Invasion of The Zebra Mussels

By J. MADELEINE NASH/MONROE

In a B movie, a marauding mollusk would probably be played by a giant clam. But the real-life monster swimming amuck in the Great Lakes is a tiny creature the size of a fingernail. With its jaunty brown stripes, a solitary zebra mussel looks cute, not threatening. The trouble is that the animal is anything but a loner, and its tendency to form colonies of thousands, even millions, makes it threatening indeed.

Unknown in North America until 1988, the zebra mussel has become a pest whose exploding population has prompted alarming predictions of millions of dollars' worth of damage to water-supply systems and the ruination of the sport-fishing industry. A year ago, the city of Monroe, Mich., lost its water supply for two full days because intake lines were plugged with zebra mussels. Earlier, Ford Motor's casting plant in Windsor, Ont., found the creatures choking off the flow of cooling water to its furnaces. Boaters, meanwhile, have watched their hulls and engines become encrusted with mussels.

Native to the Caspian Sea region of the Soviet Union, the zebra mussel spread into the canals, rivers and lakes of Western Europe more than 150 years ago. Then sometime in 1986, biologists speculate, a European cargo ship bound for Sarnia, Ont., emptied some of the water it carried as ballast into Lake St. Clair, between Lake Huron and Lake Erie. Biologists first spotted a few zebra mussels in the area three years ago -- and the race was on.

Every year each female mussel produces approximately 30,000 eggs. When fertilized, these eggs hatch into microscopic larvae that swirl with the current. Eventually the larvae find a surface to their liking and settle down, mooring themselves with sticky, hairlike threads called byssuses. They reach sexual maturity in massive colonies that pack as many mussels into a square meter as there are inhabitants in a midsize town.

Virtually unchallenged by natural predators, billions of zebra-mussel larvae left their initial colonies in Lake St. Clair and Lake Erie and drifted into Lake Ontario. By attaching themselves to boats, some adventuresome mussels even managed to move upstream into Lake Huron, Lake Michigan and Lake Superior. Similar outriders are expected to start showing up in smaller lakes and major rivers such as the Mississippi, the Susquehanna and the Hudson. "Within 20 years," predicts Margaret Dochoda, a biologist with the Great Lakes Fishery Commission, "the zebra mussel will likely have taken the entire East Coast of the U.S."

For utilities and industries, the zebra mussel represents one of the biggest maintenance challenges next to corrosion. Detroit Edison, for example, has spent $500,000 cleaning the critters from the cooling system of its Monroe power plant. "Our plant," says superintendent Sam Smolinski, "has turned into a zebra-mussel nursery. Frankly, we can't fathom things getting any worse."

Foreign organisms have invaded the Great Lakes before, but few have engendered such apprehension. "The zebra mussel is a keystone species," says zoologist David Garton of Ohio State University. "It has the power to restructure the entire ecological community." The zebra mussel can strip water of algae and other microscopic plants and thus endanger animal life. Native clams are beginning to die off, victims of the zebra mussels' habit of attaching to clamshells in such numbers that they cannot open.

Still, with a little luck (and an increase in predation by native fish like the freshwater drum), the zebra mussel may yet be brought under control. In fact, some evidence suggests that the mussel population in Lake Erie may have peaked. "There are many ways to kill the zebra mussel," observes Ohio State entomologist Susan Fisher. "The trick is to do it selectively" without wiping out other aquatic life. Fisher has recently found that minute traces of potassium, nontoxic to other organisms, reliably send zebra mussels into fatal shock. Paints laced with potassium, she speculates, might protect underwater structures from mussel infestation. Physiologist Jeffrey Ram of Wayne State University in Detroit makes an even more devious suggestion. Zebra-mussel spawning, he notes, is triggered by odors wafting from phytoplankton. These chemical cues ensure that the eggs hatch when the food supply is plentiful. But what if synthetic scents were dabbled like perfume above the mussel beds? A premature spawn, says Ram, would surely doom most of the larvae.

"I do not understand this talk of killing," counters biologist Anna Stanczkowska-Piotrowska of Poland's Agricultural-Pedagogical University. The zebra mussel, she points out, is not without virtues. Its byssuses extrude an adhesive that may have commercial value. Its appetite for foul-smelling algae can markedly sweeten the taste of drinking water. Perhaps most admirable of all, the zebra mussel has performed an act of public service by dramatizing the threat posed by tiny organisms that hitch rides around the world. Both the U.S. and Canada are moving to restrict the discharge of ballast water into the Great Lakes, a measure of ecological prudence that is long overdue.