Harmful Aquatic Hitchhikers: Mollusks: Zebra Mussel

What do zebra mussels look like?

Zebra mussels get their name from the striped pattern of their shells. However, the pattern varies greatly to where there are no stripes, only dark or light colored shells. Zebra mussels can grow to a maximum length of about 50 mm (5-10 mm in the first year) and live four to five years.
Zebra Mussel

Why are zebra mussels considered to be a nuisance?

  • Ecological Impacts - Zebra mussels have disrupted the traditional aquatic food chains of many inland lakes. Regardless of their size, inland lakes represent unique ecological systems. When zebra mussels enter into these fragile systems, their voracious filter feeding depletes the availability of microscopic organisms that play a critical part in each lake's ecological food web. As a result, valued sportfish are impacted. Zebra mussels consume considerable amounts of these beneficial microscopic organisms and this creates less food for larval and juvenile fishes that support sport and commercial fisheries.
  • Biological Impacts - Zebra mussels are similar to other mussel species because they attach themselves to hard surfaces. However, unlike other species, zebra mussels will readily attach themselves to native mussels. This behavior is known as bio-fouling and with the spread of zebra mussels, native mussel populations have been severely reduced. Some native mussels are more tolerant than others, but even for these resistant species, becoming covered by zebra mussels makes them more vulnerable to environmental stressors, such as extreme water temperatures, lack of food, or parasites and disease. As zebra mussels spread, biologists are concerned that populations of native mussels will decline, and perhaps some of the rarer species may be completely eliminated.
  • Economic Impacts - The zebra mussel attaches to hard surfaces located at moderate depths. This affinity for hard surfaces has made water intake structures, like those used for power and municipal water treatment plants, susceptible to colonization. Since 1989, some plants located in areas of extensive zebra mussel colonization have reported significant reductions in pumping capabilities and occasional shutdowns.
  • Human Health Impacts – As significant filter feeders, zebra mussels may increase human and wildlife exposure to organic pollutants (PCBs and PAHs). Early research shows that zebra mussels can rapidly accumulate organic pollutants within their tissues to levels more than 300,000 times greater than concentrations in the environment. They also deposit these pollutants in their pseudofeces. These contaminants can be passed up the food chain so that any fish or waterfowl consuming zebra mussels will also accumulate these organic pollutants. Likewise, human consumption of these same fish and waterfowl could result in further risk of exposure.
Zebra Mussel

How do zebra mussels affect recreational users?

  • Boaters - Recreation-based industries along Lake Erie have been impacted by zebra mussels. Unprotected docks, breakwalls, boat bottoms, and engine outdrives were rapidly colonized beginning in 1989. Consequently, there were numerous reports of boat engines overheating due to colonies of zebra mussels clogging cooling water inlets and mussels colonizing boat hulls.
  • Swimmers - Beaches are also affected by zebra mussels. The sharp-edged mussel shells along swimming beaches can be a hazard to unprotected feet. By autumn of 1989, extensive deposits of zebra mussel shells were on many Lake Erie beaches. The extent of these deposits varied with successive periods of high wave activity.
  • SCUBA Divers – One of the things that attract people to dive underwater are the many features such as shipwrecks, various fish species and underwater landscapes. Since zebra mussels attach to hard surfaces, many of the features are at risk of becoming obscured.

Where are zebra mussels currently found?

As a native to Russia, zebra mussels were introduced into the Great Lakes in 1985 or 1986, when one or more transoceanic ships discharged ballast water into Lake St. Clair. Being a temperate, freshwater species, the zebra mussels found the plankton-rich Lakes St. Clair and Erie to their liking. Since this time, zebra mussels have spread throughout the Great Lakes, the Ohio River Basin and the Mississippi River Basin. Colonies have been found as far west as Oklahoma and live zebra mussels have been found attached to recreational boats transported to Washington state. As of 1994, the following states had reported records of zebra mussels within their borders or in water bodies adjacent to their borders: Alabama, Arkansas, Illinois, Indiana, Iowa, Kentucky, Louisiana, Michigan, Minnesota, Mississippi, Missouri, New York, Ohio, Oklahoma, Pennsylvania, Tennessee, Vermont, West Virginia, and Wisconsin. More recently, Connecticut has been added to the list of states where zebra mussels have been found.
Zebra Mussel Map

What is the potential for zebra mussels to spread elsewhere in U.S.?

The potential for zebra mussels to spread is very high. In fact, several bills are in front of Congress that focus on preventing the westward spread of zebra mussels. These hitchhikers can spread to other inland waters either in their immature form known as veligers transported in water or as adults attached to boat hulls, engines, aquatic weeds, or other surfaces. Veligers are small-about the size of the period at the end of this sentence-and may be able to survive in any residual water source.

Adult mussels are very hardy and can survive out of water for extended periods depending upon temperature, humidity, wind, and sunlight. Maximum out-of-water survival time in ideal conditions is about 10 days for adults and 3 days for newly-settled juveniles.

Short term benefits of zebra mussels don’t override long term impacts

Originally, when zebra mussels entered Lake Erie, the public saw the positive impacts of their filter feeding that cleaned up the lake. Zebra mussels contributed to the improvement of Lake Erie's water clarity. Research shows that in the early ‘70s, water clarity was approximately 3 feet. It improved to 6 to 10 feet in the 1980s after a decade of reduced phosphorus inputs, and improved again to 10 to 17 feet in the early 1990s, after zebra mussels colonized the area. However, while the water may have better clarity, the negative impacts outweigh this one positive benefit. One result of clearer water is increased vegetative growth, which may not be the best thing for anglers, boaters, swimmers and other aquatic recreation users.

How can I prevent the spread of zebra mussels?

  • Remove any visible vegetation from items that were in the water, including the boat, trailer, and all equipment.
  • Flush engine cooling system, live wells, and bilge with tap water. If possible, use hot water.
  • Do not re-use bait if exposed to infested waters.
  • Dry boat and other equipment for at least 48 hours before using in uninfested waters.
  • Examine boat exterior for mussels if it has been docked in infested waters; if mussels are found or exterior is heavily fouled by algae, either clean fouled surfaces or leave boat out of the water for at least 5 days before entering uninfested waters.

What else can I do?

This infomation was provided by the U.S. Fish and Wildlife Service. For more information, go to their campaign website www.protectyourwaters.net.