The 39 freshwater gastropod species inhabiting the Atlantic drainages of Georgia are listed in Table 1, ranked by the number of sites at which they were recorded.  (Note that the two subpecies of P. catenaria were combined for this analysis.)  The third column of Table 1 gives "FWGNA Incidence Rank," an objective measure of the abundance of each species over U. S. Atlantic drainages.  (See "Synthesis" from the link at right for more.)  This list is compared to the spurious review of Johnson and colleagues (2013).

For the purpose of this discussion we will divide the 39 species by two criteria into three sets. We distinguish the 21 species represented in our database by more than ten records as "common in Georgia," and the remaining 18 species as "uncommon in Georgia."  Within the latter group we distinguish the 9 species with incidence ranks of I-4 or I-5 as "regionally common" and those ranked I-3 and below as "regionally uncommon."

Focusing first on the 21 species common in Georgia.  This set includes a subset that might be considered ubiquitous, found throughout the study area without regard to ecoregion: Campeloma decisum, Pleurocera catenaria (both subspecies), Physa acuta, Menetus dilatatus, Lymnaea columella, Ferrissia fragilis, Somatogyrus virginicus, and Gyraulus parvus.   

The distributions of another subset seem, on the other hand, to broadly correlate with US EPA ecoregions, as has been observed elsewhere throughout the North American Atlantic drainages.  Pleurocera proxima is characteristic of the Blue Ridge and Piedmont ecoregions, while Helisoma anceps seems rather restricted to the Piedmont, at least in Georgia.  Helisoma trivolvis, Physa pomilia, Physa carolinae, Laevapex fuscus, Amnicola limosa and Lyogyrus granum appear restricted to the Southeastern Plain / Southern Coastal Plain region.

In a general biogeographic review of freshwater gastropod distributions in southern Georgia, Thompson & Hershler (1991) observed that a region occupying the southern 10% of the state is virtually "devoid of snails."  This area, drained by the Satilla and St. Mary Rivers into the Atlantic as well as the Suwannee and Ochlochkonee Rivers into the Gulf,  is underlain by a clastic deposit of sands and clays, yielding acidic water quality and unstable substrates.  Our data tend to confirm the observations of Thompson & Hershler.  The absence of freshwater gastropod records from south-central Georgia in our database is due largely to a lack of success, not a lack of effort.  Our scattered records in the upper Satilla drainage are of the widespread Lymnaea columella and Campeloma decisum, as well as the introduced Pomacea.

Thompson & Hershler also suggested that, with the exception of Lyogyrus latus and the two Viviparus species, the Atlantic drainages of Georgia share no prosobranch species in common with the other major river system of the state, the Chattahoochee/Flint, which drains the western half of the state south to the Gulf.  Dillon & Robinson (2011) have 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.  We have also presented evidence that many of the Pleurocera populations formerly believed endemic to Chattahoochee/Flint drainges (e.g. P. albanyensis, P. viennaensis) are P. catenaria, widespread in Atlantic drainages north to Virginia.  Additional faunal similarities may be uncovered as our survey extends west.

Turning next to the set of nine species shown in Table 1 that are were uncommon in Georgia, but common enough elsewhere in Atlantic drainages to warrant FWGNA ranks of I-4 or I-5.   Lymnaea humilis, Physa gyrina, Promenetus exacuous, Viviparus georgianus, Cipangopaludina japonica and Gillia altilis are more common further north, approaching (or exceeding) the southern margins of their ranges in Georgia.  Lymnaea cubensis/viator is more common further south.  Viviparus intertextus is primarily an inhabitant of interior drainages, although not uncommon in South Carolina. Littoridinops tenuipes is an inhabitant of coastal marshes, sometimes brackish in character, a habitat type doubtless undersampled in our surveys.

Turning to finally to the set of uncommon species listed in Table 1 that demonstrate incidence ranks of I-3 or lower, we find six species apparently on the periphery of larger ranges not currently surveyed by the FWGNA project.  These six species demonstrate the phenomenon of "pseudo-rarity" (Gaston 1994), and bear FWGNA incidence ranks marked with the suffix "p."  Five of these are more common in Florida: Hebetancylus excentricus, Pomacea maculata, Pleurocera floridensis, Floridobia floridana and Biomphalaria havanensis.  Valvata tricarinata is primarily an inhabitant of northern lakes.

The remaining subset of three species (marked I-1, I-2, or I-3*) are all hydrobiids, and demonstrably rare. Two species are endemic to isolated springs and spring-fed creeks in Wilcox, Pulaski, and Laurens counties: Marstonia agarhecta and M. gaddisorum.  One species appears to be endemic (or nearly endemic) to a (roughly 100 km) run of the Ogeechee River in Georgia's eastern coastal plain: the undescribed Floridobia species A.

In addition to Floridobia species A, Table 1 shows four other hydrobiids with FWGNA incidence ranks marked by asterisks: Notogillia sathon (I-4*), Spilochlamys turgida (I-3*), Lygogyrus latus (I-3*) and Marstonia halycon (I-3*).  These four species demonstrate non-apparent rarity, having been pushed upward in their incidence rank by pseudo-rare species (Gaston 1994).  We think it also quite likely that the ranks of these four species may have been artificially inflated by conservation-biased oversampling.

Georgia's Comprehensive Wildlife Action Plan lists only three species of freshwater gatropods from Atlantic drainages as "high priority animals": Marstonia agarhecta, Somatogyrus tenax and S. alcoviensis (GaCWCS 2005).  A comparison of this (meager) list to the dataset reviewed above might prove instructive.

> References

Dillon, R. T., Jr. & J. D. Robinson (2011)  The opposite of speciation: Genetic relationships among the Pleurocera populations (Gastropoda: Pleuroceridae) in central Georgia.  Am. Malac. Bull. 29: 1 - 10. 
GaCWCS (2005)  A comprehensive wildlife conservation strategy for Georgia.  Georgia Department of Natural Resources, Wildlife Division.  Social Circle, GA.  [html].
Gaston, K. J. (1994)  Rarity.  Chapman & Hall, London.  205 pp.
Johnson, P. D., and 13 coauthors (2013)  Conservation status of freshwater gastropods of Canada and the United States.  Fisheries 38: 247-282.
Thompson, F. G. & R. Hershler (1991)  Two new hydrobiid snails (Amnicolinae) from Florida and Georgia, with a discussion of the biogeography of freshwater gastropods of south Georgia streams.  Malacological Review 24: 55 - 72.