FWGNA > Freshwater Gastropods of The Ohio > Discussion
FWGO photobar
Discussion

The 70 gastropod species and subspecies we have recovered from the waters of the Ohio River above the mouth of the Tennessee/Cumberland are tabulated by state in Table 2.  Table 2 is available at right either as a pdf file or as an excel spreadsheet, which may be selected by column and sorted by any particular state of interest.  And the species and subspecies are ordered by their number of incidences in our 5,250-record database in Table 3, giving common synonyms and overall (15-state) FWGNA incidence ranks [still TBD].

Dillon and colleagues (2019a) reported 70 species and subspecies of freshwater gastropods inhabiting Atlantic drainages from Georgia to the New York line.  This fauna is dividable into a set of cosmopolitan species, a set of endemics or near-endemics, and a set of species demonstrating north-south regionalism.  A comprehensive “FWGTN” gastropod survey is also available from the waters of East Tennessee (Dillon & Kohl 2013).  The list of 40 species and subspecies recovered by Dillon & Kohl also included cosmopolitan and endemic elements, as well as a set of species which, when compared to the Atlantic drainage fauna, demonstrated an east-west regionalism.

With the addition of survey results from the Ohio drainages, we can now examine the distribution of the North American gastropod fauna for evidence of both north-south and east-west regionalism simultaneously.  And let us combine subspecies for a continental-scale analysis, bringing the lists down to 65 species for The Ohio, 69 for the Atlantic drainages, and 39 for Tennessee.

Next let us subdivide the 69-species Atlantic drainage fauna into South-Atlantic and Mid-Atlantic halves at north latitude 36.54, the North Carolina / Virginia line.  We find that 18 species are unique to the South-Atlantic and 12 unique to the Mid-Atlantic, with an overlap of 39 species.  The 18 uniquely South-Atlantic species will be set aside for the present analysis.

Then of the 69 – 18 = 51 Atlantic species remaining, 33 are shared with the drainages of The Ohio.  Let us call these 33 species “cosmopolitan” at the scale of the present analysis: extending (at minimum) from the Carolinas both north and west across the eastern continental divide.  Then 65 – 33 = 32 Ohio drainage species demonstrate some degree of regionalism.  These 32 species are classified in Figure 2.

Figure 2 below lists 16 species as “FWGO only.”  These are 4 pulmonates (in red font) and 12 prosobranchs (in black) that have not been previously recorded by FWGNA surveys – 9 hydrobioids, 3 pleurocerids, 2 lymnaeids, 1 planorbid and 1 ancylid.  We should hasten to stipulate that we have not published any results from the Great Lakes drainages to the north nor Mississippi drainages to the west, so these 12 species are not necessarily endemic to drainages of The Ohio.  They are not, in any case, shared with drainages of the Atlantic or East Tennessee.

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A set of 6 Ohio species, all pulmonates, are shared with Mid-Atlantic states, but not with other regions  (pushing the single NW Pennsylvania population of Pleurocera virginica aside as accidental).  And compared with drainages of The Ohio, the Mid-Atlantic states have six unique species, 2 pulmonates and 4 prosobranchs.  This is a clear demonstration of East/West regionalism in the freshwater gastropods of North America.  The phenomenon might best be characterized as a gradual turnover of pulmonates proceeding east, slightly net negative, and an abrupt discontinuity of prosobranchs, strikingly net negative.

Figure 1 also shows that the Ohio fauna shares a set of 7 species, almost all pleurocerids, with Tennessee drainages to the south (pushing down the New/Kanawha populations of Pleurocera clavaeformis and P. gabbiana as accidental).  And the Tennessee list includes 13 species (6 pleurocerids, 6 hydrobioids, 1 pulmonate) not found in Ohio drainages.  This is a clear demonstration of North/South regionalism in the freshwater gastropods of North America.  The phenomenon might best be characterized as a gradual turnover in prosobranchs heading south, net neutral, and an abrupt discontinuity of pulmonates, strikingly net negative.

Narrowing our focus to biogeographic patterns within the Ohio River basin, we notice that the distributions of many freshwater gastropods seem to reflect ecoregional distinctions.  The northern Glaciated Central Lowlands host quite a few species not found elsewhere in the basin, including Marstonia lustrica, Lymnaea stagnalis, Physa vernalis, Aplexa hypnorum, Helisoma campanulata, Gyraulus deflectus, and G. circumstriatus.  Proceeding into the southern Glaciated Central Lowlands, we add Viviparus georgianus, Probythinella emarginata, Cincinnatia integra, and Lymnaea elodesPleurocera troostiana is restricted to the Unglaciated Interior Low Plateau.  Pleurocera simplex and P. laqueata range through the Unglaciated Interior Low Plateau, extending into the Appalachian Plateau.

We were surprised to discover populations indistinguishable from Pleurocera simplex and P. troostiana of East Tennessee as far north as Kentucky. 

Goodrich (1940) gave the range of Pleurocera (“Goniobasis”) simplex as “headwaters of Tennessee River system in Virginia, Tennessee and North Carolina; Beaver Fork of Bluestone River of Kanawha River, Mercer County, West Virginia."  He seems to have identified Kentucky and Middle Tennessee populations as “Goniobasis ebenum,” which he considered to range through “Cumberland River above the Falls; Smith’s Shoals, Pulaski County, KY; springs and small streams of this river (The Cumberland) downstream to Dickson County, TN.”  And indeed, Branson (1987) and Branson, Batch & Call (1987) listed Goniobasis ebenum prominently in the Kentucky malacofauna, neglecting simplex entirely.

Our observations suggest that populations bearing shells of the simplex morphology intergrade smoothly with those bearing shells of the ebenum morphology as stream size increases.  We here consider ebenum (Lea 1841) a subspecies of simplex (Say 1825) and suggest that P. simplex (now more broadly understood) extends through tributaries of the Kentucky and Green Rivers almost as far north as The Ohio River, as well as throughout the Cumberland drainage.

Our understanding of Pleurocera troostiana has also improved significantly in recent years.  Goodrich (1940) considered “Goniobasis troostiana (Lea 1839)” endemic to its East Tennessee type locality, Moss Creek in Jefferson County.  Our surveys of East Tennessee (Dillon & Kohl 2013) revealed that many more broadly-distributed species were junior synonyms of troostiana, however, the distinctions resting on phenotypically-plastic elements of the shell.

In the present survey, we report pleurocerid populations indistinguishable from troostiana through central and western Kentucky as far north as the Ohio River.  In this region they have historically been identified as Goniasis (or Elimia) plicata-striata, G. curryana, and G. curryana lyoni (Bickel 1968, Branson 1987).  We suggest that all of these specific nomina, plicata-striata (Wetherby 1876), curryana (Lea 1841), and lyoni (Lea 1863), are simple synonyms of troostiana (Lea 1839).

> References

Bickel, D.  (1968)  Goniobasis curreyana lyoni, a pleurocerid snail of west-central Kentucky. The Nautilus 82: 13 - 18.
Branson, B.A. (1987) 
Keys to the aquatic Gastropoda known from Kentucky.  Trans. Kentucky Acad. Sci. 48: 11 – 19.
Branson, B.A., D.L. Batch and S.M. Call (1987)
Distribution of aquatic snails (Mollusca: Gastropoda) in Kentucky with notes on fingernail clams (Mollusca: Sphaeriidae: Corbiculidae)  Trans. Kentucky Acad. Sci. 48: 62 – 70.
Dillon, R.T., Jr., M.J. Ashton, W.K. Reeves, T.P. Smith, T.W. Stewart, & B.T. Watson (2019a)
Atlantic drainages, Georgia through Pennsylvania.  Freshwater Gastropods of North America, Volume 1.  FWGNA Press.  199 pp.
Dillon, R. T., Jr. & M. Kohl (2013)
The Freshwater Gastropods of Tennessee. Internet address: http://www.fwgna.org/FWGTN