FWGNA > Species Accounts > Pleuroceridae > Pleurocera clavaeformis unciale
Pleurocera clavaeformis unciale (Haldeman 1841)

  • click to view larger

> Habitat & Distribution

The range of P. unciale was given by Goodrich (1940) as "upper tributaries of the Tennessee River in Virginia and eastern Tennessee."  Our survey generally confirms that populations of (what we now understand to be) Pleurocera clavaeformis bearing the more robust "unciale" shell form do indeed inhabit all the major tributaries of the Tennessee (Clinch, Holston, Nolichucky/French Broad, Little Tennessee, Hiwassee) as long as there is some solid substrate. We are not aware of any P. clavaeformis populations (of any morphology) in the largest (impounded) reaches of the Tennessee River below Knoxville.  The FWGNA incidence rank for all subspecies of P. clavaeformis combined is I-5.

> Ecology & Life History

As the "pleurocerid in residence" at the Oak Ridge National Laboratory, Pleurocera clavaeformis (bearing the typical shell form) has played a key role in quite a number of excellent studies of stream ecology (e.g., Steinman 1991, Hill 1992,  Rosemond et al. 1993, Hill et al. 2010).  Grazing by populations of P. clavaeformis can have a significant impact on algal biomass, primary productivity, and periphyton community structure.  For a review see Dillon (2000: 86 - 91).

Like other pleurocerids, P. clavaeformis 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. clavaeformis, 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. clavaeformis (bearing the typical shell form) were included in the allozyme study of Dillon & Robinson (2007a - PDF below).  The species is quite distinct genetically.  There is no evidence of hybridization with P. simplex, P. gabbiana, or P. troostiana, the other species of Pleurocera with which it often occurs.

The shells borne by populations of Pleurocera clavaeformis display a wide range of carination, as well as variation in overall length-to-width ratio and robustness.  Populations inhabiting smaller streams are more slender and may bear a single carination (of varying strength) around mid-whorl.  Such populations have been referred to the specific nomen acutocarinata (Lea 1841), the type of the genus Elimia.  Populations in moderately-sized streams are often entirely uncarinate, showing the typical shell form which I (and most other workers) have for many years called Goniobasis or "Elimia" clavaeformis.  Yet other populations, especially those inhabiting larger rivers, bear broader shells with carination shifted down toward the anterior periphery of the whorl.  Such populations have been identified as Pleurocera unciale or "uncialis" (Reeve 1861).  The phenomenon of shifting genus-level taxonomy with increasing river size was first reported by Dillon & Robinson (2007b).  See my essays of 20Feb07 and 3June13 [links below] for more.

In 2011 I extended the research of Dillon & Robinson (2007b) across east Tennessee to north Georgia, and in 2013 Dillon, Jacquemin & Pyron documented an identical phenomenon involving P. canaliculata populations further downstream.  The results of these studies have prompted a significant re-evaluation of higher taxonomic categories in the Pleuroceridae, carrying both the generic nomina Goniobasis and "Elimia" into synonymy under Pleurocera. The best entry into this long-running taxonomic controversy would be my essay of 23Mar11, entitled Goodbye Goniobasis, Farewell Elimia [link below].

It seems likely to us that the differences in shell “robustness” that led taxonomists to distinguish Goniobasis (or "Elimia") clavaeformis from Pleurocera unciale for almost 200 years may be an ecophenotypic response to differing predation pressures in big rivers and small streams.   The specimen figured at the top of this page, for example, has clearly survived an attack from some sort of crushing predator.  Although this phenomenon almost certainly has little (additive) genetic basis, we have elected to retain the nomen "uncialeas a subspecies, by virtue of its indexing function.

The shell robustness developed by unciale populations can take a wide variety of form (see the Nolichucky specimens figured below).  A complete synonymy for P. clavaeformis unciale would include all the specific nomina listed by Goodrich (1940) in his "group of Pleurocera unciale" (including hastatum and gradatum) as well as all those listed under Pleurocera curtum (including walkeri).  Goodrich also listed one specific nomen in his "group of Pleurocera canaliculatum" that seems attributable to clavaeformis unciale: P. parvum (with six synonyms) and one name in his "group of Pleurocera pyrenellum": P. viridulum.

The diploid number of P. clavaeformis (as either acutocarinata or as unciale) is 2n = 34 (Dillon 1991).

> Supplementary Resources

  • The range of shell morphological variation in the Indian Creek (Va) population of Pleurocera (formerly Goniobasis) clavaeformis, from the report of Dillon & Robinson (2007b).  Snails bearing shells of the form marked "U" have historically been referred to as "Pleurocera unciale," those marked "T" are typical Goniobasis (or "Elimia") clavaeformis, and those marked "C" were historically "Elimia" acutocarinata.Click for larger
  • Pleurocera clavaeformis unciale from the Nolichucky River, demonstrating a variety of shell form.  The three specimens at the bottom were collected at the US321 bridge in Greene County, TN.  The three at top were collected 30 km downstream, at the TN160 bridge.P. clavaeformis unciale montage


  • Pretty photo of living P. clavaeformis clavaeformis, courtesy of Chris Lukhaup.P. clavaeformis Lukhaup


  • > Essays

    • In my essay of 20Feb07 I coined the term "Goodrichian Taxon Shift" to describe the (apparently ecophenotypic) response of P. clavaeformis to increasing stream size.
    • The phenomena documented in my 20Feb07 essay were extended to included pleurocerids ranging through east Tennessee into the Mobile Basin of Alabama in my essay of 12Oct09, "Pleurocera Puzzles."
    • Taxonomic controversy has surrounded the generic nomina Pleurocera, Goniobasis, and Elimia for many years.  In 23Mar11 I posted a third essay in this series, generalizing my research findings on this subject to bid, "Goodbye Goniobasis, Farewell Elimia."  Links are available from that essay to older resources on the whole taxonomic mess.
    • In my essay of 3June13 I documented an identical "Goodrichian taxon shift" in "Pleurocera acuta is Pleurocera canaliculata."  This prompted my colleagues and me to broaden the concept and rechristen it "cryptic phenotypic plasticity."
    • Pleurocera claveformis unciale received passing mention in my essay of 4Feb14, What Is a Subspecies? 
    • The tortuous taxonomic history of the nomen "unciale" served as a primary example in my follow-up essay of 5Mar14, What Subspecies Are Not.
    • I reviewed the entire phenomenon of cryptic phenotypic plasticity in P. clavaeformis in my blog post of 2June16, The Shape-shifting Pleurocera of North Alabama.  That essay featured a good figure illustrating the range of shell morphology displayed by a clavaeformis population sampled from the Paint Rock River.


    > 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. (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. [pdf]
    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., S. J. Jacquemin  & M. Pyron (2013)  Cryptic phenotypic plasticity in populations of the freshwater prosobranch snail, Pleurocera canaliculata.  Hydrobiologia 709: 117-127. [html]  [pdf]
    Dillon, R. T., Jr., & J. D. Robinson (2007a) The Goniobasis ("Elimia") of southwest Virginia, I. Population genetic survey.  Report to the Virginia Division of Game & Inland Fisheries, 25 pp.  [PDF]
    Dillon, R. T., Jr., & J. D. Robinson (2007b) The Goniobasis ("Elimia") of southwest Virginia, II. Shell morphological variation in Goniobasis clavaeformis.  Report to the Virginia Division of Game & Inland Fisheries, 12 pp. [PDF]
    Goodrich, C. (1913)  Spring collecting in southwest Virginia.  Nautilus 27: 81-82, 91-95.  
    Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system.  Occas. Pprs. Mus. Zool. Univ. Mich., 417: 1-21.  
    Hill, W. R.  (1992)  Food limitation and interspecific competition in snail-dominated streams.  Can. J. Fish. Aquat. Sci. 49: 1257-1267.
    Hill, W. R., J. G. Smith & A. J. Stewart (2010)  Light, nutrients, and herbivore growth in oligotrophic streams.  Ecology 91: 518-527.
    Jokinen, E.H. 1992. The Freshwater Snails (Mollusca: Gastropoda) of New York State. NY State Mus Bull 482, Albany, New York.
    Rosemond, A. D., P. J. Mulholland & J. W. Elwood (1993)  Top-down and bottom-up control of stream periphyton: Effects of nutirents and herbivores.  Ecology 74: 1264 - 1280.  
    Smith, D.G. 1980. Goniobasis virginica Gastropoda Pleuroceridae in the Connecticut River USA. Nautilus 94:50-54.
    Steinman, A. D. (1991)  Effects of herbivore size and hunger level on periphyton communities.  J. Phycol.  27: 54 - 59.    
    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.