NATURAL HISTORY
Freshwater gastropods are an important and diverse component of aquatic ecosystems worldwide. These single-valved mollusks have diversified into every conceivable natural aquatic habitat including aquifers, springs, creeks, rivers, lakes, and wetlands. They are routinely found in ephemeral and man-made water bodies. Many species spend their entire lives in a few square meters of habitat, making them extremely vulnerable to localized environmental habitat degradation. Freshwater snail species richness is greatest in medium- to large-size rivers with clean, stable, substrates and high dissolved oxygen concentrations.
Aquatic snails are nature's ultimate grazer. Most graze on periphytic or epiphytic algae and biofilms, but some are suspension or deposit feeders. Freshwater gastropods often dominate stream benthos in both mass and numbers. Gastropods are an important dietary components of several predatory animals, including crayfishes, fishes, turtles, and ducks. Therefore, they have a profound impact on algal primary productivity playing a pivotal role in nutrient cycling. Gastropods were important dietary components.
Freshwater snails with an operculum—a cutaneous membrane that covers the shell opening when the snail is inside its shell—are descended from marine ancestors and extract oxygen from the water with a single gill. These operculate snails are the most diverse freshwater group, comprising about two-thirds of all North American freshwater snails. They have separate sexes, internal fertilization, and a short reproductive season. Females generally attach eggs to firm substrates in late spring and early summer. In general, these species are slow-growing, and longer-lived (often 2 to 15 years). They typically have narrow ecological tolerances, limited dispersal ability, and predominately occur found in riverine habitats. Many of these species are narrow range endemics, often isolated at a single spring, or geographically restricted to a single river basin.
In contrast, snails that lack an operculum evolved from terrestrial ancestors and breathe with a modified "lung." These species are hermaphroditic (each snail possesses both male and female reproductive organs), and have an extended breeding season. They mature quickly with short generations times (typically <2 years). This group of snails typically has broad ecological tolerances, disperses readily, and is mostly associated with lakes, ponds, bogs and ephemeral bodies of water across multiple drainage basins.
