Snails
ABOUT
Worldwide, there are roughly 300 families of freshwater snails. Only 15 of these are found in North America, these 15 accounting for 650 species. Many species of snails are now threatened, largely because of dam construction, siltation, industrial and agricultural pollution. Problems are especially severe in the southeastern states. Of the freshwater snails in North America, 60 species are presumed extinct, 20 species are now listed as “endangered and threatened,” and 290 species are considered of “conservation concern.”
Snails and limpets (Gastropoda) are freshwater mollusks with one shell. They are divided into 2 main groups (subclasses) – “gilled” (Prosobranchia) and “lunged” (Pulmonata). While the subclasses are mainly distinguished by how they breathe – using gills, or an internal air-filled lung – there are other differences as noted below. Freshwater gastropods do not undergo any discernible metamorphosis as they develop into adults: immature gastropods look like small adults. Adult shells typically have four or more coils.
DIET
Gastropods have a long, strap-like structure called a radula in their mouth that is very hard and is covered with many small, sharp teeth. The radula is dragged along the substrate and the teeth act like a file to loosen pieces of food, which accompany the radula back into the mouth. Thus, they are primarily “scrapers” in their manner of eating. A few snails use their gills to produce currents of water from which they filter out algae and fine detritus suspended in the water.
Reproduction
All lunged snails are hermaphroditic with male and female reproductive organs present in the same individual. In gilled snails, male and female are distinct. Eggs are typically laid in the spring. The number of eggs varies. Lunged snails produce more eggs than gilled snails. The entire batch of eggs is enclosed in mass and is attached to various objects in the water which may be visible and resemble a drop of mucus. Snails develop within the egg and, when hatched, look like tiny snails with only one or 2 coils.
Gilled Snails (Prosobranchia)
Gilled snails can be up to ¾ of an inch in length. The shell opening is covered by a thin plate called an operculum; with helix pointed up, shell opens to the right. These snails are intolerant of impairment. Most snails live 9-15 months, but some gilled snails live for 2 to 5 years. Gilled snails are separated into 9 families in the U.S. Of these, only 4 are important in our region: Pleuroceridae, Hydrobiidae, Bithyniidae, and Viviparidae.
Pleurocerids
Pleurocerids are mostly lotic-erosional. They usually occur in rocky riffles or sandy shoals and are often conspicuously abundant on stones in shallow, slow-moving riffles in medium-sized rivers. Pleurocerids are mostly scrapers. A pleurocerid’s shell is very thick and solid, and its operculum is often not visible due to small size and because the snail pulls it back deep in the shell. The operculum is slightly hardened, but still flexible with a continuous curved line that loops only a couple of times before ending at the edge (paucispiral). The whorls of the shell do not bulge out distinctly on the sides (flattened), which make the sides of the shell almost straight lines with only shallow incisions between the coils. Pleurocerids range from somewhat sensitive to facultative in their response to impairment.
Hydrobiids
Hydrobiids are usually found on aquatic plants and can occur in large numbers. They are mostly scrapers. The operculum of a hydrobiid is slightly hardened, but still flexible with a single continuous line that loops several times in a complete spiral before ending at the edge. The whorls of the shell bulge out distinctly on the sides (inflated), which makes the sides of the shell a series of broad curves with deep incisions between the coils. Hydrobiids are somewhat sensitive to impairment. Hydrobiidae is the most diverse gastropod family in North America with a total of 36 genera.
Bithyniids
Bithyniids usually live on aquatic vegetation. They are scrapers and collector-filterers and graze on attached algae in the normal manner for snails, but they also use internal gills to filter algae that live suspended in the water. The latter characteristic may explain their tendency to be prevalent in lakes that have become enriched with nutrients due to human activities. The operculum of a bithyniid is broadly oval to nearly round in shape with lines arranged in a series of separate, successively smaller, elongate circles that are concentric. The concentric lines are almost centered like a target with a bull’s eye. The whorls of the shell bulge out distinctly on the sides (inflated), which make the sides of the shell a series of broad curves with deep incisions between the coils. Bithyniids can live under varying conditions, meaning they have facultative tolerances to impairment.
Viviparids
Viviparids are most commonly found on sandy bottoms, and they are collector-gatherers. Their operculum is slightly hardened, but still flexible and is elongate and somewhat oval in shape with a series of oval lines that are successively concentric. The concentric lines are off center. Some have an operculum with one edge that is almost a straight line. The whorls of the shell bulge out distinctly on the sides (inflated) which makes the sides of the shell a series of broad curves with deep incisions between the coils. They are facultative in their response to impairment.
Lunged snails (Pulmonata)
Lunged snails (Pulmonata) can be up to ¾ of an inch in length. They have no operculum, and if the helix is pointed up, the shell opens to the left (with one exception, as noted below). Lunged snails are tolerant of impairment. All of the freshwater pulmonates in the northeastern United States are egg-laying hermaphrodites. Lunged snails are able to hibernate in the mud during adverse conditions (for example, in hot weather when streams dry up). Most species of lunged snails complete their life cycle in a year or less, and some can produce 2 to 3 generations per year. Lunged snails are separated into 6 families. Of these, only 4 are important in our region: Ancylidae (Limpets), Lymnaeidae, Physidae, and Planorbidae.
Ancylids (Limpets)
Ancylids (Limpets) are lotic-erosional. They are more common on firm substrates in currents, and they are mostly scrapers. Ancylids have no operculum, and their shells are not coiled, but shaped like a low, flat cone. They are somewhat tolerant of impairment. Ancylids are classified as lunged snails, but there is no air cavity in their bodies like other lunged snails and no internal gill like gilled snails. There is a projection from the foot which acts as an accessory gill and probably allows for the intake of dissolved oxygen all over the surface of the body.
Lymnaeids
Lymnaeids occur on firm surfaces, such as rocks, woody debris and aquatic plants, as well as on soft silty substrate. They are scrapers and collector-gatherers. Lymnaeids have no operculum, and their shells open on the right when the narrow end is pointing up, which is atypical for lunged snails. They are somewhat tolerant of impairment.
Physids
Physids are most common on soft, silty substrate, but are also found on any solid objects that protrude above the bottom. They are mostly collector-gatherers, but some are scrapers. Physids have no operculum, and their shells open to the left when the narrow end is pointing up. They are somewhat tolerant of impairment.
Planorbids
Planorbids are most common on soft, silty substrate, but also found on any solid objects that protrude above the bottom. They are mostly collector-gatherers, but some are scrapers. Planorbids have no operculum, and their shells are coiled flat instead of extending into a spiral. Living specimens may have a reddish body that shows through the shell. They range from facultative to somewhat tolerant in their response to impairment. Planorbids have hemoglobin in their blood, which allows them to live in some habitats with very low dissolved oxygen concentrations, such as small, warm ponds.
REFERENCES
- http://pubs.ext.vt.edu/420/420-530/420-530.html
- Voshell Jr., J. Reese. 2002. Blacksburg, VA: McDonald Woodward Publishing Company. A Guide to Common Freshwater Invertebrates of North America.
- Pennak’s Freshwater Invertebrates of the United States: Porifera to Crustacea, 4th Edition by Douglas G. Smith
For more snail pictures, click here
Research conducted by StreamWatch volunteers Tina Colom and Bob Henricks