> Discussion

The 35 gastropod species and subspecies inhabiting North Carolina Atlantic drainages are listed in Table 1, ranked by their number of records in our database. This list is compared to the result of a query to the NatureServe Explorer database performed on 6/2006.

As seems to be the case throughout the southeastern United States generally, few North Carolina freshwater gastropod species are associated with particular rivers or drainages. The distributions of several species seem, on the other hand, to broadly correlate with US EPA ecoregions. Goniobasis proxima is characteristic of the Blue Ridge and Piedmont ecoregions. Goniobasis catenaria, G. virginica, and Somatogyrus virginicus were collected from the Piedmont ecoregion east into the southeastern plains. The Southeastern Plains and Middle Atlantic Coastal Plain Ecoregions hosted Helisoma trivolvis, Gyraulus parvus, and Physa species A.  Apparently restricted to the Coastal Plain were Physa pomilia and Promenetus exacuous.

North Carolina also apparently encompasses sufficient latitude that the distributions of several of its freshwater gastropod species seem to reflect a north – south gradient. Three species more common to the south seem to reach or approach their northern limits in the state – Viviparus intertextus, Lymnaea cubensis, and Hebetancylus excentricus. Four species more common to the north seem to reach their southern limits – Leptoxis carinata, Physa gyrina, Planorbula armigera and Valvata bicarinata.

Setting aside species reaching their (otherwise broad) range limits, as well as the invasive viviparids Bellamya japonica and Viviparus georgianus, Table 1 shows that four species were represented in the database by fewer than 20 records. Physa pomilia was identified at just 10 sites, but that species is easily confused with the cosmopolitan P. acuta, and thus may be underreported. The three remaining species, Helisoma magnificum (6 records from 2 sites), H. eucosmium (3 records from 2 sites), and Floridobia sp. (2 sites) we suggest may be the legitimate objects of conservation concern. These will be treated separately in the section on recommendations.

On 6/16/06, our query of (Freshwater gastropods AND North Carolina) to the NatureServe Explorer database returned a list of 58 species. Ten of those species were associated with western drainages (either New River or Tennessee River) and as such are not the subjects of this study: five Goniobasis, two Leptoxis, two Pleurocera, and Pomatiopsis lapidaria. The remaining list of 48 species included nine species we did not confirm and ten synonyms of species we did confirm, leaving the number of matches between our list and the NatureServe list as 29.

The six species of freshwater gastropods on our list that were not returned by the NatureServe query included only one glaring omission - Amnicola grana, which we recorded from 113 sites. The five other species (Physa species A, Lymnaea cubensis, Hebetancylus excentricus, Helisoma eucosmium and Littoridinops tenuipes) are difficult to sample, on the edges of their ranges (as noted above) or suffer from some taxonomic complication.

The nine freshwater gastropod species listed by NatureServe for Atlantic drainages of North Carolina which we did not record included five introduced species: Helisoma (Planorbella) duryi, Cipangopaludina chinensis, Marisa cornuarietis, Pomacea paludosa, and Melanoides tuberculatus. Three of the NatureServe species primarily inhabit northern latitudes: Probythinella emarginata, Valvata sincera, and Gyraulus deflectus. Additional sampling on our part may well ultimately confirm the extension of these species into North Carolina. The ninth species, Neritina usnea (more usually called N. reclivata) inhabits coastal, possibly brackish, environments which were inadequately surveyed in this study. In any case, the absence of none of these nine species is a cause for conservation concern.