Physa

Physa is a genus of small, air-breathing freshwater snails in the family Physidae. They are pulmonate gastropods, meaning they possess a lung-like structure for breathing air, and they lack an operculum. The shells are typically small and commonly coiled in a sinistral (left-handed) direction, with a simple, often elongated outline. Size, color, and pattern vary among species.

The genus has a broad distribution in North America and has been introduced to many other regions, including Europe, Asia, Africa, and Oceania. Physa species inhabit still or slow-moving freshwater habitats such as ponds, ditches, and wetlands, and they often tolerate eutrophic or polluted waters. They can be abundant in disturbed or temporary habitats and may colonize new environments rapidly.

Ecology and biology: Physa snails feed on biofilm, periphyton, and detritus, contributing to the breakdown of organic matter in aquatic ecosystems. They are prey for fish, amphibians, and aquatic invertebrates. Reproduction is typically as hermaphroditic, with individuals capable of cross-fertilization. They can produce multiple clutches of eggs over a breeding season, attached to submerged substrates. Some self-fertilization can occur in certain situations, but cross-fertilization is common.

Significance: Physa species can act as intermediate hosts for various trematode parasites, linking aquatic and vertebrate hosts in parasite life cycles. They are also used in ecological and toxicological studies and are monitored as indicators of freshwater health, particularly in assessing pollution and ecosystem changes. Physa acuta is among the most widespread and frequently studied species within the genus.