This family of pulmonate freshwater snails is found world-wide, but the great majority of species are native to the western hemisphere. It is probably the most widely-dispersed and commonly encountered family of freshwater snails. The family is distinguished by the lack of an operculum to close the shell, the high spiral shape of the shell, and the sinistral ("left-handed") direction of the spiral -- when viewed from the opening, the shell spirals to the left.
Like other families in their order, physids have a single pair of sensory tentacles on the head, and an eye at the base of each tentacle. Most have comparatively small shells, and both the tentacles and foot are slender.
Like all pulmonates, they have a cavity in the shell that is used for gas exchange. Most species maintain a bubble of air in the cavity, and occasionally refresh it at the surface of the water. A few species fill the cavity with water, and live without ever approaching the surface.
Compared to other pulmonate families, physids feed more on detritus and periphyton (the film of single-celled algae that forms on solid objects in freshwater) than on filamentous algae or suspended particles in the water column. They often have relatively smaller and more complicated teeth on their radula than the other families do, and this may reflect adaptation for periphyton grazing.
Physids are simultaneous hermaphrodites, and can self-fertilize, though they will exchange sperm with other individuals if they can. They lay eggs in soft, crescent-shaped egg masses. The embryos complete their larval stages within the egg, and emerge as tiny shelled snails. Temperature strongly affects the pace of their life-cycle, warmer temperatures allow faster growth. Most species mature and reproduce in 9-15 months, but in warm regions, may complete more than one generation a year. Compared to other freshwater gastropods, physids often mature sooner and have higher fecundity. They are also able to move faster than species in other groups, and these features together give the family a higher rate of dispersal to new habitats.
Burch, John B. and Younghun Jung. 1992. Freshwater snails of the University of Michigan Biological Station Area. Walkerana 6(15).
Brown, Kenneth M. 2001 Mollusca: Gastropoda in Thorp, J.H. and A. P. Covich, Ecology and Classification of North American Freshwater Invertebrates. Academic Press, New York
Grant, Douglas G. 2001. Pennak's Freshwater Invertebrates of the United States, Fourth Edition. John Wiley & Sons, Inc. NY
Presented below is a collection of images and comments from Dr. J.B. Burch, Curator Emeritus of Mollusks, University of Michigan Museum of Zoology.
Left to right, Physella acuta, Radix natalensis and Fossaria truncatula. These three specimens demonstrate the diagnostic differences in the shells of their two families. The Physidae (represented here by Physella acuta) have left-coiled (sinistral) shells, while the Lymnaeidae (represented here by Radix natalensis and Fossaria truncatula) have right-coiled (dextral) shells.
Two species of the same genus Physella. One species is darkly pigmented, while the other lacks pigment. But there are also other differences - in the internal anatomy and in small but consistent differences in the shells.
Physella gyrina. The light band (varix) on the shell shown here was caused by the snail's much slower growth during the winter. While the shell growth was slower at the varix, a thicker shell was produced at this site (which masks the dark mantle [skin covering the viscera] of the snail's body, that elsewhere shows through the translucent shell).
George Hammond (author), Animal Diversity Web, John B. Burch (author, donor, photographer, identification), Mollusk Division, Museum of Zoology, University of Michigan-Ann Arbor.