Summer 2009

Suzanne Peyer conducted research showing that zebra mussels attach more securely to underlying material, allowing them to dominate in streams and rivers. Quagga mussels dominate in quiet water. Photo: Bob Rashid.

UW Sea Grant Outreach

Zebra Mussels Hang On While Quagga Mussels Take Over

By Carolyn Rumery Betz

The zebra mussels that have wreaked ecological havoc on the Great Lakes are harder to find these days—not because they are dying off, but because they are being replaced by a cousin, the quagga mussel. Yet zebra mussels still dominate in fast-moving streams and rivers. Research conducted by Suzanne Peyer, a doctoral candidate in the UW-Madison department of zoology, shows that physiological differences between the two species might determine which mollusk dominates in either calm or fast-moving waters.

“Zebra mussels quite rapidly colonized rivers close to the Great Lakes right after their introduction, within a year or two,” Peyer explained. “Quagga mussels were introduced in the Great Lakes around 20 years ago, but they are still not found in the rivers or tend to be present only in low numbers.”

The mussels are similar in many ways. Their habitats overlap, and both are suspension feeders that filter water to extract their food. Yet the cousin species are different in many ways, too. Zebra mussels prefer to attach to a hard surface while quagga mussels can live on soft bottoms, such as sand or silt. Zebra mussels also prefer warmer water temperatures and do not grow as big as quagga mussels.

Peyer’s research focused on the ability of the mussels to attach to underlying material. Both species attach to rocks, sand, silt, or each other by producing tiny but strong “byssal” threads composed of protein strands. These threads act as an adhesive that enable the mussels to attach to surfaces, regardless of how slippery the surface is. Byssal threads are the reason mussels are so difficult to remove from boats or water intake pipes.

Peyer collected both mussel species from Lake Michigan. In the lab, she subjected the mussels to different water velocities that simulated river flow conditions. Her research results supported her hypothesis that zebra mussels are able to produce more byssal threads than quagga mussels do, enabling them to attach more securely to underlying material. This allows them to hang on where water is flowing, such as in a river or stream.

According to Peyer’s research advisor, Prof. Carol Eunmi Lee at the UW-Madison Center of Rapid Evolution, no one has ever looked at differences in attachment between these species as an explanation for their distribution patterns in North America.

Zebra and quagga mussels have permanently changed the Lake Michigan ecosystem. Before the mussels invaded, Lake Michigan water was mostly cloudy, and millions of tiny microorganisms provided a food base for fish. Because the mussels filter the microorganisms, the waters today are surprisingly clear, allowing light to penetrate to greater depths, which in turn promotes prolific nuisance algae blooms. The mussels have also colonized shallow water, beaches, and water intake pipes in layers up to eight inches thick.

“We need to be aware of the distinct differences between the two species,” Peyer said. “If we understand the differences in their biology, we might help to make management more efficient and more effective in the end.”

The results of her UW Sea Grant-funded research were published in the July 1 issue of the Journal of Experimental Biology.

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