Snails are a member of the phylum Mollusca, which also includes freshwater mussels and clams. Mollusks are identified by their hard calcium carbonate shell which contains a soft body. All mollusks have a muscular foot used for locomotion, a visceral mass that comprises most of the body, and a mantle that secretes the materials that form the shell. Snails also have a distinct head. Snails can be distinguished from other mollusks by their single, unhinged shell. Mussels and clams have two shells that are hinged together. Most adult freshwater snails range from 2-70mm in length. While some species of snails are hermaphroditic, others have separate sexes. Juvenile snails look basically like small adults, but they can often be distinguished by the number of coils in their shells. A snail with more than 4 shell coils is usually an adult.
Most snails feed by scraping algae, detritus, and the occasional invertebrate from cobble, plants, or loose sediment. Other feeding strategies include shredding vegetation and detritus and filter feeding. Freshwater snails are most often found in ponds, lakes, ditches, swamps, and slow reaches of springs, streams, and rivers. As a group they can inhabit a wide variety of substrates, including rocks, sand, mud, vegetation, and debris, although particular species often prefer a specific substrate. Snails generally are not found in the shifting sand and gravel of swift stream reaches and wave swept zones. They also don't do well in acidic waters, such as those found in bogs, because the acidic conditions make less calcium carbonate available in the water for building their shells.
There are two main types of freshwater snails. One type, the gilled snails (prosobranchia) breathe by absorbing dissolved oxygen from the water through their gills. Because gilled snails are reliant on high concentrations of dissolved oxygen, they tend to be sensitive to pollution. The second type of snail, the lunged snails (pulmonata), takes air or sometimes water into an internal lung-like structure and absorbs the oxygen it contains. Because these snails do not rely directly on dissolved oxygen for respiration (breathing), they are generally more tolerant of polluted conditions. A water body with a large population of lunged snails and few or no gilled snails likely has low oxygen conditions which may be caused by elevated levels of pollution.
Photo: Snails (Pleuroceridae)
Source: The North American Benthological Society
Voshell, Jr., J. Reese; illustrated by Amy Bartlett Wright. 2002. A Guide to Common Freshwater Invertebrates of North America. Blacksburg (VA): The McDonald & Woodward Publishing Company. 442 p