FWGNA > Freshwater Gastropods of Mid-Atlantic States > Discussion
Mid-Atlantic photobar

The 41 gastropod species we have confirmed from the Mid-Atlantic drainages of the USA are tabulated by state in Table 1 and ordered by their number of records in our database in Table 2.  This latter list is compared to a list of freshwater gastropod species expected from the spurious review of Johnson and colleagues (2013), giving synonyms as necessary.

As has been the case through Georgia, the Carolinas, and Virginia, the ranges of (almost) none of these 41 species seem to reflect the influence of river drainage system.  Rather, the data suggest a set of cosmopolitan species, a set of species whose ranges seem influenced by latitude or climate, and a set of species whose ranges seem to be influenced by physiographic province.

Eleven of the twelve most common species listed in Table 2 are essentially ubiquitous in the study region – populations being found throughout the Atlantic drainages from Maryland to Pennsylvania.  This group is dominated by the pulmonate snails Physa acuta, Menetus dilatatus, Gyraulus parvus, Helisoma trivolvis, H. anceps, Lymnaea humilis, L. columella, Ferrissia rivularis, and F. fragilis, as well as the prosobranchs Pleurocera virginica and Amnicola limosa.  All of these species are also common in Virginia, and most range further south, as may be judged by their FWGNA incidence ranks of I-5.

FWGMA Study Area The single exception is Leptoxis carinata, populations of which reach high densities in most rivers draining the piedmont and mountains from North Carolina to eastern Pennsylvania.  Leptoxis carinata does seem to show an effect of river system, common and widespread throughout the Susquehanna but absent from the Delaware River drainage, running essentially parallel immediately to its east.  There are no obvious environmental differences between the Susquehanna and the Delaware in physiography, geology, or climate.  One's first hypothesis might be that the range of L. carinata is limited here by its dispersal capabilities.  But then how did L. carinata populations spread from wherever the species evolved through the seven major river systems draining into the Atlantic between the Broad River and the Susquehanna?   And indeed, over the continental divide into drainages of the Ohio and the Tennessee?  See the Leptoxis carinata species page for further examination of this puzzle.

A smaller set of five species listed further down in Table 2 is apparently influenced by physiographic province.  Littoridinops tenuipes is restricted to tidal marshes in the Coastal Plain.  Planorbula armigera and Promenetus exacuous also seem especially adapted to the Coastal Plain environment, populations typically inhabiting swamps, wetlands, and backwaters.  Campeloma decisum and Lyogyrus granum range through the Coastal Plain into the Piedmont, but become quite infrequent in the mountains at the western margins of the present study area, where rivers are smaller and more rapidly-flowing.

The ranges of a set of nine species seem to demonstrate evidence of influence by latitude or climate.  Physa pomilia and P. carolinae are primarily inhabitants of the southern Atlantic Coastal Plain, apparently reaching the northern margins of their ranges in the Delmarva Peninsula.  And reaching the southern margins of their (otherwise more northerly) ranges are Lymnaea catascopium, Helisoma campanulatum, Gyraulus deflectus and Valvata tricarinata – populations of which are characteristic of colder lakes (or lentic environments), and Lymnaea elodes, Aplexa hypnorum and Physa vernalis, all characteristic of northern marshes, ponds, or vernal habitats.

Reference to Table 2 shows that 12 species of Mid-Atlantic freshwater gastropods are apparently uncommon, bearing incidence ranks of I-3 or lower.  Most of these are elements of a more northerly fauna, however, as noted above: Valvata tricarinata, Helisoma campanulatum, Lymnaea catascopium, L. elodes, Gyraulus deflectus, Bithynia tentaculata, and Aplexa hypnorum.  These species become much more common in New York (Jokinen 1992), westward through Michigan (Burch & Jung 1992 ) and Canada (Clarke 1981).  Pomatiopsis lapidaria populations are more common in interior drainages to the west of the present study area, and was almost certainly undercollected by our survey techniques, in any case.  Potamopyrgus antipodarum is a recent invader of the US Mid-Atlantic drainages, much more common in the Great Lakes and the American West.

The remaining subset of four species seem to be legitimately uncommon.  Fontigens orolibas and F. bottimeri are spring dwelling (or cave dwelling) hydrobiids ranging through the Blue Ridge and the Ridge-and-Valley Provinces of Virginia into Pennsylvania.  Somatogyrus pennsylvanicus is an element of the riffle dwelling fauna of mid-size to larger rivers, a habitat that has doubtless suffered from the high sedimentation loads generated by intensive agricultural practices of the last 200 years.  Physa vernalis is an inhabitant of small or vernal ponds in northern latitudes, recently described from Connecticut by Taylor & Jokinen (1984). Although populations of P. vernalis seem quite rare, at least in our study area, little is known about the continental range of the species.

> References

Burch, J. B. & Y. Jung.  1992.  Freshwater Snails of the University of Michigan Biological Station Area.  Walkerana 6 (15): 1 - 218.
Clarke, A. H. 1981.  The Freshwater Molluscs of Canada.  National Museums of Canada, 445 pp.
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.  
Taylor, D. W. & E. H. Jokinen. 1984.  A new species of freshwater snail (Physa) from seasonal habitats in Connecticut.  Freshw. Invert. Biol. 3: 189-202.