Freshwater Molluscan Shells: Pulmonata / Physidae
The Physidae, sinistral pond snails, are commonly referred to as tadpole snails or pouch snails. They are widespread, abundant, and tolerant of pollution. Burch (1982) indicates that they are most abundant in the New World. They have evidently found a shell morphology suitable for their life station, as he goes on to say "...the physids have undergone considerable diversification, much of which is not clearly exhibited in their shells. Many of the species [and genera] are not easy to identify on shell characters alone."
They have been used in studies of ecophenotypic plasticity, a so-called phenoplastic switch. Burt Vaughan of Washington State University indicates several studies in M. J. West-Eberhardt's recent compendium of research, "Developmental Plasticity & Evolution" (Oxford Press, 2003, pp. 307-362). A typical example involved rearing Physa gyrina, or P. heterostropha in controlled pair groups in either water in which crayfish co-existed or water in which only fish co-existed. Within a month, differences in shell morphology appeared; i.e., snails exposed to shell-crushing fish predators showed wide apertures and very much strengthened, rotund shells. Snails exposed to crayfish only showed narrow-apertured, thin elongate shells, with barricading teeth.
Banarescu lists the following classification for the Physidae:
Family Physidae (with number of taxa listed by Burch (1982) for North America:)
Genus Physa most of Northern Hemisphere. (2 sp. plus 1 additional morph)
Genus Physella, Holarctic,containing subgenera;
Physella s.s. (16 species, 15 subspecies, and several additional morphs), subgenus
Costatella, (14 species with 13 additional subspecies and morphs, and subgenus
Petrophysa (1 species).
Genus Aplexa most of Northern Hemisphere. (1 species plus 1 morph)
Genus Stenophysa Central America Mexico, introduced in Texas. (2 species)
Physella species, probably Physella gyrina
aurea (Lea, 1838). East and central Unite
States. Many subspecies and morphs of Physella
gyrina have been named.
Physella heterostropha halei
(Lea, 1864) Central and Southern
United States, ponds.
Unidentified Physella species,
small western Montana creek.
Aplexa hypnorum, Moscow, Russia Physa albertisi Clessin,
1866. Papua New Guinea