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Zebra Mussels
Ridge of zebra mussel shells accumulates on the beach.

The large light colored ridge along the shoreline is not composed of sand or stones. It is a "drift" of zebra mussel shells washed up in the last month or two. The empty shells accumulate in the Fall on some areas along the Green Bay shoreline (they are by no means this numerous everywhere along the shoreline). These drifts can reach depths of over 3 feet and widths of perhaps 10 feet. The shells are actually very diverse in size and color.

Zebra Mussels (Dreissena polymorpha) are bivalve molluscs (referring to the two hinged shells) that attach to solid objects under the water. They are mostly under 1 inch in length and feed by filtering small particles of organic matter from the water. They do best when the water is moving a little to supply them with a constant supply of food.

Zebra Mussels are an invasive species which has been introduced to Lake Michigan only in the last 10-12 years (first recognized in the U.S.A. in 1988). They have become an expensive pest because of the large numbers which cling to man-made objects such as water intake pipes of power plants, water utilities, factories, etc. If not treated they can accumulate to fill in the pipes and inhibit or completely block the flow. They also pose a threat to native molluscs and may pose a threat to entire ecosystems by altering the food web.

Zebra Mussels have amazing abilities to increase their numbers and can be spread quickly by movement of the water. They also are highly capable of attaching themselves to boat hulls and being transported to other bodies of water, or of traveling in bilge water or bait buckets from one site to the next. Another common means of spread is on vegetation clinging to boats or trailers as they move between lakes.

Text and photos prepared by Gary Fewless

© 2001-2004 The Cofrin Center for Biodiversity and the University of Wisconsin Green Bay, All Rights Reserved
Last updated on May 12, 2014