> Habitat & Distribution

Pleurocera proxima populations range through the mountains and piedmont from north Georgia through the Carolinas into central Virginia on both sides of the continental divide, inhabiting tributaries of the James River south to the Oconee River on the Atlantic side, the upper New River of the Ohio, and Tennessee River tributaries from the Holston to the Hiwassee (Dillon & Robinson 2009).  We have confirmed one extralimital population (or set of nearby populations) in southeastern Pennsylvania.  (See my essay of 11Oct12 from the link below for more.)

Through this vast and rugged territory, populations of P. proxima reach maximum abundance in smaller streams with high percent groundwater and good flow over rock & cobble substrate (Foin & Stiven 1970, Foin 1971, Dillon & Keferl 2000).  Water quality is typically soft and low in nutrients, but the environment quite stable and predictable.  FWGNA incidence rank I-5.

> Ecology & Life History

Grazing by populations of pleurocerids can have a significant effect on energy flow in small streams (Dillon 2000: 86 - 91, see also Dillon & Davis 1991).  The snail populations are perennial and iteroparous, two years being required for maturity (Stiven & Walton 1967, cycle Hi of Dillon 2000: 156 - 162).  I am not aware of any detailed data on reproductive energetics, but it seems quite likely to me that P. proxima populations are S-adapted in the sense of Dillon (2000: 131- 136), demonstrating allocation to reproductive effort an order of magnitude below expectation based on body size, or less.

The sex ratio seems balanced in some P. proxima populations (Dillon 2000: 104 – 106) although female-biased in others.  Individual movement seems to average around 10 m/yr upstream and 5 m/yr down, but apparently minor barriers may have significant effects (Crutchfield 1966, Dillon 1988a, 1988b, 2020, Stiven & Kreiser 1994).  See my essay of 12Oct21 from the link below for a review of our research bearing on intrapopulation gene flow in P. proxima, barriers to dispersal and isolation by distance.  

Populations of P. proxima often carry substantial loads of trematode parasites (Lang 1968, Wyderko et al. 2015, Zemmer et al. 2020).

> Taxonomy & Systematics

Strikingly high levels of interpopulation divergence in morphology, allozyme frequency, and DNA sequence among populations of P. proxima are well-documented (Dillon, 1984a; 1984b; 1988; Dillon and Reed, 2002; Dillon and Frankis, 2004; Dillon and Robinson, 2009; 2011), correlated with overland distance but independent of drainage system or water distance.  Dillon and Robinson (2009) speculated that the distribution of P. proxima may date to the Paleozoic orogeny of the Appalachians.  (See my essay of 16Mar09 from the link below for more.)  Dillon and Davis (1980) recognized three races: Race A being the typical form found in the mountains and foothills, Race B inhabiting harder waters, and Race C inhabiting streams of lower flow in the eastern piedmont.  Goniobasis (or Elimia) symmetrica (Haldeman, 1841) is a synonym. The diploid number is 2N = 34 (Dillon 1991).

This species has travelled through three genera in thirty years.  Although predominantly assigned to Goniobasis through most of the 20th century, in the 1980s many workers began placing it in the resurrected generic nomen, "Elimia."  Both Goniobasis and Elimia were subsumed under Pleurocera by Dillon (2011).  See my essay of 23Mar11 from the link below for more.

> Maps and Supplementary Resources

• Distribution in tributaries of The Ohio (2019)
• Pleurocera proxima distribution in Atlantic drainages (2023)
• Distribution in the Tennessee/Cumberland (2022)
• Virginia species account with county distribution (2011)
• Pleurocera proxima grazing in the splash midstream.
  
• Pretty photo of living P. proxima, courtesy of Chris Lukhaup.
  

> Essays

• Taxonomic controversy has surrounded the generic nomina Pleurocera, Goniobasis, and Elimia for many years.  The best entry into the subject would be my essay of 23Mar11, entitled Goodbye Goniobasis, Farewell Elimia.  Links are available from that essay to older resources.
• See my FWGNA post of 16Mar09, The Snails The Dinosaurs Saw, for more on the genetics, taxonomy, and distribution of P. proxima.
• The discovery of Pleurocera proxima populations in southeastern Pennsylvania was reported in my essay of 11Oct12, The Most Improbable Invasion.  That post also included a brief review of the life history of P. proxima, and several photos of the animal and its habitat.
• I briefly reviewed my old (1984a) dissertation research on the relationship between geographic distance and genetic divergence among populations of P. proxima in my blog post of 24Apr17, Accelerating The Snail's Pace.
• See my essay of 7Sept21, Intrapopulation Gene Flow: King Arthur's Lesson, for a review of research generally relevant to the question of intrapopulation gene flow in freshwater gastropods, with a special nod toward Pleurocera proxima.
• I reviewed my dissertation research on gene flow within a particularly well-studied North Carolina population of P. proxima in a blog post of 12Oct21, Intrapopulation Gene Flow: The Polymorphic Pleurocera of Naked Creek.  I then compared my 1980s P. proxima results to more recent findings from Alabama in a follow-up post of 2Nov21, Intrapopulation Gene Flow, the Leptoxis of the Cahaba, and the Striking of Matches. 

> References

Crutchfield, P. (1966)  Positive rheotaxis in Goniobasis proxima. Nautilus, 79: 80-86.
Dillon, R. T., Jr.  (1984a)  Geographic distance, environmental difference, and divergence between isolated populations. Syst. Zool., 33: 69-82.  
Dillon, R. T., Jr.  (1984b)  What shall I measure on my snails?  Allozyme data and multivariate analysis used to reduce the non-genetic component of morphological variance in Goniobasis proxima. Malacologia, 25: 503-511.  
Dillon, R. T., Jr. (1986)  Inheritance of isozyme phenotype at three loci in the freshwater snail, Goniobasis proxima: Mother-offspring analysis and an artificial introduction. Biochem. Genet., 24: 281-290.  
Dillon, R., T., Jr. (1988a)  Evolution from transplants between genetically distinct populations of freshwater snails. Genetica, 76: 111-119.  
Dillon, R. T., Jr. (1988b)  Minor human disturbance influences biochemical variation in a population of freshwater snails. Biol. Conserv., 43: 137-144.  
Dillon, R. T., Jr. (1989)  Karyotypic evolution in pleurocerid snails: I. Genomic DNA estimated by flow cytometry. Malacologia, 31: 197-203.  
Dillon, R. T., Jr. (1991)  Karyotypic evolution in pleurocerid snails: II. Pleurocera, Goniobasis, and Juga. Malacologia, 33: 339-344.  
Dillon, R. T., Jr. (2000)  The Ecology of Freshwater Molluscs. Cambridge, Cambridge University Press.  509 pp.  
Dillon, R. T., Jr. (2011)  Robust shell phenotype is a local response to stream size in the genus Pleurocera (Rafinesque, 1818).  Malacologia 53: 265-277.
Dillon, R. T., Jr. (2020) Fine scale genetic variation in a population of freshwater snails. Ellipsaria 22(1) 24 - 25.
Dillon, R. T., Jr. & Davis, G. M. (1980) The Goniobasis of southern Virginia and northwestern North Carolina: Genetic and shell morphometric relationships. Malacologia, 20, 83-98.  
Dillon, R., T. Jr., & K. B. Davis. (1991)  The diatoms ingested by freshwater snails: temporal, spatial, and interspecific variation. Hydrobiologia, 210: 233-242.  
Dillon, R., T, Jr., & R. C. Frankis (2004)  High levels of mitochondrial DNA sequence divergence in isolated populations of the freshwater snail genus Goniobasis. Amer. Malac. Bull., 19: 69-77.  
Dillon, R. T., Jr. & Keferl, E. (2000) A survey of the pleurocerid gastropods of South Carolina.  In Freshwater Mollusk Symposia Proceedings, Part II, eds. Tankersley, Warmolts, Watters, Armitage, Johnson & Butler, pp. 153 - 160.  Columbus: Ohio Biological Survey.  
Dillon, R. & Reed, A. (2002) A survey of genetic variation at allozyme loci among Goniobasis populations inhabiting Atlantic drainages of the Carolinas.  Malacologia 44: 23 - 31. 
Dillon, R.T., Jr. & Robinson, J. D. (2009)  The snails the dinosaurs saw: Are the pleurocerid populations of the Older Appalachians a relict of the Paleozoic Era?  J. N. Am. Benthol. Soc. 28: 1-11. 
Dillon, R.T., Jr. & Robinson, J. D. (2011) The opposite of speciation: Genetic relationships among populations of Pleurocera in central Georgia.  Amer. Malac. Bull. 29: 1 - 10.
Foin, T. C. (1971)  The distribution pattern of the freshwater prosobranch gastropod Oxytrema proxima (Say). J. Elisha Mitchell Sci. Society, 87: 1-10.  
Foin, T.C. & A. Stiven  (1970)  The relationship of environment size and population parameters in Oxytrema proxima (Say) (Gastropoda: Pleuroceridae). Oecologia (Berl.), 5: 74-84. 
Goodrich, C. (1942) The Pleuroceridae of the Atlantic coastal plain. Occas. Pprs. Mus. Zool. Univ. Mich., 456, 1-6.  
Lang, B. Z. (1968)  Note on ecology of Goniobasis proxima in North Carolina.  Nautilus 82: 3 – 5.  
Stiven, A. E. & B.R. Kreiser (1994)  Ecological and genetic differentiation among populations of the gastropod Goniobasis proxima (Say) in streams separated by a reservoir in the piedmont of North Carolina.  J. Elisha Mitchell Sci. Society 110: 53 – 67.  
Stiven, A., & C. Walton. (1967)  Age and shell growth in the freshwater snail, Goniobasis proxima (Say). Am. Midl. Natur., 78: 207-214.
Wyderko, J. A., E. F. Benfield, J. C. Maerz, K. C. Cecala, and L. K. Belden (2015)  Variable infection of stream salamanders in the southern Appalachians by the trematode Metagonimoides oregonensis (family: Heterophyidae).  Parasitol. Res. 114: 3159 - 3165.
Zemmer SA, Detwiler JT, Sokol ER, Da Silva Neto JG, Wyderko J, Potts K, et al. (2020) Spatial scale and structure of complex life cycle trematode parasite communities in streams. PLoS ONE 15(11): e0241973. https://doi.org/10.1371/journal.pone.0241973