> Habitat & Distribution

The range of Lithasia geniculata geniculata was given by Goodrich (1940) as the main Cumberland River between Burnside (KY) and Nashville, plus the Duck River of Middle Tennessee.  Tiemann & Cummings (2010) uncovered several additional historical records from the main Tennessee River and the main Ohio, adding a fresh report from the Ohio River at Mound City, downstream from its confluence with the Tennessee/Cumberland. Our modern surveys have confirmed an extensive Duck/Buffalo River population, with a remnant population around the Cumberland/Tennessee/Ohio River junction.

Populations of L. geniculata reach maximum densities on rocky shoals, and are not typically recorded in the (often lengthy) stretches of slackwater through this (rather circumscribed) range.  Considering L. geniculata geniculata (ss) together with its subspecies L. geniculata fuliginosa and L. geniculata pinguis, the FWGNA incidence rank of L. geniculata is I-4.

> 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) hypothesis 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.  See my essays 20Feb07, 3June13 and 11July14 from the links below for a review of the CPP phenomenon, as demonstrated by the Duck River Lithasia populations and elsewhere.

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. 

Two individual L. geniculata fuliginosa sampled from the Buffalo River (a tributary of the Duck) differed, however, from the main Duck populations by 2.8%.  This prompted Minton (2013) to describe the Buffalo Lithasia as a new species, L. bubala.  Minton & Lydeard also uncovered 4.3% divergence between the common Duck River Lithasia sequence and individual L. geniculata fuliginosa sampled from Garrison Fork (of the upper Duck) and Red River (of the Cumberland), the significance of which was not clear at the time.  The allozyme study of Dillon (2020a) has confirmed, however, that the L. geniculata population of the Duck River and its tributaries comprises a single biological species, rates of gene flow attenuated both by the isolation by distance, and by the occasional barriers to dispersal typical of pleurocerid populations generally.

Going beyond their failure to detect any mtDNA sequence divergence in their six L. geniculata samples from the main Duck River, Minton & Lydeard were unable to detect any genetic difference between L. geniculata and five Duck River samples of Lithasia armigera subspecies, for which allozyme data confirm reproductive isolation (Dillon 2020a,b).  This failure calls into question the efficacy of mtDNA sequence data, especially as applied to pleurocerid systematics.  It also led Minton et al. (2008) and Minton et al. (2018) to lump all subspecies of geniculata together with reproductively isolated populations of L. armigera subspecies into combined studies of shell morphometric variation subsequently conducted down the length of the Duck.

See the series of essays I posted on the FWGNA blog from December 2021 to March 2022 from the links below for more about the Duck River Lithasia, contextualized within our understanding of pleurocerid evolution generally.

> Maps and Supplementary Resources

• Lithasia distribution in drainages of The Ohio (2019)
• Lithasia distribution in the Tennessee/Cumberland (2022)

> Essays

• Goodrich's (1934) paper on the Duck River Lithasia geniculata population 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.
• Calvin Goodrich's brilliant 1934 insight into the evolutionary relationships among the Duck River Lithasia populations is an old, old story, but I told it again in my blog post of 4Sept19, "CPP Diary: The Spurious Lithasia of Caney Fork."
• I reviewed my (Dillon 2020a) allozyme survey of isolation-by-distance in the L. geniculata population of the Duck River in my essay of 7Dec21, "Intrapopulation gene flow: Lithasia geniculata in the Duck River."
• On 4Jan22 I used the allozyme divergence I discovered between the Lithasia geniculata ("GEN") and L. armigera duttoniana ("DUT") populations of the Duck RIver (Dillon 2020b) to answer the question, "What is character phase disequilibrium?"
• And on 3Mar22 I reviewed the mtDNA sequence study of Minton & Lydeard (2003) in light of my (Dillon 2020a,b) allozyme survey, concluding that their findings on the Duck River Lithasia constitute "The third-most amazing research results ever published for the genetics of a freshwater gastropod population."

> 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. (2020a) Population genetic survey of Lithasia geniculata in the Duck River, Tennessee.  Ellipsaria 22(2): 19 - 21 [pdf].
Dillon, R. T., Jr. (2020b) Reproductive isolation between Lithasia populations of the geniculata and duttoniana forms in the Duck River, Tennessee.  Ellipsaria 22(3): 6 - 8.  [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.
Tiemann, J. S. & K. S. Cummings (2010)  New record for the freshwater snail Lithasia geniculata (Gastropoda: Pleuroceridae) in the Ohio River, IL, with comments on potential threats to the population.  Southeastern Naturalist 9: 171 - 176.
Tryon, G. W. (1873) Land and Fresh-water shells of North America.  Part IV, Strepomatidae.  Smithsonial Miscellaneous Collections 253: 1- 434.