> Discussion

Although the sites visited by myself and my colleagues throughout the state of South Carolina probably numbered in excess of one thousand, only 376 were positive for freshwater gastropods.  Figure 1 shows the 376 sites mapped on the five USEPA Ecoregions of South Carolina, and Table 1 gives the number of sites at which each of the 31 species were collected.  The cosmopolitan “tadpole snail” Physa acuta was the most widespread species, collected from a variety of environments at 123 sites throughout the state.  Campeloma decisum was found at 82 generally lotic (or in any case sandy) sites statewide, Lymnaea columella was collected in 48 primarily lentic habitats, and Laevapex fuscus also appeared at 48 sites, on plant matter and woody debris.

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 distributions of Goniobasis proxima and Ferrissia rivularis seem closely correlated to the Blue Ridge Ecoregion.  These two species may range into the upper Piedmont, but are entirely absent from the Coastal Plain.  Goniobasis proxima and F. rivularis seem especially common in well-oxygenated rivers and streams with good flows of well-oxygenated water.  The environment need not be rich or productive.

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

Goniobasis catenaria dislocata appears to be restricted to rivers and streams of the Southeastern Plains Ecoregion.  It does not require rocky substrate, but is often found clinging to woody debris, or grazing on sandy substrate 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 species A, and the two Amnicola species.  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 species A appear especially adapted to swamps – rich but shallow and possibly ephemeral habitats near the coast.

In summary, the freshwater gastropod fauna of South Carolina includes a small set of species of statewide range and a rather larger set of species showing distributions apparently related to climate and topography.  South Carolina also includes a set of species with limited or local distributions.  These will be reviewed in the section on conservation recommendations.