Saskatchewan Snails, Limpets and Clams
(Gastropoda and Pelecypoda: Mollusca)

Pond snail

The "basic" pond snails and clams are well known by the anyone who pokes around in lakes, ponds or rivers in Saskatchewan. In fact the obvious pond snails and larger clams make up only a small percentage of the 35 species of snails, single species of limpet and 34 species of clams found in the province. The snails and "pea clams" are usually more diverse in standing water habitats such as the edges of lakes and ponds. Larger clams and limpets, along with snails are found in flowing waters.

Some snail groups have a "lung" and must breathe air. Others have a "gill" and can breathe under water. Clams all have gills which they also have developed into structures for filtering out small organisims and organic particles which they feed on. Snails usually "graze" on substrates by scraping off small organisims and algae using a toothed radula as they slide along on their foot on a film of mucous.

"Typical" pond snails are hermaphroditic. After mating the eggs are laid in a translucent jelly mass attached to a submerged rock, branch or plant. Development is rapid and is interesting to observe under a good magnifying glass or microsope. Generation times are usually less than a year. Life spans are usually less than a couple of years.

The "typical" large clams found along the shorelines of rivers and streams have separate sexes. After mating the eggs and young larvae are brooded in the female until the larvae, called glochidia, are released into the water. The glochidia are external parisites of fish. Each clam species is parasitic on a specific species of fish. After an interval up to 10 weeks, during which time they change into little clams, they fall off their host to begin free living. It is thought the parisitic stage is more of a dispersal mechanism rather than a feeding stage. The larger clams/mussels live for many years before they become sexually mature. The life span can exceed 20 years in some species.

Below is a basic taxonomic key to the families of Mollusca found in SK.

1a: Shell in two pieces.--(Clams: Pelycypoda)--Go to 2

1b: Shell with only one piece. May be coiled in a spire or flat like a rope or in the shape of a cone.--(Snails and Limpets: Gastropoda)--Go to 3

Clams:

2a: Shells greater than 25 mm. In mature specimens very much larger.--Unionidae (Pearly Mussels)

2b: Shells less than 25 mm.--Sphaeriidae (Pea Clams)

Snails and Limpets:

3a: Shell in the form of a flat cone with no visible spiralling or coiling.--Ancylidae (Freshwater Limpets)

3b: Shell coiled flat or in a spiral spire.--4

4a: Shell coiled flat like a rope coil.--Planorbidae (Rams Horn Snails)

4b: Shell coiled in a spire although in some groups it may be relatively low.--5

5a: Spire low only about one whorl high. Shell has an operculum that closes the shell opening.--Valvatidae (Valve Snails)

5b: Spire taller, more than one whorl high. Operculum can be present or absent.--6

6a: With the spire pointing to your left the shell opening will be downward when looking into the shell opening.--Physidae (Tadpole Snails)

6b: With the spire pointing to your left the shell opening will be upward when looking into the shell opening.--7

7a: Mature snails larger than 6 mm tall. The tip of the shell spire very pointed. Operculum absent.--Lymnaeidae (Pond Snails)

7b: Mature snails are smaller than 6 mm and have. The shell is blunt at the tip and an operculum (B) is present.--Hydrobiidae (Spire Snails)

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References

Clarke, A.H. 1981. The Freshwater Molluscs of Canada. National Museum of Natural Sciences, Ottawa, Canada.

Prescott, D.R.C. and M.M. Curteanu. 2004. Survey of Aquatic Gastropods in the Central Parkland Subregion of Alberta. Alberta Sustainable Resource Development, Fish and Wildlife Division, Alberta Species at Risk Report No. 92, Edmonton, AB.