FWGNA > Species Accounts > Pleuroceridae > Lithasia armigera armigera
Lithasia armigera armigera (Say 1821)
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> Habitat & Distribution

Goodrich (1940) gave the range of L. armigera bearing typical shell morphology as the lower Ohio River and lower Wabash River, the main Tennessee River in the vicinity of Florence (AL), and the main Cumberland River, from Burnside (KY) to its mouth, with major tributaries such as the Harpeth River and the Stones River (TN).

Our surveys have indeed confirmed substantial populations in the lower Ohio and the lower Wabash and would add scattered records 900 miles up the Ohio River almost as far as Pittsburgh.  More modern TVA surveys have also confirmed records up the main Tennessee to the tailwaters of the Pickwick Dam, near Florence.  Tiemann et al. (2013) have reported L. armigera populations in the main Mississippi River.

We do not, however, have records of Lithasia armigera populations bearing the typical shell morphology in the Cumberland River further upstream than the Lake Barkley tailwaters (RM 30).  The more lightly-shelled populations inhabiting Cumberland tributaries upstream, such as the Harpeth and the Caney, we would assign to Lithasia armigera duttoniana.

As is true for the genus generally, populations of L. armigera reach maximum abundance grazing on solid substrates (including gravel and cobble), which can be few and far between in big rivers such as The Ohio and the main Tennessee.  Divers sometimes report populations on rocks at substantial depths.  Populations are not reported in muddy slackwaters. FWGNA incidence rank for all subspecies together I-4.

> Ecology & Life History

Pleurocerids are heavily-shelled, conspicuous freshwater gastropods, typically grazing nonspecifically on 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.  Where present in high density, pleurocerids can have significant effects on energy flow in rivers and streams (Dillon 2000: 86-91).

> Taxonomy & Systematics

Thomas Say's (1821) Melania armigera was the first species described in what is now known as the genus Lithasia.  Tryon (1873) recognized 25 species in the genus Lithasia/Angitrema, primarily inhabiting the Tennessee/Cumberland but ranging throughout the interior drainages of seven states, among which were armigera and two very similar species, duttoniana (Lea 1841) and jayana (Lea 1841).  Goodrich (1940) boiled these 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.  All three species, armigera, duttoniana, and jayana, survived the winnowing process unscathed.

Minton & Lydeard (2003) obtained mtDNA CO1 sequence data from 5 populations they identified as L. armigera:  Ohio R (2 individuals), Wabash R (2), Tennessee R (1), Stones R (6), and Harpeth R (3).  All 14 individuals sampled, from 5 populations, were genetically indistinguishable.  But certain inconsistencies between the CO1 results of Minton & Lydeard and the 16S results of Holznagel & Lydeard (2000) led Minton and his colleagues (2005) to focus on the Harpeth River population of L. armigera and ultimately, on the basis of CO1 sequence and radular wear, describe a new species, L. spicula.  See my 2016 series on mitochondrial superheterogeneity from the links below for a better explanation of apparent mtDNA inconsistency in the Pleuroceridae.

Minton & Lydeard did, however, document 4% divergence between their 14 Lithasia armigera sequences and a set of 19 CO1 sequences sampled from the Duck River of Middle Tennessee.  The snails from which these 19 nearly-identical sequences were obtained were identified by five separate names on the basis of shell morphology: L. geniculata pinguis (6), L. geniculata fuliginosa (7), L. geniculata geniculata (1), L. jayana (1), and L. duttoniana (4).  Minton & Lydeard suggested, therefore, that all five nomina be synonymized under Lithasia geniculata (Haldeman 1840).  Minton et al. (2008, 2018) went on to lump all nominal taxa of Duck River Lithasia into combined studies of shell morphology.

The allozyme surveys of Dillon (2020), however, confirmed reproductive isolation between Duck River populations identified as L. geniculata (all subspecies) and those identified as L. duttoniana.  No evidence of reproductive isolation was apparent between populations identified as L. duttoniana and L. jayana.  See my 2022 series of essays from the links below.

Populations conventionally identified as Lithasia duttoniana are more lightly-shelled and inhabit smaller rivers but are otherwise indistinguishable from typical L. armigera.  Indeed, Lithasia bearing shells of the duttoniana form in upper regions of the Duck River intergrade smoothly with heavily-shelled populations in the lower Duck identified as L. jayana.  And L. jayana is entirely indistinguishable from L. armigera in all respects.

In light of these results, we have suggested that both duttoniana (Lea 1841) and jayana (Lea 1841) be lowered to subspecific rank under Lithasia armigera (Say 1821).  And we noted, as we did, that the shell morphological basis for distinctions such as these have often been attributable to cryptic phenotypic plasticity.  See my 2013-14 essays on CPP from the links below to learn more about this important phenomenonon.

> Supplementary Resources

> Essays

> References


Burch, J. B. (1989)  North American Freshwater Snails.  Malacological Publications, Hamburg, MI.
Davis, G.M. (1974) 
Report on the rare and endangered status of a selected number of freshwater Gastropoda from southeastern U.S.A. U.S. Fish & Wildlife Service. Washington, DC. 51 p.
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. (2020) 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.
Holznagel, W. E. & C. Lydeard (2000)  A molecular phylogeny of North American Pleuroceridae (Gastropoda: Cerithioidea) based on mitochondrial 16S rDNA sequences.  Journal of Molluscan Studies 66: 233 - 257.
Minton, R. L. (2002)  A cladistic analysis of Lithasia (Gastropoda: Pleuroceridae) using morphological characters.  The Nautilus 116: 39-49.
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
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Minton, R. L., S. P. Savarese & D. C. Campbell (2005) A new species of "Lithasia" (Mollusca: Caenogastropoda: Pleuroceridae) from the Harpeth River, Tennessee, USA.  Zootaxa 1054: 31 - 42.
Tiemann, J. S., W. R. Posey, K. S. Cummings, K. J. Irwin, and B. Turner (2013)  First occurrences of Lithasia armigera and Lithasia verrucosa (Gastropoda: Pleuroceridae) in the Mississippi River.  Southeastern Naturalist 12: N35 - N39.
Tryon, G. W. (1873) Land and Fresh-water shells of North America.  Part IV, Strepomatidae.  Smithsonial Miscellaneous Collections 253: 1- 434.