> Marstonia agarhecta Thompson 1969
 
> Habitat & Distribution
This is one of three nominal species of Georgia hydrobiids endemic (or nearly endemic) to springs and spring-fed tribuarties of the Ocmulgee River in the vicinity of Hawkinsville.  They inhabit clear water with a slight current, predominantly in diatomaceous ooze on top of submerged logs, and less commonly in silt containing large amounts of diatoms (Thompson 1969, Thompson 1977, Watson 2000). 
   

> Ecology & Life history
No specific life history data are as yet available for M. agarhecta.  Males bear a relatively large, bladelike penis above their heads, females lay single, hemispherical egg capsules (Hershler 1994).    

> Taxonomy & Systematics
Like Spilochlamys, Notogillia, and Floridobia, the genus Marstonia belongs to the hydrobiid subfamily Nymphophilinae, males being characterized by a lobe-shaped penis (or verge) with surficial glandular patterns (Kabat and Hershler 1993, Hershler et al. 2003, Thompson 2004).  The penis of M. agarhecta has been figured by Thompson (1969).

Baker (1926) originally proposed Marstonia as a subgenus of Amnicola.  Thompson (1969, 1977) redefined and expanded the taxon, placing several newly-described southeastern species in it.  Hershler and Thompson (1987) synonymized Marstonia under Pyrgulopsis 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 to it all eastern North American species previously placed in Pyrgulopsis.  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).

Of the three nominal Marstonia species inhabiting Georgia Atlantic drainages, M. agarhecta and M. gaddisorum are nearly indistinguishable.   Marstonia halcyon is more distinctive, by virtue of its thinner and more delicate shell.  Marstonia halcyon is, however, strikingly similar to M. castor (Thompson 1977) of the Flint drainage. 

> Map of Marstonia distribution (PDF)


> 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.   Hershler, R. (1994)  A review of the North American freshwater snail genus Pyrgulopsis (Hydrobiidae). Smithsonian Contributions to Zoology 1994; 0(554):1-115.   Hershler, R., H. Liu, and F.G. Thompson (2003)  Phylogenetic relationships of North American nymphophiline gastropods based on mitochondrial DNA sequences.  Zoologica Scripta 32:357-366.  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. (1968)  The Aquatic Snails of the Family Hydrobiidae of Peninsular Florida. University of Florida Press, Gainesville, Florida, USA.   Thompson, F.G. (1969) Some hydrobiid snails from Georgia and Florida. Quarterly Journal of the Florida Academy of Sciences 32:241-265.   Thompson, F.G. (1977) The hydrobiid snail genus Marstonia.  Bulletin of the Florida State Museum 21(3):113-158.  Thompson, F.G. (2004)  An identification manual for the freshwater snails of Florida.   Watson, C. N.  (2000)  Results of a survey for selected species of Hydrobiidae (Gastropoda) in Georgia and Florida.  pp. 233 - 244 in Freshwater Mollusk Symposium Proceedings (Tankersley et al, eds)  Ohio Biological Survey, Columbus.   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.