> Io fluvialis (Say 1825)
  

> Habitat & Distribution
Historically Io fluvialis ranged throughout the large rivers of East Tennessee, as far west as Muscle Shoals (Lewis 1876, Adams 1915).  But the impoundment of the Tennessee River and most of its major tributaries, together with more localized but severe pollution, had by the middle 20th century extirpated the species from all but the upper Powell River, the upper Clinch River, and a small region of the lower Nolichucky (Stansbery & Stein 1976, McLeod & Moore 1978).  In the 1970s and 1980s the species was reintroduced to the North Fork Holston River in Virginia (Ahlstedt 1991).  Within its rather restricted range, Io fluvialis inhabits rocky riffles with good flow.

> 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).  

> Taxonomy & Systematics
Variation in the spire height and spinosity of the shells born by Io fluvialis led earlier workers to author a large number of Latin nomena for the species, now long synonymized (Lutz & Weese 1951).  The taxonomic status of fluvialis as the only species in the monotypic genus Io is now uncontroversial.  An Io shell serves as the logo of the American Malacological Society.

>Pretty photo
Living Io fluvialis, courtesy of Gary Peeples, US Fish & Wildlife Service (www.forestryimages.org)

 
>References
Adams, C.C. (1915)  The variations and ecological distribution of the snails of the genus Io.  Mem. National Acad. Sci. Ahlstedt, S. A. (1991)  Reintroduction of the spiny riversnail, Io fluvialis (Say, 1825) (Gastropoda: Pleuroceridae) into the North Fork Holston River, southwest Virginia and northeast Tennessee.  Amer. Malac. Bull. 8: 139-142.  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. (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.  Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system.  Occas. Pprs. Mus. Zool. Univ. Mich., 417: 1-21.  Lewis, J. L. (1876)  Io and its habits.  Amer. Natur. 10: 321-326.    Lutz, L. & A. O. Weese (1951)  Variation in Io, fifty years later.  Proc. Okla. Acad. Sci 32: 1 - 3.  McLeod, M. J. & J. D. Moore (1978)  Change in the gastropod Io spinosa (Pleuroceridae: Mollusca) in 70 years.  Am. Midl. Natur. 99: 198 - 205.  Stansbery, D. H. & C. B. Stein (1976)  Changes in the distribution of Io fluvialis (Say 1825) in the upper Tennessee River system.  Bull. A.M.U. 1976: 28-33.  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.