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The Exotic Zebra Mussel
by Amy J. Benson
The word "exotic" conjures up thoughts of faraway places where strange animals inhabit sun-drenched islands. But in the biological sciences realm, this word has come to be associated with unwelcome visitors to our shores. Over the past 500 years, more than 4,500 foreign species, including many harmful plants, vertebrates, invertebrates, and pathogens, have established populations in the United States.
One of these newcomers, the zebra mussel (Dreissena polymorpha), poses widespread ecological and economic threats. This small freshwater mollusk originated in the Black, Caspian, and Azov seas region of the former Soviet Union. By the late 18th and early 19th centuries, the construction of extensive canal systems enabled the spread of zebra mussels to almost all major drainages of Europe.
In the United States, the first account of an established population occurred in 1988 from Lake St. Clair, located between Lake Huron and Lake Erie. By 1990, zebra mussels had been found in all five Great Lakes. Over the next two years they made their way out of the Great Lakes through canals and into the Illinois, Hudson, Arkansas, Cumberland, Hudson, Mississippi, Ohio, and Tennessee rivers. As of 2011, the following states had reported zebra mussels within, or in waters adjacent to, their borders: Alabama, Arkansas, California, Colorado, Connecticut, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nebraska, New York, North Dakota, Ohio, Oklahoma, Pennsylvania, Tennessee, Texas, Utah, Vermont, Virginia, West Virginia, and Wisconsin. The mussels have also been documented in over 600 lakes and reservoirs in the United States.
Zebra mussels probably entered the Great Lakes when ships arriving from Europe discharged ballast water containing a variety of aquatic organisms, including zebra mussel larvae. The species rapid dispersal throughout the Great Lakes and major river systems was due to its ability to attach to boats navigating these waters. Zebra mussels have an even more troubling characteristic: the ability to stay alive out of water for several days under moist and reasonably cool conditions. Thus, overland dispersal is another possible means of range expansion. An increasing number of small lakes near, but not connected to, the Great Lakes are now inhabited by zebra mussels. Beginning in 1993, many trailered boats crossing into California and other western states were found to have zebra mussels attached to their hulls. These mussels, discovered at agricultural inspection stations by informed officials, were removed before the boats were allowed to continue.
Zebra mussels can grow to a maximum length of about 50 millimeters (1.9 inches) and live 4 to 5 years. Their common name was inspired by their dark, zebra-like stripes. Although they are freshwater animals, zebra mussels have recently been found living in brackish water with salinity levels of 1 to 2 parts per thousand. Females generally reproduce in their second year. More than 40,000 eggs can be laid in a reproductive cycle and up to 1 million in a spawning season. The larvae emerge within 3 to 5 days after the eggs are fertilized and are free-swimming for up to a month. Dispersal of larvae is normally passive downstream. The mussels begin their juvenile stage by settling to the bottom, where they crawl about by means of a foot searching for a suitable firm surface or substratum upon which to anchor. Although hard, calcareous materials, such as limestone, concrete, and the shells of other mussels are preferred substrates, they will attach to various surfaces, including water intake valves and pipes, and have even been found on vegetation. Juvenile zebra mussels attach themselves by an external organ called a byssus, which consists of many threads that adhere to a surface. Adults zebra mussels filter about 1 liter (2.1 pints) of water per day while feeding primarily on algae.
Most of the biological impacts of zebra mussels in North America may yet to be realized. However, they have the potential to harm native mussels by interfering with their feeding, growth, movement, respiration, and reproduction. Researchers are observing some of these effects as they study interactions between zebra mussels and native mussels in the Great Lakes. In one study, biologists found that where zebra mussel densities were highest, in Lake St. Clair and in the western basin of Lake Erie, native mussels had declined after only two years of zebra mussel colonization. Other studies have shown an inverse correlation between zebra mussel biomass and the density of native mussels. Scientists in the Great Lakes region are using models that may predict the degree of loss based on zebra mussel densities. Unfortunately, research shows zebra mussels prefer to attach to the shells of live mussels rather than to dead ones or to stones. Some native mussels have been found with more than 10,000 zebra mussels attached to them. Native species may not survive if zebra mussels continue to colonize Lake St. Clair.
Another exotic invader, the quagga mussel (Dreissena rostriformis bugensis), probably arrived at the same time as the zebra mussel. Although the quagga mussel closely resembles its cousin, it is not expected to have as great an impact on native mussels because it does not show a preference for using them as substrates. Hoever, in the Great Lakes, the quagga mussel appears to be outcompeting the zebra mussel to near exclusion. Quagga mussel were contained to primarily the Great Lakes until 2007 when a very large population was discovered in Lake Mead on the Colorado River in Nevada. Its appearance was mostly likely the result of a contaminated boat entering the lake coming from infested waters. This species is spreading in western states and overall has been reported from the following states: Arizona, California, Colorado, Illinois, Indiana, Michigan, Minnesota, Missouri, Nevada, New Mexico, New York, Ohio, Pennsylvania, Utah, and Wisconsin.
The rapid spread of both mussel species has researchers working together to help track this invading exotic. Many government and private organizations are cooperating with U.S. Geological Survey researchers at the Southeast Ecological Science Center in Gainesville, Florida, by reporting information on new sightings. This information becomes part of the Nonindigenous Aquatic Species Information System, which distributes general information available to government agencies, private groups, and the public. The system also provides sound scientifc information to State and Federal agencies responsible for the management of public lands. It is available at http://nas.er.usgs.gov/taxgroup/mollusks/zebramussel/. By working together, it may be possible to control the continued spread of non-native pests like the zebra mussel.
Amy J. Benson is a fishery biologist at the U.S. Geological Survey's Southeast Ecological Science Center in Gainesville, Florida.
Last updated: April 1, 2014