What is Dreissena polymorpha

Zebra mussels are temperate, freshwater bivalve molluscs. Adults range from 0.5 to 3.5 cm in length. External coloration of the shell is highly variable (thus the species name polymorpha), though most exhibit some form of the characteristic dark brown and cream concentric banding that gives the organism its common name. The anterior end of the shell is greatly reduced; the posterior is inflated. The ventral side is wide and flat.

Biology

Zebra mussels are dioecious (either male or female) broadcast spawners. Mature females can produce 30,000 to one million eggs per year. Spawning occurs when water temperatures exceed 12°C and peaks at 15° to 17°C (Nichols 1993; Claudi and Mackie 1993). The veliger larvae are planktonic, and it is during this stage that currents can easily transport larval zebra mussels from one body of water to another. Following the planktonic larval stage, which can last from three days to three months depending on water temperature (Nichols 1993), the zebra mussel larvae settle to the bottom; their survival depends on settling on a hard surface. Zebra mussels will colonize almost anything solid: rocks, aquatic plants, boat hulls, pier pilings, buoys, water intake screens and the shells of other molluscs are common points of attachment. They often grow in large colonies as young mussels settle on the older, larger zebra mussels, forming a clump called a "druse" (Claudi and Mackie 1993). Attachment is by strong byssal threads.

Temperature, salinity and calcium are limiting factors. Zebra mussels grow and reproduce best in water with a temperature range of 12 to 26°C and a calcium content of 25 to 35 mg/L. Calcium is important for growth and maintenance of the shell. Food is obtained through filter feeding; phytoplankton, small zooplankton, bacteria and detritus in the size range of 15 to 40 mm make up the bulk of their food. The filtering capacity of zebra mussels has been estimated at 10 to 100 ml/individual/hour (Claudi and Mackie 1993).

Distribution

D. polymorpha is native to eastern Europe, including the Black, Azov and Caspian seas. It spread through western Europe via canals and inland waterways that were connected to facilitate trade during the Industrial Revolution (Morton 1993). Although a freshwater organism, the zebra mussel can survive in slightly brackish water.

Zebra mussels were first documented in North America in Lake St. Clair in 1988. They were most likely transported unintentionally in the ballast water of transoceanic ships. By the fall of 1989 they were widespread in Lake Erie (Leach 1992), and by December 1993 zebra mussels were in all the Great Lakes, 18 states, and two provinces. By 2000, zebra mussels inhabited most stretches of the Mississippi, Illinois, Ohio, Mohawk, Hudson, St. Lawrence, Cumberland, Tennessee and Arkansas rivers as well as tributaries including the Missouri, Allegheny, Monongahela, Wabash and St. Croix rivers. Zebra mussels have colonized New York's Finger Lakes, Lake Champlain, Wisconsin's Lake Winnebago, Kentucky Lake and nearly 100 smaller inland lakes in seven of the eight states bordering the Great Lakes. A second Dreissena species, D. bugensis or the quagga mussel (named after an extinct zebra-like horse), appeared in North America in 1991 in Lake Erie and Lake Ontario. It is now found in the St. Lawrence River as far downstream as Montreal as well as in Lake Michigan and Lake Huron (University of Wisconsin Sea Grant Institute, Sea Grant Great Lakes Network).

In the summer of 2000, an established population of zebra mussels was found in the northernmost part of the Chesapeake Bay watershed, in Eaton Brook Reservoir in Madison County, New York. This population is currently being monitored by Dr. Thomas Horvath from the State University of New York-Onoenta and Scott Ingmire of the Madison County Planning Department (Scott Ingmire pers. comm.).

Ecological Concerns

Zebra mussels filter large amounts of phytoplankton from the water, which can lead to significant changes in ecosystem trophic dynamics. Such filtering activity has been observed to improve water clarity, which can increase light penetration; Pillsbury and Lowe (1994) found, for example, a proliferation of aquatic plants and changes in species dominance, including shifts from benthic diatoms to benthic filamentous green algae.

Zebra mussels can tolerate moderate pollutant stress and bioaccumulate contaminants; there is evidence they are involved in the trophic transfer of pollutants, particularly where waterfowl feed on zebra mussels (de Kock and Bowmer 1993). They can also cause major changes in the structure of benthic colonizing surfaces as they rapidly cover all available rock or similar hard substrate. The resulting mass of zebra mussel shells presents a more complex, three-dimensional substrate, which can lead to a change in the populations of benthic organisms that inhabit the area (Pillsbury and Lowe 1994).

Zebra mussels are a major threat to endangered North American freshwater mussels: their growth on the shell of unionids causes stress and mortality due to feeding interference. Zebra mussels rapidly encrust the native mussel's shell, competing with the unionid for food and impairing its ability to open and close its shell and to move and burrow (Parker et al. 1998).

Economic Concerns

Zebra mussels are major biofouling organisms. Their rapid colonization of solid surfaces causes serious problems at water treatment facilities, power plants, marinas, docks and boatyards. Intake pipes and screens become clogged with massive clumps of zebra mussels; buoys have sunk from the weight of the shells, and piers, pilings and boat hulls have become heavily fouled. In addition, recreational beaches have been altered when the shells of dead zebra mussels wash up in large drifts. The U.S. Geological Survey has estimated that the cost of controlling zebra mussels in the Great Lakes region alone, may soon reach $5 billion annually. It is estimated that the mussel has cost the power industry $3.1 billion since 1993; the estimated impact on industries, businesses and communities is over $5 billion (New York Sea Grant 1994).

Control Strategies

The need for controlling zebra mussels has spawned a multi-million dollar industry. Removal of established zebra mussels is usually a temporary solution. Treatments must be repeated periodically as recolonization occurs unless Dreissena is completely removed from the entire body of water, which is relatively impossible in most cases.

Methods of controlling zebra mussels include:

• Chemical controls
• Biological controls
• Oxygen deprivation
• Thermal treatment
• Exposure and dessication
• Radiation
• Manual scraping
• High-pressure jetting
• Mechanical filtration
• Removable substrates

Describing these methods is beyond the scope of this briefing paper. For research abstracts and other information on each of the control strategies listed, see the excellent bibliographic database on the Sea Grant National Aquatic Nuisance Species website http://www.cce.cornell.edu/aquaticinvaders/.

Federal Regulations

The Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990 amended the Lacy Act to include the zebra mussel to the list of injurious fish, molluscs and crustaceans. The U.S. Fish and Wildlife Service (USFWS) has also amended its regulations to include the zebra mussel. Effective December 9, 1991, the importation of live zebra mussels, veligers or viable eggs into the United States, or transportation between the continental United States, the District of Columbia, Hawaii, the Commonwealth of Puerto Rico, or any territory or possession of the United States by any means is prohibited except by permit for zoological, educational, medical or scientific purposes. This prohibition includes any species of the genus Dreissena. Permits are issued by the director of the USFWS.

Management Efforts in the Chesapeake Bay Watershed

Delaware

No identified established populations of zebra mussels have been confirmed in Delaware. The Department of Natural Resources and Environmental Control is the contact agency.

Maryland

No identified established populations of zebra mussels have been confirmed in Maryland The Maryland Department of Natural Resources Fisheries Service is the contact agency for sightings and for information requests.

Pennsylvania

Zebra mussels are established in western Pennsylvania; there has been one confirmed point sighting in eastern Pennsylvania (west of the Delaware River). The Pennsylvania Department of Environmental Protection Zebra Mussel Monitoring Program functions as a tracking and information referral service. As new sightings are reported, local water users are notified so they can implement preventive measures in their area.

Virginia

No identified established populations of zebra mussels have been confirmed in Virginia. The Department of Game and Inland Fisheries is the contact agency for reports of zebra mussel sightings. The Virginia Sea Grant Marine Advisory Program at the Virginia Institute of Marine Science assists in responding to reports of zebra mussel sightings.

References

Claudi, R. and G.L. Mackie. 1993. Practical Manual for Zebra Mussel Monitoring and Control. Boca Raton: Lewis Publishers.

de Kock, W. Cr. and C.T. Bowmer. 1993. Bioaccumulation, biological effects and food chain transfer of contaminants in the zebra mussel Dreissena polymorpha. In T.F. Nalepa and D.W. Schloesser, eds., Zebra Mussels: Biology, Impacts and Controls, p. 531. Boca Raton: Lewis Publishers.

Leach, J.H. 1993. Impacts of the zebra mussel Dreissena polymorpha. on water quality and fish spawning reefs in western Lake Erie. In T.F. Nalepa and D.W. Schloesser, eds., Zebra Mussels: Biology, Impacts and Controls, p. 392. Boca Raton: Lewis Publishers.

Morton, Brian. The Anatomy of Dreissena polymorpha. and the evolution and success of the heteromyarian from in the Dreissenoidea. In T.F. Nalepa and D.W. Schloesser, eds., Zebra Mussels: Biology, Impacts and Controls, pp. 190-192. Boca Raton: Lewis Publishers.

New York Sea Grant. 1994. "Policy issues." Dreissena polymorpha. information review. Zebra Mussel Clearinghouse Newsletter 5(1):14-15.

Nichols, S.J. 1993. Spawning of zebra mussels (Dreissena polymorpha) and rearing of veligers under laboratory conditions. In T.F. Nalepa and D.W. Schloesser, eds., Zebra Mussels: Biology, Impacts and Controls, pp. 315-319. Boca Raton: Lewis Publishers.

O'Neill, C. 1997. Economic impact of zebra mussels - Results of the 1995 National Zebra Mussel Information Clearinghouse Study. Great Lakes Research Review 3(1).

Parker, B.C., M.A. Patterson and R.J. Neves. 1998. Feeding interactions between native freshwater mussels (Bivalvia-Unionidae) and zebra mussels (Dreissena polymorpha) in the Ohio River. American Malacological Bulletin 14:173-179.

Pillsbury, R. W. and R.L. Lowe. 1994. The impact of zebra mussels on benthic algal communities in Saginaw Bay, Lake Huron, pp. 575-591. Proceedings: 4th International Zebra Mussel Conference. Madison, Wisconsin.

Sea Grant National Aquatic Nuisance Species website. 2001.
www.cce.cornell.edu/aquaticinvaders/

Snyder, F., M.B. Hilgendorf and D.W. Garton. 1994. Zebra mussels in North America: The Invasion and its Implications. The Ohio State University, OHSU-FS-045 (1994).

University of Wisconsin Sea Grant Institute 2001. Zebra mussels and other nonindigenous species. www.seagrant.wisc.edu/greatlakes/glnetwork/exotics.html

Wu, L. and D.A. Culver. 1991. Zooplankton grazing and phytoplankton abundance: An assessment before and after invasion of Dreissena polymorpha. J. Great Lakes Res.

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