> Habitat & Distribution

Dillon (2019) gave the range of P. shenandoa as "small, rich, hardwater creeks with rocky bottoms and good flow in the Great Valley of Virginia from Augusta County south through the drainages of the Shenandoah, James, and Roanoke Rivers to the New River drainage in Wythe County.  Also ranges westward through the New River drainage from Wythe County to Greenbriar County, WV."  Pleurocera shenandoa demonstrates non-apparent rarity in our 17-state study area, FWGNA incidence rank I-3*.

Earlier versions of this website, online until late 2018, identified populations of P. shenandoa inhabiting U.S. Atlantic drainages as P. semicarinata (Say, 1829).  We have always been aware, however, that Goodrich (1940, 1942) and subsequent authorities restricted the range of P. semicarinata to Ohio drainages much further west, assuming (as we did) that semicarinata populations must also extend from the Ohio up the Kanawha/New through West Virginia, into SW Virginia, and north down the Great Valley.  Our more recently completed survey of Ohio drainages has shown this assumption to be false, however.  The Virginia populations we had previously identified as semicarinata proved to be isolated by over 200 km of rugged terrain from the range of bona fide semicarinata in Kentucky, Ohio, and Indiana.

> 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).

Like other pleurocerids, P. shenandoa is dioecious, eggs being deposited on hard substrates from spring to mid-summer. Eggs are spirally arranged in masses of 2-15 or more, with a tough, membranous outer covering to which sand grains typically adhere (Smith 1980, Jokinen 1992). Although we are unaware of any study specifically directed toward the life history of P. shenandoa, it seems reasonable to expect that two years will be required for maturity, and that several years of iteroparous reproduction can be expected thereafter, as is the case for pleurocerids generally (Dazo 1965). This is life cycle Hi of Dillon (2000: 156 - 162)

> Taxonomy & Systematics

Four populations of P. shenandoa, identified as "Goniobasis semicarinata,"  were included in the allozyme survey of Dillon & Davis (1980) - three from tributaries of the upper New River and one from a tributary of the North Fork Roanoke.  These four populations were quite genetically and morphometrically distinct from populations of P. proxima and P. simplex, the two other species sharing that range.  One of the New River populations (labelled Gs) was also analyzed in the mtDNA sequence study of Dillon & Frankis (2004), and (relabelled SV) the allozyme surveys of Dillon et al. (2013) and Dillon (2014).

The allozyme survey of Dillon (2014) returned at least two surprising results.  The first was that Midwestern pleurocerid populations previously identified as livescens (Menke, 1830) and obovata (Say 1829) were conspecific with Pleurocera semicarinata, the distinctions in shell morphology apparently a result of ecophenotypic plasticity.  The second was that our "control" population SV of Pleurocera semicarinata was the most genetically divergent population in the entire analysis, more similar to Pleurocera canaliculata than to any nominal conspecific. Virginia population SV became the type population of Pleurocera shenandoa (Dillon 2019).

> Maps and Supplementary Resources

• Pleurocera shenandoa in the drainage of The Ohio (2019)
• Pleurocera shenandoa distribution in Atlantic drainages.
• Pleurocera shenandoa in the headwaters of the North Fork Roanoke River.
  

> 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.
• On 11July14 I posted an essay elaborating the results of my (2014) paper on cryptic phenotypic plasticity in P. semicarinata, entitled "Elimia livescens and Lithasia obovata are Pleurocera semicarinata."  The role of "control population SV" in that paper was played (anonymously) by the type population of Pleurocera shenandoa.
• I reviewed the longstanding confusion between Pleurocera shenandoa n.sp. and Pleurocera semicarinata in my blog post of 11Mar19, and described the process by which the two species were ultimately disentangled.

> References

Dazo, B. C. 1965. The morphology and natural history of Pleurocera acuta and Goniobasis livescens (Gastropoda: Cerithiacea: Pleuroceridae). Malacologia 3: 1 - 80.
Dillon, R. T. Jr. (2014)  Cryptic phenotypic plasticity in populations of the North American freshwater gastropod, Pleurocera semicarinata.  Zool. Stud. 53:31.  [pdf]
Dillon, R. T. Jr. (2019)  Description of a new species of freshwater snail (Caenogastropoda: Pleuroceridae) from the Great Valley of Virginia.  Appendix (pp 189 - 193) IN: The Freshwater Gastropods of North America, Volume I: Atlantic Drainages, Georgia through Pennsylvania.
Dillon, R. T. Jr., & G. M. Davis (1980)  The Goniobasis of southern Virginia and northwestern North Carolina: Genetic and shell morphometric relationships.  Malacologia 20: 83-98.
Dillon, R. T. Jr., S. J. Jacquemin & M. Pyron (2013)  Cryptic phenotypic plasticity in populations of the freshwater prosobranch snail, Pleurocera canaliculata.  Hydrobiologia 709: 117-127. [pdf]
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., & 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 freshwater snails of the genus Goniobasis Lea, 1862.  Am. Malac. Bull. 19: 69-77.  
Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system.  Occas. Pprs. Mus. Zool. Univ. Mich., 417: 1-21.
Goodrich, C. (1942)  The Pleuroceridae of the Atlantic Coastal Plain.  Occas. Pprs. Mus. Zool. Univ. Mich. 456: 1 - 6.
Jokinen, E.H. 1992. The Freshwater Snails (Mollusca: Gastropoda) of New York State. NY State Mus Bull 482, Albany, New York.
Smith, D.G. 1980. Goniobasis virginica Gastropoda Pleuroceridae in the Connecticut River USA. Nautilus 94:50-54.
Stewart, T. W., & R. T. Dillon, Jr.  (2004)  Species composition and geographic distribution of Virginia's freshwater gastropod fauna: A review using historical records.  Am. Malac. Bull. 19: 79-91.