> Habitat & Distribution

Thompson described Marstonia pachyta as endemic to Limestone and Piney Creeks, a pair of adjacent tributaries of the Tennessee River in North Alabama.  This quite circumscribed distribution was passed along verbatim by Hershler (1994) and confirmed by Haggerty and Garner (2008).  The FLMNH also holds records from Round Island Creek 10 miles to the west and Bradford Creek 6 miles to the east.  To that range our surveys have added the Flint River, 18 miles east of Bradford Creek.  In addition, FWGNA surveys have found populations bearing shells with a slight angulation on the body whorl, Marstonia pachyta angulobasis, widely but very patchily sprinkled throughout Tennessee.

At the Limestone Creek type locality, Thompson reported that “Snails were found on dead leaves and tree rootlets in still shallow water at the edge of a pool.”  Our observations suggest that they can also be common in riffles and rapids, especially among aquatic bryophytes.  We do not typically find the little snails on soft bottoms or on bare rock surfaces,  however.  All the streams from which populations are known are rich and hard.  

FWGNA incidence rank for both subspecies considered together I-3*, non-apparently rare. Marstonia pachyta was listed as “endangered” by the US Fish & Wildlife service on 02-25-2000.

> Ecology & Life History

The cryptic habitat of Marstonia pachyta seems to suggest a diet that does not ordinarily include algae, but rather very fine organic matter or bacteria.  One might speculate that temperature, light, current, and other environmental factors in such a habitat would be fairly constant, suggesting constant (perhaps low) levels of reproduction year round.  Hydrobiids are typically dioecious, the males being characterized by a penis that arises from the neck.  Eggs are generally laid singly, attached in a spare capsule to a solid substrate.

> Taxonomy & Systematics

Marstonia pachyta is another entry in the rather long list of small-bodied, wide-ranging, and obscure hydrobiids of eastern North America bearing narrow shells and (often) a carinate body whorl.  Thompson’s original (1977) description of typical Marstonia pachyta included figures of shell, operculum, and penial morphology.  He raised Baker’s (1926) subgenus Marstonia to the full genus level, placing underneath it three previously-described species and five species he newly described, including M. pachyta.  This eight-species model was transmitted by Burch (1989).

Hershler and Thompson (1987) synonymized Marstonia under Pyrgulopsis (Call & Pilsbry 1886) on the basis of similarities in penial morphology.  But after review of female reproductive anatomy, Thompson and Hershler (2002) resurrected Marstonia to generic status and allocated back into it M. pachyta, as well as (by that point) eleven other species from around the eastern USA.  Female reproductive anatomy, specifically occurrence of a large extension of the albumen gland into the pallial roof, is a distinctly Marstonia characteristic (Hershler 1994, Hershler et al. 2003).  Recent molecular-based phylogenetic analyses has also supported the distinction between Marstonia and Pyrgulopsis (Liu and Hershler 2005), as well as the retention of both genera in the Hydrobiidae sensu strictu (Wilke et al. 2013).

Hershler and colleagues (2003) reported a CO1 sequence for a singleton M. pachyta collected from its type locality.  That sequence was over 98% similar to CO1 sequences reported from typical Marstonia lustrica populations sampled from lakes across the northern United States.  For context in which to place this surprising result, see my essay of 3Aug20 from the link below.

Our discovery of typical M. pachyta in the Flint River drainage 25 miles east of the Limestone/Piney led to the recognition that populations inhabiting the Paint Rock River just 10 miles beyond, described by Thompson (2005) as Marstonia angulobasis, are a shell variant of M. pachyta.  See my essay of 3Nov22 from the link below for more.

> Maps and Supplementary Resources

• Marstonia distribution in the Tennessee/Cumberland (2022)

> Essays

• Earlier versions of this website, online until August of 2016, adopted the large, broadly-inclusive concept of the Hydrobiidae (sl) following Kabat & Hershler (1993).  More recently the FWGNA project has shifted to the Wilke et al. (2013) classification system, distinguishing a much smaller Hydrobiidae (ss) and elevating many hydrobioid taxa previously ranked as subfamilies to the full family level.  For more details, see my 18Aug16 essay,  The Classification of the Hydrobioids.
• In my essay of 3Aug20 I reviewed T. W. Coote's (2019) important survey of Mitochondrial heterogeneity in Marstonia lustrica across 20 populations, 6 states, and one Canadian province.  This essay provides context for sequence variation in a variety of other hydrobioids, including M. pachyta.
• For background info on the subgroup of Marstonia that Thompson referred to as “small narrow hydrobiids that have in common a carinate body whorl,” see my essay of 4Oct22, The SNHTHICACBY Marstonia 5: scalariformis.
• See my 3Nov22 essay, The SNHTHICACBY Marstonia 6: pachyta, for a review of the reasoning behind our suggestion that Thompson’s angulobasis be lowered to the subspecific level.  That essay includes several nice figures illustrating variation in pachyta shell morphology as well.

> References

Baker, F. C. (1926) Nomenclatural notes on American fresh water Mollusca. Transactions of the Wisconsin Academy of Sciences, Arts, and Letters 22:193-205.
Burch, J. B. (1989)  North American Freshwater Snails.  Malacological Publications, Hamburg, MI. 365 pp.
Call R. E. & Pilsbry H. A. (1886) On Pyrgulopsis, a new genus of rissoid mollusk, with description of two new forms". Proceeding Davenport Academy of Natural Sciences V.: 9-14.
Coote, T. W. (2019) A phylogeny of Marstonia lustrica (Pilsbry 1890) (Gastropoda: Hydrobiidae) across its range.  Northeastern Naturalist 26: 672 – 683.
Haggerty, TM and JT Garner (2008)  Distribution of the armored snail (Marstonia pachyta) and slender Campeloma (Campeloma decampi) in Limestone, Piney, and Round Island Creeks, Alabama.  Southeastern Naturalist 7(4) 729-736.
Hershler, R. (1994)  A review of the North American freshwater snail genus Pyrgulopsis (Hydrobiidae).  Smithsonian Contributions to Zoology 554: 1 - 115.
Hershler,R., Liu,H-P. and Thompson,F.G. (2003) Phylogenetic relationships of North American nymphophiline gastropods based on mitochondrial DNA sequences.  Zool. Scr. 32 (4), 357-366.
Hershler, R., and F.G. Thompson (1987)  North American Hydrobiidae (Gastropoda: Rissoacea): redescription and systematic relationships of Tryonia Stimpson, 1865 and Pyrgulopsis Call and Pilsbry, 1886. The Nautilus 101:25-32.  
Kabat, A.R., and R. Hershler (1993) The prosobranch snail family Hydrobiidae (Gastropoda: Rissooidea): review of classification and supraspecific taxa. Smithsonian Contributions to Zoology 547:1-94.
Liu, H., and R. Hershler (2005)  Molecular systematics and radiation of western North American nympholine gastropods. Molecular Phylogenetics and Evolution 34:284-298. 
Thompson, F.G. (1977) The hydrobiid snail genus Marstonia.  Bulletin of the Florida State Museum 21(3):113-158.
Thompson, FG (2005) Two new species of hydrobiid snails of the genus Marstonia from Alabama and Georgia.  The Veliger 47: 175 – 182.
Thompson, F. G. & R. Hershler (2002)  Two genera of North American freshwater snails: Marstonia Baker, 1926, resurrected to generic status, and Floridobia, new genus (Prosobranchia: Hydrobiidae: Nymphophilinae).  The Veliger 45: 269 - 271.
Wilke T., Haase M., Hershler R., Liu H-P., Misof B., Ponder W. (2013)  Pushing short DNA fragments to the limit: Phylogenetic relationships of “hydrobioid” gastropods (Caenogastropoda: Rissooidea).  Molec. Phyl. Evol. 66: 715 – 736.
Misc. Publ. Mus. Zool. Univ. Mich. 57: 1 - 68.