> Goniobasis clavaeformis (Lea 1841)
    "Elimia" clavaeformis

> Habitat & Distribution
The range of G. clavaeformis was given by Goodrich (1940) as "tributaries of the upper Tennessee River in Virginia, Tennessee and North Carolina; does not make the westward turn around Walden Ridge" (at Chattanooga).  The species is common in the main stems of the Clinch, Powell, and Holston Rivers of southwest Virginia and throughout their tributaries, in areas with rocky bottom.

> Ecology & Life history
As the "pleurocerid in residence" at the Oak Ridge National Laboratory, Goniobasis clavaeformis 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).  Grazing by populations of G. clavaeformis can have a significant impact on algal biomass, primary productivity, and periphyton community structure.  For a review see Dillon (2000: 86 - 91).  

> Taxonomy & Systematics
Four populations of G. clavaeformis were included in the allozyme study of Dillon & Robinson (2007a).  The species is quite distinct genetically.  There is no evidence of hybridization with either G. simplex or G. arachnoidea, the other two species of Goniobasis with which it sometimes co-occurs.

The shells borne by populations of Goniobasis clavaeformis display a wide range of carination, as well as variation in overall length-to-width ratio.  Populations inhabiting smaller streams are more slender and may bear a single carination (of varying strength) around mid-whorl.  Such populations have been identified as G. acutocarinata (Lea 1841), the type of the genus Elimia.  Other populations are entirely uncarinate.  Yet other populations, especially those inhabiting larger streams and 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).  See Dillon & Robinson (2007b) for a more complete discussion of shell morphological variation in G. clavaeformis.  The diploid number of G. clavaeformis (as either acutocarinata or as unciale) is 2n = 34 (Dillon 1991).

Burch resurrected the name "Elimia" to include clavaeformis and approximately 80 other pleurocerid species traditionally assigned to Goniobasis (Lea 1862).  But Elimia (H. & A. Adams 1854) is a composite group, explicitly rejected by Tryon, Walker, Pilsbry and Goodrich (Dillon 1989).  Details are available from the link below.

> Essay #1
See my 28Sept04 post to the FWGNA web site for a review of the Goniobasis/Elimia taxonomic controversy.

> Essay #2
See my 20Feb07 post to the FWGNA web site for a discussion of "Goodrichian" taxon-shift in Goniobasis clavaeformis.

>Pretty photo
Living G. clavaeformis, courtesy of Chris Lukhaup.

 
>References
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., & K. B. Davis (1991)  The diatoms ingested by freshwater snails: temporal, spatial, and interspecific variation. Hydrobiologia 210: 233-242.  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.   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.  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.  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.  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.