Freshwater snails of North America (Mollusca:Gastropoda): A Tale of Taxonomic Impact on Species Richness and Diversity

Pyrgulopsis californiensis

In 1982, Burch published a much needed manual on North American freshwater gastropods.  It had been over a hundred years since W. G. Binney published his account, so as Burch indicates, the new manual was greatly needed.  Burch’s (1982) manual and identification guide represented his own interpretation of species-level taxonomy as well as reliance on prior revisions for various groups.  However, Burch acknowledged that the manual was not a major systematic revision of the fauna.  Indeed, he noted that the systematics was not well worked out in many groups, which made a definitive listing of species somewhat arbitrary at the time.  Nevertheless, the manual was seminal and was used as an important reference for many systematists, conservation biologists and ecologists interested in freshwater mollusks.   

Based on Burch (1982), approximately 63% of the freshwater gastropod fauna is found in the southeastern United States, which has been identified as an aquatic hotspot of diversity for freshwater gastropods and other aquatic organisms such as turtles, fishes, and freshwater mussels, and crayfishes (Lydeard and Mayden, 1995; Neves et al., 1997; Benz and Collins, 1997).    A significant proportion of the fauna is considered imperiled and indeed 38 gastropod species are presumed extinct in the Mobile River basin of Alabama, Mississippi, and Georgia alone (Lydeard and Mayden, 1995; Neves et al., 1997). 

Although the southeastern gastropod fauna certainly is in a state of imperilment and warrants considerable conservation efforts, the supposition that most gastropod diversity is found in the southeastern United States is based on the assumption that the fauna as a whole is well understood.  Although this certainly is the case for vertebrates and perhaps freshwater mussels, at the time Burch (1982) wrote his account, it was not the case for gastropods.  For example, for the Hydrobiidae, although taxonomic studies had been conducted on the fauna of the southeastern Hydrobiidae (Thompson, 1968, 1969, 1977, 1984; Thompson & McCaleb, 1978), the hydrobiid fauna of the western United States was virtually unstudied (Hershler and Thompson, 1987).  This situation changed in 1986 when Robert Hershler and his collaborators began publishing taxonomic studies on the hydrobiids of the southwestern and western United States (Hershler and Longley, 1986a, b) and started to describe the fauna.    

Since 1986, Hershler and his collaborators have described 131 species raising the number of hydrobiids from 152 to 283 species making it the most diverse family in North America instead of the family Pleuroceridae with 153 species.  Additionally, he and his collaborators research have raised the number of Pyrgulopsis from five to 98 species making it the most diverse gastropod genus in North America instead of the genus Elimia with 83 species, which reaches its greatest diversity in the southeastern United States.   The taxonomic treatment of the western assemblage reduces the percent of gastropod species occurring in the southeastern United States from 63% to 50%.   Southeastern gastropods are deserving of on-going conservation efforts and reflect a fauna of rivers and streams, but so do the many newly described, narrowly endemic hydrobiids of the springs and ground waters of the western United States.

We are still in an age of discovery of new species of freshwater gastropods even in seemingly ‘developed’ nations like the United States.  To effectively use North American freshwater gastropods in larger ecological, conservation and evolutionary studies, we must have sound taxonomic infrastructure (sensu Bieler et al., 2013), which includes an understanding of the valid species, their geographic distribution and their geologic history. Species lists are only as good as the quality of the taxonomy and geographic coverage from which they are based.  For example, the European marine fauna “lost” all of its 16 species of Discodoris species after a recent worldwide revision demonstrated there was not a single Discordis species in the region (Dayrat, 2011).  Strong et al. (2008) estimate that there may be twice as many as the 4,000 valid species currently known in freshwater species globally.  It remains to be seen what the final count will be in North America.

LITERATURE CITED

Benz, G. W., and D. E. Collins (eds).  1997.  Aquatic Fauna in Peril: The Southeastern Perspective.  Special Publication 1, Southeast Aquatic Research Institute, Lenz Design & Communications, Decatur, GA.

Bieler, R. P. M. Mikkelsen, and G. Giribet.  2013.  Bivalvia – A discussion of known unknowns.  American Malacological Bulletin 31:123-133.

Burch, J. B.  1982.  Freshwater Snails (Mollusca: Gastropoda) of North America. U. S. Environmental Protection Agency, Cincinatti, Ohio. 294 pp.

Dayrat, B.  2011.  A warning for ecologists and conservation biologists using species checklists: How the European marine fauna ‘lost’ all of its 16 Discodoris species (Mollusca: Gastropoda).  Org. Divers. Evol. 11:75-82.

Hershler, Robert and Thompson, F. G. 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.

Neves, R. J., A. E. Bogan, J. D. Williams, S. A. Ahlstedt, and P. W. Hartfield.  1997.  Status of aquatic mollusks in the Southeastern United States: A downward spiral of diversity.  In Benz and Collins (eds) Aquatic Fauna in Peril: The Southeastern Perspective.  Southeast Aquatic Research Institute, Lenz Design & Communications, Decatur, GA.

Strong, E. E., O. Gargominy, W. F. Ponder, and P. Bouchet.  2008.  Global diversity of gastropods (Gastropoda: Mollusca) in freshwater.  Hydrobiologia 595:149-166.

Thompson, F. G. 1968. The aquatic snails of the family Hydrobiidae of peninsular Florida, University of Florida Press, Gainesville, FL, xv -I- 268 p.

Thompson, F G, 1969. Some hydrobiid snails from Georgia and Florida, Quarterly Journal of the Florida Academy of Science 32:242-265.

Thompson, F. G, 1977.  The hydrobiid snail genus Marstonia.  Bulletin of the Florida State Museum, Biological Sciences 21:113-158.

Thompson, F, G, 1979.  The systematic relationships of the hvdrobiid snail genus Nymphophilus Taylor 1966 and the status of the subfamily Nymphophilinae, Malacological Review 12:41-49.

Thompson, F. G.  1984.  North American freshwater snail genera of the hydrobiid subfamily Lithoglyphinae.  Malacologia 25:109-141.

Thompson, F. G. and J. E. McCaleb. 1978 A new freshwater snail from a spring in eastern Alabama.  American Midland Naturalist 100:350-358.