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(Clams and mussels)
Soil & Water Conservation Society of Metro Halifax (SWCSMH)
Updated: October 09, 2013
Photograph: (clam)- Family: Corbiculidae
This group includes clams and mussels which typically occur in most freshwater habitats and may be particularly abundant in certain streams. Although the clams and mussels have a wide range of tolerances to pollution with some species being very sensitive to water quality, habitat and biological conditions, a number of species of this group (especially clams) can tolerate somewhat degraded conditions.
This group is distinguished by:
- Characteristic two shells attached by an external hinge which enclose the body of the bivalve.
- No eyes or distinct head.
- Soft, fleshy body (foot) may be seen extending from shell.
As a general rule, mussels are large and have a flat, more oblong shell shape, while freshwater clams are smaller (3/4 inch) and typically more round. In addition, freshwater clams are usually symmetrical with the umbo (the highest point on the shell) equally distant from both ends. Mussel shells are usually lopsided with the umbo closer to the end.
All freshwater clams are filter feeders, subsisting on phytoplankton, zooplankton, detritus, and bacteria. Some species of fish consume clams regularly, and several species of mammals, most notably muskrats and racoons prey heavily on unionaceans. The shells of unionaceans are used in the Japanese cultured-pearl industry.
- The corbiculid clams (superfamily Sphaeracea, family Corbiculidae) are represented in North America by a single species, Corbicula fluminea. A hermaphrodite, C. fluminea lives two to three years. The species is said to be intolerant of low winter temperatures, so it is not clear how widely it will be able to establish itself in the northeastern United States. C. fluminea often occurs at spectacularly high density (more than 10,000 individuals/sq.m.). It has been accused of competitively displacing native bivalves and can cause serious economic problems by attaching to the inside of and clogging pipes that carry cooling water to power plants.
- The sphaeriid clams (superfamily Sphaeracea, family Sphaeriidae) are tiny (3-20 mm) bivalves known as fingernail clams or pea clams. These are hermaphrodites as well. Sphaeriids live for a year or two. These clams are especially abundant in standing waters, both permanent and temporary; a few species are common in running waters.
- The two families of clams of the superfamily Unionacea that occur in the northeastern United States, the Unionidae and Margaritiferidae, have similar biological characteristics. The unionaceans are the large (3-20 cm) pearly mussels of lakes, streams, and rivers. Many species live for 20 years or more, and one species, Margaritifera margaritifera, has been reported to live for more than a century.
Diagnostic features (Mackie, 1998)
There are two families of bivalves native to North America, the Sphaeriidae (fingernail clams) and the Unionidae (freshwater pearly mussels), and two families that were introduced from Europe, the Corbiculidae (Asian clams) and the Dreissenidae (zebra and quagga mussels). All bivalves are filter-feeding organisms.
- The Asian clam is a warm water species and cannot survive waters that freeze. They are common in enriched waters and can tolerate water with as little as 50% oxygen saturation, but not for prolonged periods.
- Fingernail clams are closely related to the Asian clams. There are four genera, but only three, Sphaerium, Musculium and Pisidium, are common. Most species of Musculium can be found in temporary aquatic habitats. Most Sphaerium species are large, about 8 to 20 mm; Pisidium species are the smallest, most ranging in shell length from about 2 to 6 mm; and most Musculium species are intermediate in size, about 8 to 10 mm in shell length, and the shells are thin and fragile.
- One species, Musculium transversum, is an enrichment indicator, reaching its largest densities in organically enriched waters that may have as little as 25% oxygen saturation.
- Most other fingernail clams require clean water with high oxygen tensions. In fact, some fingernail clams are oligotrophic indicators. While most fingernail clams are not assigned to any indicator group, they seem to be most abundant in sandy bottoms and waters with atleast 75% oxygen saturation.
- Sphaerium nitidum and Pisidium conventus reach their greatest densities in the profundal zones of oligotrophic lakes or in the shallow waters of lakes in high northern latitudes.
- Some, like Sphaerium simile, Sphaerium striatinum, Pisidium casertanum, Pisidium compressum, and Pisidium adamsi are abundant in organic sediments but the waters are usually well saturated with oxygen, as in many river and stream environments.
- The most familiar bivalves are the freshwater pearly mussels. Most species are large (30-150 mm), but some may grow to nearly 250 mm in shell length.
- Zebra mussels were first discovered in 1988 in Lake St. Clair but probably first arrived in 1985. Quagga mussels were first discovered in 1990 in Lake Ontario but probably first arrived in 1988 or 1989.
- Because of the quagga mussels' ability to reproduce in cooler waters and survive in soft substrates, they will be found in deeper, colder waters of deep lakes and occur further north than zebra mussels.
- Conversely, zebra mussels will probably prevail on hard substrates in the shallow waters of lakes and will be the main species in the southern United States where water temperatures are warmer than found at higher latitudes.
- But both species will cause the same kinds of problems. Because zebra mussels are so prolific in numbers and are so efficient at filtering the water, there has been a noticeable increase in the clarity of water in the Great Lakes since their arrival in 1985. For example, the Secchi depth in Lake Erie had increased from about 1.5 m to about 3.5 m in the eight years that the mussels have been in the Great Lakes. The water clarity is suspected to have a profound impact on larval species of fish that feed upon the plankton. This includes several zooplankton species, larval species of fish that feed upon the zooplankton, and planktivorous adult fish.
This group includes clams and mussels which typically occur in most freshwater habitats and may be particularly abundant in certain streams. Although the clams and mussels have a wide range of tolerances to pollution with some species being very sensitive to water quality, habitat and biological conditions, a number of species of this group (especially clams) can tolerate somewhat degraded conditions (Peckarsky et al, 1990).
Mussels have larval stages that are parasitic on specific fish species and are dependent on this host fish species for dispersal within aquatic systems. As a result, problems such as barriers to fish movement, or the reactions of mussels or host fish species to environmental conditions may cause complex and variable responses in mussel populations. Because of their long life span and sensitivity to environmental change, most species of mussels are good indicators of water quality.
"Dead" clams or mussels (empty shells) do not accurately reflect water quality because shells can persist for long periods regardless of water conditions. The life is long, 1 to 15 years in clams, and productivity is relatively low (Kellogg, 1994).
References and web URLs:
- Kellogg, L.L. 1994. Save Our Streams. Monitor's Guide to Aquatic Macroinvertebrates. Second Ed. Izaak Walton League of America. 60p.
- Mackie, G.L. 1998. Applied Aquatic Ecosystem Concepts. University of Guelph Custom Coursepack. 12 chapters.
- Mackie, G.L. 2001. Applied Aquatic Ecosystem Concepts. Kendall/Hunt Publishing Company. xxv, 744 pp. ISBN 0-7872-7490-9
- Narf, R. 1997. Midges, bugs, whirligigs and others: The distribution of insects in Lake "U-Name-It". Lakeline. N. Am. Lake Manage. Soc. 16-17, 57-62.
- Pennak, Robert W. 1978. Fresh-Water Invertebrates of the United States. Second Edition. John Wiley & Sons. ISBN: 0-471-04249-8. xviii, 803p.
- Peckarsky, B.L., P.R. Fraissinet, M.A. Penton, and D.J. Conklin, Jr. 1990. Freshwater Macroinvertebrates of Northeastern North America. Cornell Univ. Press. ISBN: 0-8014-9688-8. xii, 442pp.
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