> Discussion
Georgia Atlantic Drainages

The 37 freshwater gastropod species and subspecies inhabiting the Atlantic drainages of Georgia are listed in Table 1, ranked by the number of sites at which they were recorded.

The fauna includes a set of about 8 - 10 species that might be characterized as ubiquitous and a set of 6 species (entirely hydrobiids) that appear to be endemic or narrowly-restricted. Treatment of these species (Notogillia, Spilochlamys, Marstonia halcyon, M. agarhecta, M. gaddisorum, and Floridobia species A) will be deferred to the section on "Recommendations."

Setting aside the 6 hydrobiids, few of Georgia freshwater gastropod species seem to be associated with particular rivers or drainages. The distributions of several species seem, on the other hand, to broadly correlate with US EPA ecoregions, as has been observed elsewhere throughout the southeast.  Goniobasis proxima is characteristic of the Blue Ridge and Piedmont ecoregions, while Helisoma trivolvis, Physa pomilia, Physa species A, Laevapex fuscus and Hebetancylus excentricus appear restricted to the Southeastern Plain / Southern Coastal Plain region.

Table 1 shows that 13 species were represented in our database by five records or fewer.  Three of these species are Floridian, reaching the northern edge of their ranges: Goniobasis floridensis timidus, Floridobia floridana, and the exotic Pomacea insularum.  Most of the other nominally rare species are more common elsewhere in North America, and also apparently on the edges of their much larger distributions: Hebetancylus excentricus, Lymnaea cubensis, Viviparus intertextus, Promenetus exacuous, Physa gyrina, and Lymnaea humilis.  Our single record of Biomphalaria from Georgia may represent an artificial introduction. The status of the remaining species (Marstonia agarhecta, M. gaddisorum, and Floridobia sp A) we suggest may be cause for some conservation concern.

On 1/25/07, our query of (Freshwater gastropods AND Georgia) to the NatureServe Explorer database returned a list of 72 species.  A total of 32 of those species are associated with western drainages (of the Gulf) and as such are not the subjects of this study.  The remaining list 40 species is compared to the 37 species confirmed in this study in the right column of Table 1.

Combining synonyms, the NatureServe list of freshwater gastropods from Georgia Atlantic drainages reduces from 40 to 29.  Missing from the NatureServe list were two common species - Amnicola limosa and Lyogyrus granum.  But most of the species overlooked on the NatureServe list were either quite sporadically distributed in Georgia, or suffer some taxonomic complication (Goniobasis catenaria dislocata, Physa species A).  The NatureServe list also included one species we have not collected from Georgia Atlantic drainages - Planorbula armigera.  This is another wide ranging but sporadically-distributed species, possibly dispersed by birds, and easy to miss in a systematic survey.  The absence of P. armigera from our database is certainly no cause for conservation concern.

In their 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 sample sites in south-central Georgia apparent in Figure 1 is due largely to a lack of success, not 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.  We have reported, however, the occurrence of Goniobasis floridensis in Atlantic drainages around Hawkinsville, a species widespread in the Chattahoochee/Flint and other Gulf drainages south into Florida.  Additional faunal similarities may be uncovered as our survey extends westward.

> References
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.