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?Force the overhaul of water facilities and cause damage to fisheries that one expert estimates could amount to $4 billion or more in the Great Lakes region alone.
"The zebra mussel has caught North America completely off guard," says Don Schloesser, a biologist at the U.S. Fish and Wildlife Service National Fisheries Research Center in Ann Arbor, Mich. "Last year we thought it would take these mussels five years to reach all the Great Lakes, but in less than a year we've found them in every one of the lakes." Indeed, the spread of this pernicious Caspian Sea shellfish beyond the waters of the northern U.S. and the southern half of Canada, where it is already wreaking havoc, is not a question of if, but when.
Zebra mussels grow to no more than 1� inches in length, but they make up in numbers what they lack in size. Billions, probably trillions, of them are now in Lake Erie, so far the most seriously affected of the Great Lakes. Like most marine mussels, the zebra uses its beard (a cluster of as many as 100 or more elastic threads) to attach itself to any firm surface—including the shell of another mussel. In a matter of weeks, they can form colonies that look like gobs of Cracker Jack. As the colonies grow, their weight can sink navigational floats. Zebra mussels can severely inhibit the flow of water to factories, power plants and water utilities. They clog the cooling systems of both inboard and outboard engines. They coat pilings, the hulls of ships, concrete walls, anchor chains. They can form a living barrier around the rocks and limestone reefs that game fish such as walleyes need for spawning.
Zebra mussels attach themselves to the eyes and pincers of crayfish, causing this favorite form of forage for bass, trout and other game fish to starve to death. These immigrant bivalves kill native unionid mussels—a key element of the freshwater food web—by latching onto the natives so that they cannot open their shells and therefore smother.
Schloesser has counted 10,000 tiny zebra mussels stacked on a single 2�-inch-long unionid mussel. "We don't know what the zebra mussel means to the unionid mussels," he says, "but we believe it's going to wipe them out in some habitats."
Says Alan Greenberg, chief of water purification for the city of Cleveland, which draws 320 million gallons of water a day from Lake Erie: "Ordinarily, we get native mussels on our filter screens, but we haven't isolated one in a year. They're goodbye, gone, wiped out, in my estimate. The same goes for snails. We haven't seen anything normal for a year. All we get is zebra mussels."
The zebra mussel, scientifically known as Dreissena polymorpha, is one of many highly peculiar creatures that hail from the Caspian, the largest enclosed body of water in the world that is considered saline. But because it has no links to other bodies of salt water, the influx of fresh water, particularly from the Volga and Ural rivers, creates near-freshwater conditions in parts of the Caspian. Over the centuries, a number of its indigenous marine animals, including the zebra mussel, have adapted to freshwater conditions. However, the zebra mussel has retained two characteristics reflecting its saltwater origin. First, it is tremendously fecund, typical of many marine bivalves. Second, unlike other freshwater mussels, the zebra mussel retains its beard when sexually mature, which enabled members of the clan to hitchhike from the Caspian to colonize new waters.
Some 200 years ago, zebra mussels began moving up the Volga, presumably by attaching themselves to the hulls of vessels. The bivalves eventually made their way to the Baltic Sea, and by the mid-1880s they had become a nuisance across Europe, jamming the water supply pipes in Hamburg and Rotterdam. In 1895 the decaying meats of dead mussels made such a stink in Berlin that the city's waterworks had to be shut down for 27 days until a new water supply could be located. As a result, Europeans began building water systems with intake pipes buried in sand to keep out the bivalve. Backwashing capability was also built into water systems so that the mussels could be periodically flushed out. There was no reason to go to such expensive precautions in North America, because the zebra mussel was unknown here.
Now the zebra mussel has made the transatlantic migration. Dr. Jon Stanley, the director of the U.S. Fish and Wildlife Service National Fisheries Research Center in Ann Arbor, calculates that over the next 10 years the redesign, retrofitting and maintenance of water-supply facilities alone will cost Great Lakes municipalities, manufacturers and electric utilities $1.2 billion. On top of that, he projects the cost to shipping, pleasure boating and fisheries in the Great Lakes to be nearly $3 billion more over the next decade.