The freshwater gastropods of Georgia Gulf drainages

We here in South Carolina think of Georgia as our younger brother, now outgrown us.  The colony to our immediate south was founded in 1732 to protect us from Spaniards, and in that function, at least, has been largely successful.  Our port cities, Charleston and Savannah, have grown up as sisters.  Our capital cities, Atlanta and Columbia, were both burned by Sherman.  We take turns beating Alabama for national championships.  Georgia grows more onions; we grow more peaches.

But if there is any honor of which our younger brother boasts that we here in South Carolina might envy, it is topographic diversity.  From North Georgia arise real mountains, the southern terminus of the ancient Appalachians.  And from those mountains are born rivers flowing in four directions through six USEPA Level III Ecoregions: north to the Tennessee/Ohio, west to the Alabama/Mobile Basin, south to the Gulf and east to the Atlantic.  The luxuriant aquatic biodiversity of Georgia more than compensates for any shortfall it might suffer in production of cannable fruit.

FWGGA v1.0, release 26Mar07

The Freshwater Gastropods of Georgia (FWGGA) web resource debuted online (at our old cofc.edu address) way back in 2007 with a survey of the Atlantic drainages of Georgia only, approximately 45% of the state.  Our database at that time comprised 845 records of 37 species.  It was migrated (as v2.0) to its present address in 2010, and saw minor upgrades (with additional data and fresh maps) in 2013 (v2.1) and 2024 (v2.2).  So as of last month, the FWGGA website v2.2 reported 960 records of 41 species and subspecies.  But that was for the Atlantic drainages alone.

Speaking now for myself and my coauthors Martin Kohl, Will Reeves, and Tim Stewart, today we are pleased to announce Version 3.0 of the FWGGA web resource, now expanded to include the Gulf drainages of Georgia, extending through 11 counties of the Florida panhandle between the Apalachicola and Suwanee Rivers.  Our database has grown to 1,608 records, documenting 56 species and subspecies of freshwater gastropods in the 85% of Georgia now covered.  Check it out today!

[FWGGA v3.0]

 

Most of our new records were gleaned from the extensive and well-curated mollusk collection held by the University of Florida Museum of Natural History in Gainesville.  Our initial search of the FLMNH online database returned 577 freshwater gastropod records from the Gulf drainages of Georgia and 495 from the 11 Florida counties downstream, for a total of 1,074 records in our enlarged study area.  Simple duplicates – records differing only by date or method of preservation, for example – were removed.  The records were then screened by three primary criteria: (1) dated since 1955, (2) locality data of sufficient quality to be plottable, and (3) habitat not brackish, as far as could be determined.

 

That last requirement turned out to be surprisingly restrictive.  Dr. Fred G. Thompson, who was the senior curator of mollusks at the FLMNH for 50 years, specialized in the hydrobioid snails.  And the museum cabinets that survive him in Gainesville to this day are packed with teensy-little vials full of teensy-weensier little gastropods, collected across every square foot of the Sunshine State, curated with great care.

 

FWGGA v3.0, release 19May25

The hydrobioid snails – especially the cochliopids – are famous for their adaptation to varying salinity.  Ultimately, however, we found it necessary to eliminate Heleobops, Onobops, and Littoridinops monroensis/palustris from our survey, because we could not find any records of any of these inhabiting entirely fresh waters in the area under study here.  That last omission was especially surprising, because Littoridinops monroensis was described from the (entirely fresh) upper St. Johns River system on the Atlantic side of Florida [1].  But this is the Freshwater Gastropods of North America Project, not the brackish, and we must draw a line somewhere, even if our study organisms do not.

So.  This past February I printed the resulting list of 454 panhandle Florida + 255 Gulf Georgia = 709 FLMNH records on a clipboard and made an appointment with our good friend and colleague John Slapcinsky, the collection manager way down south in Gainesville.  And we must take a moment to thank, from the bottom of our hearts, our buddy John for his infinite patience, great good humor, and skillful manipulation of the local parking authority hosting us during our most recent sojourn on his sprawling campus.  All identifications were verified, or (if the corresponding museum lot could not be located), deleted.

All qualified, verified lots of freshwater gastropods from our GA/FL Gulf drainage 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 a total of 257 Florida + 230 Georgia = 487 unique, modern, verified, georeferenced FLMNH freshwater gastropod records from the Gulf drainages of our two-state study area.  These were added to the 392 older Atlantic-drainage records, to yield the total of 879 FLMNH records analyzed in FWGGA version 3.0.

 

The additional 161 records newly reported in the FWGGA expansion were almost entirely collected by RTD using simple untimed searches 2003 - 2025, specifically targeting freshwater gastropod habitat [2].  Ultimately our survey covered approximately 645 discrete sample sites, located across the Atlantic and Gulf drainages of Georgia, extending through the 11 counties of the Florida panhandle between the Apalachicola and the Suwanee.  See the map above.  No “absence stations” are shown.  If freshwater gastropods were not collected at a site, then no record resulted. 

Marstonia castor UF22178

Our entire 1,608 record database is available (as an excel spreadsheet) from yours truly upon request.

The list of 56 species and subspecies of freshwater snails we have documented from our study area omits Marstonia castor, described by Thompson in 1977 as endemic to Cedar Creek in Crisp County, Ga [3].  Although the FLMNH collection also includes more recently-collected lots of M. castor from Swift Creek (Crisp Co) and Mercer Mill Ck (Worth Co), our 2023 efforts to locate a viable population anywhere in the region were unsuccessful.  The FWS listed M. castor as extinct in 2017, and we concur.

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 [4] 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 observations of Thompson & Hershler [4] 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 travelled 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 [5], 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 [6].

To live and die in Dixie...

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 [7] 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 and colleagues [8] reported that the best predictor of current macroinvertebrate diversity in East Tennessee river systems is not present 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.

Although relatively minor in areal extent, the expansion of FWGNA coverage to include this diverse little drip of North American freshwater nevertheless resulted in the addition of 9 new gastropod species and subspecies to the 136 previously included in our coverage, bringing our continental total to 145.  Come visit us again, for the first time!

 

Notes:

 

[1] Von Frauenfeld, G. R. (1863) Verhandel. Kais. Konig. Zool. Botan. Ges. Wein 13: 1023.

 

[2] Dillon, R.T., Jr. 2006. Freshwater Gastropoda. pp 251 - 259 In The Mollusks, A Guide to their Study, Collection, and Preservation. Sturm, Pearce, & Valdes (eds.) American Malacological Society, Los Angeles & Pittsburgh.

 

[3] Thompson, F.G. (1977) The hydrobiid snail genus Marstonia.  Bulletin of the Florida State Museum, Biological Sciences 21: 113 – 158.

 

[4] Thompson, F.G. & R.H. Hershler. 1991. Two new hydrobiid snails (Amnicolinae) from Florida and Georgia, with a discussion of the biogeography of freshwater gastropods of South Georgia streams. Malac. Rev. 24:55-72.

 

[5] Aldrich, T.H. (1911) Notes on some Pliocene fossils from Georgia with descriptions of new species.  Nautilus 24: 131 – 132, 138 – 140.

 

[6] For a review of the effects of calcium concentration and related water chemical variables on the distribution of freshwater gastropods, see pages 326 – 338 in:

• Dillon, R.T. Jr (2000) The Ecology of Freshwater Molluscs.  Cambridge University Press. 509 pp.

[7] Clench, W.J. & R.D. Turner. 1956. Freshwater mollusks of Alabama, Georgia, and Florida from the Escambia to the Suwannee River. Bull. Fl. State Mus., Biol. Sci. 1:95-239.

 

[8] Harding, J.S., E.F. Benfield, P.V. Bolstad, G.S. Helfman and E.B.D. Jones (1998) Stream biodiversity: The ghost of land use past. Proceedings of the National Academy of Sciences 95: 14843 - 14847.