FWGNA > Freshwater Gastropods of South Carolina > Discussion
Discussion

Although the sites we visited throughout the state of South Carolina certainly numbered in excess of one thousand, only approximately half were positive for freshwater gastropods.   Table 1 shows the 35 species confirmed for the state, ordered by their number of records in our database.  (Note that the subspecies of P. catenaria were combined for this analysis.)  This list is compared to the (1913) catalogue of Mazyck, and to a list from the spurious review of Johnson and colleagues (2013).

Among the more interesting findings of the present survey is the apparent independence of freshwater gastropod distribution and river drainage.  Individual examination of the range maps for all species collected in this study does not reveal a single instance where a species seems constrained to a river system or set of tributaries.  Rather, the distributions of many elements of the freshwater gastropod fauna of South Carolina leave one with the impression that, at a regional scale, the most important factor is climate and topography.

The distribution of Pleurocera proxima seems closely correlated to the Blue Ridge Ecoregion.  Populations of this species may range into the upper Piedmont, but are entirely absent from the Coastal Plain.

Characteristic of the Piedmont Ecoregion are Pleurocera catenaria catenaria, Helisoma anceps, and (perhaps) Somatogyrus virginicus.  These species seem to prefer lotic environments – Pleurocera and Somatogyrus are especially associated with rock substrate – and waters that are at least moderately productive.  Judging from its (currently fragmented) distribution, it is our suspicion that P. catenaria may have been especially impacted by sedimentation from agricultural development in South Carolina.

Pleurocera catenaria dislocata appears to be restricted to rivers and streams of the Southeastern Plains Ecoregion.  Populations do not seem to require rocky substrate, but are often found clinging to woody debris, or grazing on sandy substrates in the eddys.

Helisoma trivolvis seems to be found throughout the three ecoregions that comprise the Coastal Plain province.   And characteristic of the Middle Atlantic Coastal Plain Ecoregion are Viviparus intertextus, Physa carolinae, Amnicola limosa and Lyogyrus granum.  All five of these species seem most common in lentic habitats with dark waters, high in dissolved organics although not (necessarily) acidic.  Viviparus intertextus and Physa carolinae appear especially adapted to swamps – rich but shallow and possibly ephemeral habitats near the coast.

Table 1 also shows that 15 species of freshwater gastropods were collected at fewer than ten of the approximately 500 snail-positive sites in South Carolina, and that another 4 species were predicted by the works of either Mazyck or Johnson, but not confirmed.  All 19 of these species are significantly more common elsewhere, as demonstrated by their FWGNA incidence ranks of I-4 and I-5, or by the suffix "p" indicating peripheral status.

Hebetancylus excentricus, Pomacea maculata, P. paludosa and Biomphalaria havanensis become more common in Florida and points south of the present study area.  Becoming more common to the north of the present study area are Viviparus georgianus, Promenetus exacuous, Valvata bicarinata, Lioplax subcarinta, Lymaea humilis, Gillia altilis, Valvata tricarinata, Physa gyrina, Gyraulus deflectus, Ferrissia rivularis and Planorbula armigera.  More common in the interior drainages to the west are Viviparus subpurpureus and Pomatiopsis lapidaria.  

The fauna of South Carolina does not seem to include any species of freshwater gastropod that might be considered legitimately rare.  Three species have listed as "priorities" on the state wildlife action plan: Somatogyrus virginicus, Lioplax subcarinata and Gillia altilis.  Clench (1962) reported L. subcarinata in Atlantic drainages from New York to South Carolina, although Jokinen (1992) suggested that the species may have been extirpated in New York.  The nominal range of G. altilis is similar: Atlantic drainages from New Jersey to South Carolina (Walker 1918), although in this case Jokinen (1992) reported a true range extending further north.  Our FWGNC survey (elsewhere on this website) suggests that all three of these species are more common in North Carolina, although populations seem to be rare in Virginia and in Atlantic drainages further north.

> References

Clench, W.  1962.  A catalogue of the Viviparidae of North America with notes on the distribution of Viviparus georgianus, Lea. Occas. Pprs. on Mollusks, Mus. Comp. Zool. Harvard 2: 261-287.  
Johnson, Bogan, Brown, Burkhead, Cordeiro, Garner, Hartfield, Lepitzki, Mackie, Pip, Tarpley, Tiemann, Whelan & Strong 2013.  Conservation status of freshwater gastropods of Canada and the United States.  Fisheries 38: 247- 2824.  
Jokinen, E.  1992.  The freshwater snails (Mollusca: Gastropoda) of New York State. New York State Museum Bulletin 482: 1-112.  
Mazyck, W.  1913.  Catalog of Mollusca of South Carolina, P. Rea (eds.). Contributions from the Charleston Museum, vol. II. Charleston, SC, Charleston Museum.  
Walker, B.  1918.  A Synopsis of the Classification of the Freshwater Mollusca of North America, North of Mexico. Misc. Pubs., vol. 6. Ann Arbor, University of Michigan Press.  
Watson, C.  2000.  Results of a survey for selected species of Hydrobiidae (Gastropoda) in Georgia and Florida. Pp 233 - 244 In Freshwater Mollusk Symposia Proceedings, Tankersley, Wamolts, Watters, Armitage, Johnson, & Butler (eds). Columbus, Ohio Biological Survey.