Spring 2008

Zebra mussels (bottom) are rapidly being replaced in the Great Lakes by quagga mussels (top). Quagga mussels may have a more detrimental impact on the ecosystem due to their adaptive nature.
UW Sea Grant Outreach

Quagga Mussels Outpace Zebras as Invader

By Carolyn Betz

It is unusual to find a zebra mussel in Lake Michigan today. This is not a happy ending to the story of the invasive species’ takeover of the lake, but rather a surprise twist. The quagga mussel (Dreissena bugensis), a cousin to the zebra (Dreisssena polymorpha), has rapidly become the dominant mussel species today.

While the two mussels are similar in many respects, it is important to differentiate between them because the quagga may have greater impacts on the Great Lakes environment. Zebra and quagga mussels are both natives of the Caspian and Black Seas brought to the Great Lakes in the ballast water of ocean-going ships. They are both thumbnail-size, hinged mussels with zebra-like stripes. However, the quagga is easily outcompeting its cousin because it can adapt more readily to the Great Lakes environment.

The zebra must attach to hard surfaces in relatively warm water, which has made it the most common in near-shore areas. In contrast, the quagga mussel can thrive on any surface except mud and prefers silt and sand. Quaggas can also tolerate both colder and warmer water temperatures, can spawn in colder water, feed year around, and are found at depths of 300 feet and greater.

Phil Moy, Fisheries and Invasive Species Specialist with the University of Wisconsin Sea Grant Institute, says because the quagga mussel can thrive in colder, deeper waters, we need to get a handle on it as quickly as possible so that it does not dominate Lake Superior waters, a place where zebras do not thrive.

“The $64,000 question is why the quaggas have taken over so quickly,” says Moy. He thinks it may be because zebra mussels have paved the way for the new invaders by making the waters clearer and providing the opportunity for quaggas to thrive.

Quagga Mussels Impact

Zebras and quaggas alike are continuing to plague water utilities and other industries, clogging pipes in layers up to eight inches thick. However, quagga mussels may have a greater impact on food web dynamics in the Great Lakes. Quaggas may be the reason Diporea, a small shrimp-like species that serves as a food source for larger fish, is no longer abundant. The whitefish that feed on this amphipod are growing to less than half of their expected size and are showing changes in tissue and fatty acid composition. Algal outbreaks associated with the improved water clarity from the mussel invaders make recreational uses of the lakes less attractive to the public.

Despite these challenges, Moy feels optimistic that the quagga’s invasion of Wisconsin’s inland lakes can be controlled. The public must take seriously the importance of “clean boating” measures such as draining all water from bilges, ballast, live wells, bait buckets and other containers to avoid carrying unwanted invasives from lake to lake.

The Aquatic Sciences Center is the administrative home of the
University of Wisconsin Sea Grant Institute & University of Wisconsin Water Resources Institute.

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