Quagga Mussel

According to an Associated Press article, the population of an invasive mussel species in Lake Michigan is declining - but only because a more prolific relative appears to be taking its place.

The quagga mussel (Dreissena bugensis) seems to have nearly annihilated the zebra mussel population, said Russell Cuhel, a senior scientist at the University of Wisconsin-Milwaukee Great Lakes WATER Institute. "Right now, if you go out and suck up 1,000 mussels, you're lucky to find a couple of zebra mussels," he said.

The zebra mussel (Dreissena polymorpha), a native of the Caspian and Black Sea region, was first found in the Great Lakes in the late 1980s. The mussels quickly made their presence known, hogging the plankton upon which so many fish species depend. They also filtered nearshore waters to unnatural clarity that spawned excessive algae outbreaks, and they clogged industrial water intake pipes.

Quaggas have done the same thing on a larger scale, blanketing the lake bottom in many of the deep, cold places that the more delicate zebras can't survive. "Everybody used to say, 'Oh no, zebra mussels!'" Cuhel said. "Well, zebras don't hold a candle to what these guys are going to do to Lake Michigan."

Quaggas were first found in the lake in 1997 and are believed to have invaded the Great Lakes from the Caspian and Black Sea region from contaminated ballast water discharged from overseas freighters. Their shells closely resemble those of zebra mussels but the quaggas are more hardy and prolific. That worries experts who fear the lake could face serious ecological shock.

"With quagga mussels getting in and becoming established, it's a much different situation," said Tom Nalepa, a researcher at the National Oceanic and Atmospheric Administration's Great Lakes Environmental Research Laboratory. "The changes occurring to Lake Michigan are going to be more significant because the mussel biomass is much greater."

A recent survey of 160 sites across Lake Michigan showed a reversal in the invasive mussel population, Nalepa said. In 2000, zebras comprised 98.3 percent of the mussel population. In 2005, quaggas made up 97.7 percent.

The quagga explosion may be responsible for the plummeting population of diporeia, a shrimp-like creature that is an important food source for some fish. The average density of diporeia dropped from 1,836 per square meter five years ago to 293 today. While no direct connection has been established between the rise of quaggas and the decline of diporeia, few doubt there's a link.

Experts say invasive species continue to find their way into the Great Lakes, with a new species discovered about every 6 1/2 months. Most arrive in the bellies of overseas freighters. Legislation to improve control over contaminated ballast water spills has stalled in Congress for more than three years.

For more information:
U.S. Geological Survey

Photo credit: Quagga mussel (Dreissena bugensis), B.May
The Sea Grant Nonindigenous Species Site (SGNIS)