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They cut your feet, slice your hands, clog industrial pipes and foul water-treatment plants.

But diver Don Martin says the freshwater invasion of zebra mussels has done wonders for treasure-hunters trying to find sunken wrecks in the St. Lawrence River."The clarity of the water since they came here a few years ago has become simply phenomenal. We now get as high as 100 feet vertical visibility when 4 feet used to be common," said Martin, who operates Scuba Adventures.

Biologists and industrial managers hardly see it that way. From Quebec to Oklahoma, from Minnesota to the mouth of the Mississippi, this black-and-white striped mollusk the size of a thumbnail is laying siege to rivers, lakes, reservoirs, power and chemical plants and steel mills.

Unlike scuba divers, who love zebra mussels because each one filters a quart of water a day, greatly increasing the clarity of whatever water it's in, biologists liken zebra mussels to the fire ant for its negative impact. Others simply consider it a cancer.

Zebra mussels reproduce so quickly and so prolifically that tens of thousands soon clog water-intake systems and municipal water-treatment plants if left untreated.

Swimmers and scuba divers, beware, too: Their sharp edges will cut hands and feet, even protective diving suits, like a razor.

Four-foot-high piles of zebra mussels are not uncommon at affected plants; parts of Lake Erie are covered 2 feet deep with them; huge clumps of mussels, with rotting meat, sometimes wash up in a stinking mess on beaches.

"They're kind of amazing animals," said biologist Cliff Kraft, a zebra mussel specialist in Green Bay, Wis. "There is nothing else like them in fresh water. Zebra mussels establish a new mark for species."

Scientists believe a Russian freighter first dumped larvae of the zebra mussel, a native of the Caspian Sea region, into the Great Lakes in 1986 when it cleaned its ballast tanks. Full-grown zebra mussels were found two years later in Lake St. Clair near Detroit.

U.S. and Canadian laws now ban bilge-water dumping to combat the introduction of aquatic species from ships carrying ballast from foreign waters.

Ships entering the St. Lawrence and Hudson rivers must now dump their ballast 200 miles offshore. Sen. John Glenn of Ohio is sponsoring a bill to expand the National Invasive Species Act to all saltwater and freshwater U.S. ports.

Most biologists believe it's too late to rid North America of zebra mussels.