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 arena, 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 Balkans, Poland, and 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 came in 1988 from Lake St. Clair, located between Lake Huron and Lake Erie. By 1990, zebra mussels had been found in all the Great Lakes. The following year, they made their way into the Illinois and Hudson rivers. By 1992, zebra mussels had established populations in the Arkansas, Cumberland, Hudson, Illinois, Mississippi, Ohio, and Tennessee rivers. As of 1994, the following States had reported zebra mussels within, or in waters 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.

 

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. In 1993, several trailered boats crossing into California 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 5 years. Their common name was inspired by their dark, zebra-like rays. Although they are freshwater animals, zebra mussels have recently been found living in brackish water with salinity levels of 1 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 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. 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 are not yet known. However, information from Europe tells us 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.

 

The National Park Service is very concerned about the St. Croix River, a National Wild and Scenic River in the upper Mississippi River basin, because it contains the only viable population of the winged mapleleaf clam (Quadrula frugosa). Zebra mussels could wipe out this already endangered species if they become established in the river. In an attempt to save these native mussels, biologists are placing them in temporary refugia or transplanting them into waters free from zebra mussels.

 

Another exotic invader, the quagga mussel (Dreissena 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.

 

The zebra mussel's rapid spread has researchers working together to help track this invading exotic. Many government and private organizations are cooperating with National Biological Service researchers at the Southeastern Biological 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 scientific information to State and Federal agencies responsible for the management of public lands. It is available by Internet on the World Wide Web (http://www.nfrcg.gov). 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 National Biological Service's Southeastern Biological Sciences Center in Gainesville, Florida.

7920 NW 71st Street
Gainesville, FL 32653
Phone: (352) 378-8181
Fax: (352) 378-4956

E-mail: amy@nfrcg.gov

 

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