FWGNA > Freshwater Gastropods of Virginia > Discussion
Discussion
Virginia Atlantic Drainages

The 40 gastropod species we have confirmed from Virginia Atlantic drainages are listed in Table 1, ranked by their number of records in our database. (Note that the two subspecies of Pleurocera catenaria have been combined for this analysis.)  This list is compared to the historical review of Stewart & Dillon (2004), as well as to the spurious review of Johnson and colleagues (2013).

We will first focus our attention on the set of 24 freshwater gastropod species represented in our database by more than ten records.  As would seem to be the general situation throughout the United States Atlantic drainages, none of these species is obviously associated with or limited to a particular river drainage. Rather, our survey suggests a set of species that range throughout the state and a set whose distributions seem broadly correlated to ecoregion or physiographic province.

Species ranging widely throughout the Old Dominion include the cosmopolitan pulmonates Physa acuta, Menetus dilatatus, Gyraulus parvus, Helisoma anceps, Lymnaea columella, L. humilis, and Ferrissia fragilis as well as the prosobranch Campeloma decisum and the introduced Bellamya japonica, rather patchily-distributed.

The fauna of the coastal plain includes Physa pomilia, Physa carolinae, and (generally) Amnicola, Lyogyrus, and Helisoma trivolvis.  Characteristic of the Piedmont ecoregion are four pleurocerid species: Pleurocera virginica, P. catenaria, P. proxima, and Leptoxis carinata, as well as the pulmonates Physa gyrina and Ferrissia rivularis, the last four of which range westward. Fontigens orolibas appears to be narrowly endemic to the Blue Ridge ecoregion. The other Fontigens species (the common F. nickliniana as well as several more rare) appear restricted to the Ridge and Valley portion of our study area, as does Pleurocera semicarinata.

Over the course of the present survey we recorded 16 freshwater gastropod species at fewer than ten sites, and failed to confirm 4 additional species that might have been expected to inhabit Virginia Atlantic drainages, based on previous reviews (shaded in Table 1).

This set of 20 can be subdivided into two subsets.  A subset of 15 are common elsewhere in North America, as reflected by their FWGNA incidence rank of I-4, or a suffix of "p" designating peripheral status.  Common further south of the present study area are Lioplax subcarinata, Somatogyrus virginicus, Hebetancylus excentricus, Gillia altilis, Viviparus georgianus and Valvata bicarinata.  Common further north are Bithynia tentaculata, Promenetus exacuous, Planorbula armigera, Valvata tricarinata, Gyraulus deflectus, Aplexa hypnorum, and Bellamya chinensis.

Two of the species shaded in Table 1 were certainly undercollected by our sampling methods – Littoridinops tenuipes (I-4) and Pomatiopsis lapidaria (I-3p).  Littoridinops populations inhabit brackish and tidal freshwater habitats broadly distributed along the Atlantic coast but inadequately covered by this survey.  Pomatiopsis is amphibious, primarily inhabiting moist and muddy habitats above water level, and is more widespread in interior drainages than in regions draining east to the Atlantic.

The remaining subset of five species appear to be legitimately rare.  This subset includes four epigean or phreatic hydrobiids (Holsingeria unthanksensis, Fontigens morrisoniF. bottimeri and F. tartarea) and the obscure pulmonate Stagnicola neopalustris.

Stagnicola neopalustris
has not been re-collected since its original description from Orange County, Virginia, by F. C. Baker (1911).  If it was indeed a Stagnicola, a Virginia population would be extralimital.  Snails of the subgenus Stagnicola (as Baker applied the taxon) typically range through the northern states and Canada, inhabiting marshes, bogs, and lake margins.  All 24 species and subspecies Baker assigned to the taxon in 1911 were synonymized under the circumboreal Lymnaea palustris by Hubendick (1951).  Its also seems possible that Baker’s neopalustris may have represented a senescent or deformed Lymnaea columella.  In either case, it seems unlikely to us, even should a population of oddly-shaped lymnaeids be rediscovered in Orange County in the present day, that “Stagnicola neopalustris” would prove to be a valid specific nomen.

Virginia's Wildlife Action Plan was developed in 2005 under the US Fish & Wildlife Service's State Wildlife Grant program. The plan lists 20 species of freshwater gastropods as being needy of conservation, ranking them into four tiers - Tier 1 (critical conservation need), Tier 2 (very high), Tier 3 (high), and Tier 4 (moderate). Eleven of these 20 species might range into Virginia Atlantic drainages, and as such are covered by the present report (Table 2).  A comparison of the VDGIF conservation tiers assigned to these 11 species and their FWGNA Incidence ranks (calculated regionally) might prove instructive.

> References

Baker, F.C. (1911) The Lymaeidae of North and Middle America, Recent and Fossil. Chicago Academy of Sciences Special Publication No. 3., Chicago, Illinois.
Hubendick, B. (1951)
Recent Lymnaeidae. Their variation, morphology, taxonomy, nomenclature, and distribution. Kungl. Svenska Vetensk. Akad. Handl., 3, 1-223.
Virginia Department of Game and Inland Fisheries (VDGIF 2005) Virginia Wildlife Action Plan. VA: Richmond.
Johnson, P. D., and 13 coauthors (2013)
 Conservation status of freshwater gastropods of Canada and the United States.  Fisheries 38: 247 - 282.
Stewart, T.W. & R.T. Dillon, Jr. (2004)
Species composition and geographic distribution of Virginia’s freshwater gastropod fauna: a review using historical records. American Malacological Bulletin 19:79-91.