FWGNA > Species Accounts > Physidae > Aplexa hypnorum
Aplexa hypnorum (Linneaus 1758)

  • click to view larger

> Habitat & Distribution

Aplexa hypnorum is a more northern species, ranging coast to coast from New York to Washington, north to Alaska and south to Colorado (Clarke 1981, Burch 1989, Wu 1989, Jokinen 1992).  Its range extends across the northern latitudes of Europe and Asia.  We have just one single modern record of the species in our study area, from a vernal pond in mountainous central Pennsylvania.  The national collections include quite a few historic Click for Largerrecords, however, ranging as far south as Virginia (Beetle 1973, Stewart & Dillon 2004).  Populations of A. hypnorum typically inhabit marshes and weedy ditches, as well as the edges of intermittent ponds and slow-moving streams.  Turner & Montgomery (2009) reported that populations of Aplexa are entirely restricted to temporary or (in any case) fishless ponds in Western Pennsylvania, attributing the phenomenon to predator avoidance.  Aplexa hypnorum is pseudo-rare in US Atlantic drainages, FWGNA incidence rank I-1p.

We strongly suspect that scattered records of Aplexa in southern Atlantic drainages are attributable to confusion with Physa carolinae, which is a similar species both morphologically and ecologically, apparently the result of convergence.

> Ecology & Life History

Brown (1982) characterized A. hypnorum as a specialist in both habitat and diet when compared to the other pulmonate snails of the Crooked Lake Biological Station in northern Indiana, feeding primarily on detritus.  Den Hartog & De Wolf (1962) reported an annual, semelparous life cycle in a Dutch population of A. hypnorum, as is typical for larger-bodied pulmonates in northern latitudes (Dillon 2000: 156-162).  The animals overwinter as juveniles in the frozen soil, growing rapidly in the spring, reproducing in late summer or fall.  Den Hertog (1963) reported a correlation between the abundance of Aplexa and certain soil types, characterized by cyclic periods of inundation and drying.

> Taxonomy & Systematics

Recent studies of anatomy (Wethington 2004), allozyme frequency (Dillon & Wethington 2006), and mtDNA sequence (Wethington & Lydeard 2007) have confirmed that Aplexa is the most genetically distinctive of the North American physids.  See my essay of 12Oct07 (below) for more on the systematics of the Physidae.

Thomas Say considered American populations of Aplexa distinct from those of the Old World, authoring the nomen A. elongata in 1821.  But we join Baker (1928) and Clarke (1981) in finding no difference between North American populations and those of the Palearctic, considering elongata a junior synonym of hypnorum.

> Supplementary Resources [PDF]

> Essays

> References

Baker, F. C. (1928) Freshwater Mollusca of Wisconsin, Part I, Gastropoda. Bull. Wisc. Geol. Natur. Hist. Survey, Vol. 70. University of Wisconsin Press, Madison.  
Beetle, D. (1973)  A checklist of the land and freshwater mollusks of Virginia. Sterkiana, 49:21-35.  
Brown, K. M. 1979. The adaptive demography of four freshwater pulmonate snails. Evolution 33:417-432. 
Brown, K. M. 1982. Resource overlap and competition in pond snails: an experimental analysis. Ecology 63:412-422.  
Brown, K. M. 1997. Temporal and spatial patterns of abundance in the gastropod assemblage of a macrophyte bed. Ameri Malac Bull 14:27-33.  
Burch, J. B. (1989)  North American Freshwater Snails. Malacological Publications, Hamburg, MI.   
Clarke, A.H. 1981. The Freshwater Molluscs of Canada. National Museum of Natural Sciences, National Museums of Canada, Ottawa, Canada.   
den Hartog, C. (1963) The distribution of the snail Aplexa hypnorum in Zuid-Beveland in relation to soil and salinity. Basteria, 27:8-17.  
den Hartog, C. & De Wolf, L. (1962) The life cycle of the water snail Aplexa hypnorum. Basteria, 26:61-88.   
Dillon, R.T., Jr. 2000. The Ecology of Freshwater Molluscs. Cambridge University Press, Cambridge, United Kingdom. 
Dillon, R.T., Jr. and A.R. Wethington. 2006.  The Michigan Physidae revisited: A population genetic study. Malacologia 48: 133-142. [PDF]
Dillon, R. T., A. R. Wethington, and C. Lydeard (2011)  The evolution of reproductive isolation in a simultaneous hermaphrodite, the freshwater snail Physa.  BMC Evolutionary Biology 11:144. [PDF] [html
Hanley, R.W., and G.R. Ultsch. 1999.  Ambient oxygen tension, metabolic rate, and habitat selection in freshwater snails. Arch Hydrobiol. 144:195-214. 
Jokinen, E.H. 1992.  The Freshwater Snails (Mollusca: Gastropoda) of New York State. NY State Mus Bull 482, Albany, New York. 
Jokinen, E.H. 2005.  Pond molluscs of Indiana Dunes National Lakeshore: then and now. Amer Malac Bull 20:1-9. 
McKillop, W.B. 1985.
Distribution of aquatic gastropods across the Ordovician dolomite -Precambrian granite contact in southeastern Manitoba, Canada. Can J Zool 63:278-288. 
Stewart, T.W., and R.T. Dillon, Jr. 2004. Species composition and geographic distribution of Virginia's freshwater gastropod fauna: a review using historical records. Amer Malac Bull 19:79-91. 
Stewart, T.W. 2006.  The freshwater gastropods of Iowa (1821-1998): species composition, geographic distributions, and conservation concerns. Amer. Malac. Bull. 21: 59 -75. 
Te, G. A. 1978.  The systematics of the family Physidae (Basommatophora: Pulmonata). Ph.D. Dissertation, University of Michigan, pp. 325. 
Te, G. A. 1980.  New classification for the family Physidae (Pulmonata: Basommatophora). Arch. Moll. 110:179-184. 
Turner, A. M. & S. L. Montgomery.  2009.  Hydroperiod, predators and the distribution of physid snails across the freshwater habitat gradient.  Freshwater Biology 54: 1189-1201.
Walker, B. 1918. A synopsis of the classification of the freshwater molusca of North America, north of Mexico. Univ. Mich. Museum of Zool. Misc. Publ. 6. 
Wethington, A. R. 2004 Phylogeny, taxonomy, and evolution of reproductive isolation in Physa (Pulmonata: Physidae) Ph.D. dissertation, University of Alabama, Tuscaloosa.  
Wethington, A. R. & C. Lydeard 2007.  A molecular phylogeny of Physidae (Gastropoda: Basommatophora) based on mitochondrial DNA sequences.  J. Molluscan Stud. 73: 241 - 257 [PDF] .
Wu, S.-K. (1989) Colorado Freshwater Mollusks. Natural History Inventory of Colorado, Vol. 11.  Univ. Colorado Museum, Boulder.