> Habitat & Distribution

Aplexa elongata is a species of northern latitudes, ranging coast to coast from New York to Washington, north to Alaska and south to Colorado (Clarke 1981, Burch 1989, Wu 1989, Jokinen 1992).  Populations are widely scattered throughout Nebraska and The Dakotas, although absent from Kansas.

We have just five modern records in our Eastern study area, however, all from the northern Glaciated Central Lowlands, three in Pennsylvania and two in Ohio.  We suspect that scattered literature records of Aplexa from Atlantic drainages in Virginia (Beetle 1973, Stewart & DIllon 2004) and further south are attributable to confusion with Physa carolinae, which is similar both morphologically and ecologically, apparently the result of convergence.

Populations of A. elongata 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 elongata is pseudorare in our 17-state Eastern study area, FWGNA incidence rank I-2p.

> Ecology & Life History

Brown (1982) characterized A. elongata 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.

A detailed study of the shell morphology of European Aplexa hypnorum, together with a review of the ecology and life history of populations quite similar to American A. elongata, has been contributed by Cieplok et al (2022).  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

The evolutionary relationship between North American populations identified as Aplexa elongata and Palearctic populations described by Linneaus in 1758 as Aplexa hypnorum is so close that for many years the FWGNA Project followed Baker (1928) and Clarke (1981) considering the former nomen a junior synonym of the latter.  Indeed, we are not aware of any morphological or ecological distinction between New World and Old World Aplexa populations whatsoever, even to the present day.  Mitochondrial COI sequence data recently made available through the NCBI Genbank have persuaded us, however, that the levels of genetic divergence between European populations of A. hypnorum and their cognates on this side of the Atlantic are of sufficient magnitude to warrant distinguishing A. elongata at the specific level.

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.

> Maps and Supplementary Resources

• Physid distribution in drainages of The Ohio (2019)
• Aplexa distribution in Atlantic drainages (2023)
• Aplexa distribution in The Great Plains (2024)
• Virginia species account (2011)

> Essays

• The phylogenetic analysis of Wethington & Lydeard prompted me to review The Classification of the Physidae in my post of 12Oct07.
• Aplexa made a cameo appearance as an outgroup in the Dillon et al. (2011) paper on the evolution of reproductive isolation in Physa.  See my blog post of 12July11, "What is a Species Tree?"
• See my post of 26Sept14 for good, comparative figures illustrating "The egg masses of freshwater pulmonate snails."

> 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.
Cieplok, A., R. Anderson, M. Gawlak, T. Kaluski, & A. Spyra (2022) Morphological diversification of alien and native aquatic snails of the genus Physa and Aplexa (Gastropoda: Physidae) of Western and Central European range.  Zootaxa 5168 (2): 101 - 118.
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.