FWGNA > Species Accounts > Pleuroceridae > Lithasia geniculata pinguis
Lithasia geniculata pinguis (Lea 1852)
  • Lithasia geniculata pinguis

> Habitat & Distribution

The range of Lithasia geniculata (s.l.) was given by Goodrich (1940) as the Cumberland River between Burnside (KY) and Nashville, together with its major tributaries the Harpeth River and the Red River, plus the Duck/Buffalo Rivers of middle Tennessee.  For the subspecies L. geniculata pinguis, Goodrich listed only "Caney Fork and branches; Duck River, Coffee County, TN."  Like Lithasia populations generally, L. geniculata pinguis seems to reach maximum density on rocky shoals with good current.  Considering all subspecies together, the FWGNA incidence rank of Lithasia geniculata is I-2, rare.

> Ecology & Life History

Pleurocerids are heavily-shelled, conspicuous freshwater gastropods, typically inhabiting firm substrates in shallow waters.  Males are aphallic; females can be distinguished by an egg laying groove on the right side of their foot.  Most populations are perennial and iteroparous, typically requiring more than a year to mature and living several years (Life cycle Hi of Dillon 2000: 156-162).  Eggs are affixed to hard substrates singly or in small clusters from spring to midsummer.  Pleurocerids are generalized grazers, and where present in high density can have significant effects on energy flow in streams (Dillon 2000: 86-91).

> Taxonomy & Systematics

Tryon (1873) catalogued 25 species in the genus Lithasia/Angitrema, primarily inhabiting the Tennessee/Cumberland but ranging throughout the interior drainages of seven states.  Among Calvin Goodrich's greatest contributions to science was his (1934) documentation that three of these nomina: geniculata (Haldeman), fuliginosa (Lea) and pinguis (Lea), were shell variants of a single species inhabiting the Duck River of middle Tennessee in clinal series.  This observation presaged our understanding of "cryptic phenotypic plasticity" in the North American Pleuroceridae (Dillon 2011, 2014; Dillon et al. 2013) by 80 years.  Goodrich lowered fuliginosa and pinguis to subspecific rank under geniculata, ultimately (in 1940) boiling Tryon's 25 species down to 10 species and 14 subspecies.  Burch (1989) left Goodrich's system almost untouched, trimming out one species and 8 subspecies, but adding one species more recently described, to bring the total back to 10.

Minton & Lydeard (2003) obtained mtDNA sequences from 11 L. geniculata populations: 1 identified as geniculata geniculata (1 individual), 8 as geniculata fuliginosa (17 individuals) and 2 as geniculata pinguis (8 individuals).  The 6 populations sampled from the Duck River, regardless of subspecific designation, were indistinguishable genetically.

Minton & Lydeard (2003) also did not uncover, however, any genetic difference between L. geniculata and L. duttoniana populations in the Duck River, for which allozyme data confirm reproductive isolation.  This failure calls into question the efficacy of mtDNA sequence data, especially as applied to the systematic biology of the Duck River Lithasia.  It also led Minton et al. (2008) and Minton et al. (2018) to lump all subspecies of geniculata together with duttoniana in two separate studies of shell morphometric variation subsequently conducted down the length of the Duck.

Minton & Lydeard did uncover such substantial mtDNA divergence between their Lithasia samples (of all species) and the sample they identified as L. geniculata pinguis from the Collins/Caney Fork that they concluded the latter populations were not Lithasia.  Our field observations suggest a convergence in shell morphology between Pleurocera simplex populations inhabiting the Collins/Caney system and L. geniculata pinguis.

> Supplementary Resources

> Essays

  • Goodrich's (1934) paper on the Duck River Lithasia was the primary inspiration behind my essay of 20Feb07, Goodrichian Taxon Shift.  See that essay for a detail from Goodrich's Figure 1.
  • I generalized the concept of Goodrichian taxon shift to "cryptic phenotypic plasticity" in two subsequent essays on pleurocerid systematics, Pleurocera acuta is Pleurocera canaliculata (3June13) and Elimia livescens and Lithasia obovata are Pleurocera semicarinata (11July14).   That latter essay featured a scan of Goodrich's (1934) figure 1 in its entirety.
  • In my essay of 4Sept19, "CPP Diary: The spurious Lithasia of Caney Fork," I documented a striking case of cryptic phenotypic plasticity in the P. simplex population of the Caney/Collins River system in Middle Tennessee.  The shell phenotype of that population ranges from typical in the headwaters to ebenum in the middle reaches to a form that has been mistaken for Lithasia geniculata pinguis.

> References

Burch, J. B. (1989)  North American Freshwater Snails.  Malacological Publications, Hamburg, MI.
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. (2011)  Robust shell phenotype is a local response to stream size in the genus Pleurocera.  Malacologia 53: 265-277.  [pdf]
Dillon, R. T., Jr.  (2014) Cryptic phenotypic plasticity in populations of the North American freshwater gastropod, Pleurocera semicarinata.  Zoological Studies 53:31. [html] [pdf]
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]
Goodrich, C. (1934) Studies of the gastropod family Pleuroceridae - I.  Occas. Pprs. Mus. Zool. Univ. Mich. 286: 1 - 17.
Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system.  Occas. Pprs. Mus. Zool. Univ. Mich., 417: 1-21.
Goodrich, C. (1941) Studies of the gastropod family Pleuroceridae VIII.  Occas. Pprs. Mus. Zool. Univ. Mich. 447: 1 - 13.
Minton, R. L. (2002)  A cladistic analysis of Lithasia (Gastropoda: Pleuroceridae) using morphological characters.  The Nautilus 116: 39-49.
Minton, R. L. (2013) A new species of Lithasia (Gastropoda: Pleuroceridae) from the Buffalo River, Tennessee, USA.  The Nautilus 127: 119 - 124.
Minton, R. L., K. C. Hart, R. Fiorillo, & C. Brown (2018)  Correlates of snail shell variation along a unidirectional freshwater gradient in Lithasia geniculata (Haldeman, 1840) (Caenogastropoda: Pleuroceridae) from the Duck River, Tennessee, USA.  Folia Malacologica 26: 95 - 102.
Minton, R. L. & C. Lydeard (2003) Phylogeny, taxonomy, genetics, and global heritage ranks of an imperiled, freshwater snail genus Lithasia (Pleuroceridae)  Molecular Ecology 12: 75-87.
Minton, R. L., A. P. Norwood & D. M. Hayes (2008) Quantifying phenotypic gradients in freshwater snails: a case study in Lithasia (Gastropoda: Pleuroceridae)  Hydrobiologia 605: 173-182
Tryon, G. W. (1873) Land and Fresh-water shells of North America.  Part IV, Strepomatidae.  Smithsonial Miscellaneous Collections 253: 1- 434.