Pleistocene malacofauna catalogued by Aldrich from the lower Satilla across the entire DFS Zone. But the hypotheses that Thompson & Hershler advanced regarding the influence of bedrock and soil type on water chemistry, and the influence of water chemistry upon freshwater gastropod distributions, are well supported (Dillon 2000: 326-338).

The Satilla, the St Marys, the upper Suwanee and the upper Ochlockonee drainages in South Georgia are underlain by Cretaceous gravels and sands, yielding soft, acidic, low-carbonate surface waters to which freshwater gastropod populations are often poorly adapted. And to the inhospitable water chemistry of this region, we would hasten to add the inhospitable water physics. Silt.

Clench & Turner (1956) suggested that the greatest source of damage to the freshwater mollusk fauna of the Georgia Gulf drainages seems to be land erosion and consequent silting of the rivers. For over a century, most of the state was intensively farmed for cotton, stream bank to stream bank. Harding et al. (1998) reported that the best predictor of current macroinvertebrate diversity in East Tennessee river systems is not current land use, but rather land use prior to 1950. We suggest that the intensive burdens of silt that have been carried, and that continue to be carried, by the rivers of South Georgia, together with the softness, acidity, and poor buffering capacity of the regional surface waters, account for the phenomenon we here describe as the DFS Zone.

> Acknowledgements

We thank John Robinson and Tom Smith for their help with the field work. Gracious hosts included John Slapcinsky and Fred Thompson at the Florida Museum of Natural History, Liz McGhee at the Georgia Museum of Natural History, Art Bogan and Jamie Smith at the North Carolina State Museum, Bob Hershler and Jerry Harasewych at the USNM, and Gary Rosenberg, Paul Callomon, and Amanda Lawless at the Academy of Natural Sciences of Philadelphia. The early success of this project in large part depended on the GIS and data analysis skills of Dr. Doug Florian, and continues to depend on the webmastery of Mr. Steve Bleezarde.

> References

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Clench, W.J. 1962. A catalogue of the Viviparidae of North America with notes on the distribution of Viviparus georgianus Lea. Occas. Pprs. Mollusks 2:261-285.
Dillon, R.T., Jr. 2000. The Ecology of Freshwater Molluscs. Cambridge University Press. 509 pp.
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Thompson, F.G. 1969. Some hydrobiid snails from Georgia and Florida. Quarterly J. Fl. Acad. Sci. 32:241-265.
Thompson, F.G. 1977. The hydrobiid snail genus Marstonia. Bull. Fl. State Mus. 21:113-158.
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Watson, C. 2000. Results of a survey for selected Hydrobiidae (Gastropoda) in Georgia and Florida. Proc. First Freshwat. Moll. Conserv. Soc. Sympos. 233 - 244.