> Physa gyrina (Say 1821)
    "Physella gyrina"

> Habitat & Distribution
The range of the species extends from California to the Gulf of Mexico, and northward to Quebec and the Northwest Territories (Clarke 1981, Jokinen 1992, Dillon and Wethington 2004).  Physa gyrina has also been introduced to the British Isles (Anderson 1996).   It is most common in the northern part of our study area - especially western Virginia.  Physa gyrina is rare in North Carolina and absent from South Carolina.  We have one Georgia record - a spring in a DeKalb County Park.

Physa gyrina occurs in almost any permanent or intermittent freshwater habitat type, including ponds, lakes, creeks and rivers. Substrata occupied include mud, sand, gravel, rock, and plants (Jokinen 1992, Dillon 2000, Dillon and Wethington 2004, Stewart 2006). In northern states, populations are often very abundant in macrophyte stands (Pip 1991, Brown 1997). 

> Ecology & Life History
Physa gyrina seems to thrive under a variety of environmental conditions, and is considered a dietary and habitat generalist (Brown 1982, Dillon 2000).  Dillon (2000:363) considered Connecticut populations of P. gyrina to be S-adapted, capable of tolerating low food and perhaps osmoregulatory stress in exchange for environmental predictability. This snail is an important component of many aquatic foodwebs.  Populations consume tremendous quantities of detritus, diatoms, filamentous algae, fungi, and living animal and vascular plant material (Newman et al. 1996). Populations of P. gyrina have documented effects on algal and macrophyte biomass, and can alter primary producer community structure through grazing (Hunter 1980, Sheldon 1987).

In turn, this snail can serve as an important prey for crayfish, fish, and other predaceous invertebrates and vertebrates. There is experimental evidence that crayfish can measurably reduce P. gyrina densities (Hanson and Chambers 1995). Physa gyrina populations have evolved a variety of antipredator adaptations, including detecting predators through chemical cues and subsequently avoiding them by crawling to structurally-complex protective habitats and reducing activity levels (Turner et al. 2000, McCarthy and Dickey 2002). This species also exhibits phenotypic plasticity with respect to shell shape and thickness; shell thickness may be increased in populations under heavy predation pressure by shell-crushing fish, whereas snails with narrow apertures can be found in habitats with crayfish that extract snails from their shells (DeWitt et al. 2000).

Physa gyrina is more pollution-tolerant than most freshwater snails (Clarke 1981). However, a survey by Pip (2000) revealed declines in Manitoba from 1978 to 1998, presumably due to habitat degradation caused by intensive agriculture and mining activities. Harman (2000) also found that P. gyrina went extinct in Oneida Lake, New York State, following years of anthropogenic eutrophication.
Physids are monecious, capable of cross- or self-fertilization. An individual P. gyrina can begin reproducing at 3-5 weeks of age, and produce 200 or more eggs in its lifetime (Brown 1979, Dillon 2000, Dillon & Wethington 2004). Egg masses are gelatinous, transparent or whitish, and crescent-shaped. A variety of life history patterns have been reported, including types A, B, and G (McKillop 1985, Jokinen 1992, Hanley and Ultsch 1999, Dillon 2000: 156-162).

> Taxonomy & Systematics
The shell morphological variability exhibited by physid populations has led to the description of a large number of species that will prove to be junior synonyms of P. gyrina. Dillon and Wethington (2004) reported no postzygotic reproductive isolation between populations of P. gyrina, P. ancillaria, P. aurea, P. microstriata and P. utahensis collected at or near their type localites. The results of the Dillon and Wethington (2006) genetic survey of Michigan physid populations suggested that P. sayii and P. parkeri are synonyms of P. gyrina as well. Additional junior synonyms include crocata, elliptica, inflata, hildrethiana, microstoma, lordi, and oleacea.

Until recently it was believed that the North American Physidae numbered more than 40 species, and a variety of elaborate classification schemes have been proposed (eg, Te 1978, 1980; Burch 1989). Physa gyrina has lately been referred to the genus “Physella.” It is now clear that most of this nominal diversity is attributable to phenotypic plasticity, and that the true number of American species is closer to ten (Wethington 2004, Wethington & Lydeard 2007). The simple two-genus system favored by earlier workers (Walker 1918) would seem sufficient, all southeastern species referable to the genus Physa.  See my essay of 12Oct07 (below) for more on the systematics of the Physidae.

> Essay 
The phylogenetic analysis of Wethington & Lydeard (2007) prompted me to review the systematics of the Physidae on 12Oct07.

> Maps of Physa distribution
Click the small map to enlarge it, or download the state-specific PDF



North Carolina (PDF)

> References
Anderson, R. 1996. Physa gyrina (Say), a North American freshwater gastropod new to Ireland, with a key to the British Isles Physidae. Irish Naturalists‘ Journal 25:1-6. 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, Michigan. Clarke, A.H. 1981. The Freshwater Molluscs of Canada. National Museum of Natural Sciences, National Museums of Canada, Ottawa, Canada. DeWitt, T.J., B.W. Robinson, and D.S. Wilson. 2000. Functional diversity among predators of a freshwater snail imposes an adaptive trade-off for shell morphology. Evol Ecol Res 2000:129-148. Dillon, R.T., Jr. 2000. The Ecology of Freshwater Molluscs. Cambridge University Press, Cambridge, United Kingdom. Dillon, R.T., Jr. and A.R. Wethington. 2004. No-choice mating experiments among six nominal taxa of the subgenus Physella (Basommatophora: Physidae). Heldia 6:1-10. Dillon, R.T., Jr. and A.R. Wethington. 2006. The Michigan Physidae revisited: A population genetic study. Malacologia 48: 133-142. Hanley, R.W., and G.R. Ultsch. 1999. Ambient oxygen tension, metabolic rate, and habitat selection in freshwater snails. Arch Hydrobiol 144:195-214. Hanson, J.M., and P.A. Chambers. 1995. Review of effects of variation in crayfish abundance on macrophyte and macroinvertebrate communities of lakes. ICES Marine Science Symposium 199:175-182. Harman, W.N. 2000. Diminishing species richness of mollusks in Oneida Lake, New York State, USA. Nautilus 114:120-126. Hunter, R.D. 1980. Effects of grazing on the quantity and quality of fresh water aufwuchs. Hydrobiologia 69:251-260. 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. McCarthy, T.M., and B.F. Dickey 2002. Chemically mediated effects of injured prey on behavior of both prey and predators. Behaviour 139:585-602. 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. Newman, R.M., W.C. Kerfoot, and Z. Hanscom III. 1996. Watercress allelochemical defends high-nitrogen foliage against consumption: effects on freshwater invertebrate herbivores. Ecology 77:2312-2323. Pip, E. 1991. Macrophyte and associated mollusc communities in a Meteor Crater Lake on the Precambrian Shield of Manitoba. Can Field-Nat 105: 483-487. Pip, E. 2000. The decline of freshwater molluscs in southern Manitoba. Can Field-Nat 114:555-560. Sheldon, S.P. 1987. The effect of herbivorous snails on submerged macrophyte communities in Minnesota USA lakes. Ecology 68:1920-1931. 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., R.J. Bernot, and C.M. Boes. 2000. Chemical cues modify species interactions: the ecological consequences of predator avoidance by freshwater snails. Oikos 88:148-158. 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.