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 other species subsequently reported
(Johnson et al. 2013), with synonyms commonly encountered in the
literature.
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.