FWGNA > Species Accounts > Pleuroceridae > Pleurocera modesta
Pleurocera modesta (Lea 1845)
Goniobasis georgiana or "Elimia" gerhardtii

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> Habitat & Distribution

Populations of this rather neglected species inhabit small streams in the (often rugged) upper Hiwassee drainage in extreme western North Carolina, north Georgia, and SE Tennessee.  Goodrich (1940) included this species (known by him only from Georgia) as "Goniobasis gerhardtii," in his list of "Goniobases Representing Invasions From the Alabama River System."  Populations of P. modesta are indeed also quite common in tributaries of the upper Coosa draining south through Georgia into the Mobile Basin.

> 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. modesta 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. modesta, 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

We agree with Thompson (2000) that Isaac Lea's (1845) nomen modesta seems to be the oldest name available for this much-confused taxon.  Both Tryon and Goodrich apparently missed modesta entirely, preferring the Lea (1862) nomina georgiana and gerhardtii to describe pleurocerids with elongated, unsculptured shells from north Georgia.  Goodrich (1941) lowered georgiana to subspecific status under caelatura (Conrad 1849), keeping gerhardtii distinct.  Thompson (2000) synonymized gerhardtii under the resurrected modesta, while retaining georgiana as a subspecies of lecontiana (Lea 1841).

All five of these nomina refer to very similar snails.  The only consistent difference seems to be that modesta, georgiana and gerhardtii are applied to pleurocerids bearing unsculptured shells, while caelatura and lecontiana are applied to snails with shells bearing some plication or striation.  Both caelatura and lecontiana are probable synonyms of Pleurocera catenaria (Say 1822), the widespread Atlantic drainage species with a trans-Appalachian population in the Hiwassee (Dillon & Robinson 2009).  It is possible that modesta, georgiana and gerhardtii may prove to be synonyms of P. catenaria as well.

We do, however, have personal observations of plicate and unsculptured populations co-occurring sympatically in Coosa tributaries of North Georgia which incline us to accept their specific status, at least for the present.

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.

> Supplementary Resources

> 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.

> 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. (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. & 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.  
Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system.  Occas. Pprs. Mus. Zool. Univ. Mich., 417: 1-21.
Goodrich, C. (1941) Pleuroceridae of the small streams of the Alabama River system.  Occas. Pprs. Mus. Zool. Univ. Mich., 427: 1-10.
Jokinen, E.H. (1992) The freshwater snails of New York State. New York State Museum Biological Survey, New York State Museum Bulletin 482.
Smith, D.G.  (1980) Goniobasis virginica (Gastropoda: Pleuroceridae) in the Connecticut River USA. Nautilus 94:50-54.
Thompson, F. G. (2000) Freshwater snails of the genus Elimia from the Coosa River system, Alabama.  Walkerana 11: 1-54.