STILLWATER, Minn. - In the growing annals of invasive species, the fingernail-size zebra mussel hardly seems as fearsome as others that have recently made headlines - the bighead carp that have been leaping onto fishing boats in the Mississippi River, the "Frankenfish" snakehead that ate a Maryland pond.
But what the zebra mussel lacks in style, it makes up for in destructive ability: Tiny but prolific, they can quickly take over a body of water, clogging power plant intake pipes, stealing food and oxygen from other species and even suffocating the native mussels that they attach themselves to and eventually encrust.
In the Mississippi, they've pushed a species of native mussel closer to extinction. In Lake St. Clair on the Michigan-Ontario border, they might be poisoning a kind of duck known to eat them. And in Lake Erie, they are suspected of contributing to an oxygen-deprived "dead zone" where no fish can survive.
Native to the Caspian Sea, zebra mussels were detected in North America in 1988, apparently having made their way over in the ballast tanks of oceangoing freighters. Within a few years, the species had established itself throughout the Great Lakes and the Mississippi River and its tributaries, and now is found in waters from Minnesota to Louisiana. Zebra mussels have been found in the upper reaches of the Chesapeake Bay watershed, in parts of the Susquehanna River in New York, raising fears that they could pose a threat to municipal water systems and power facilities in Maryland such as the Conowingo Dam.
The St. Croix River, whose waters are glassily still as it passes through this small town, is the latest battleground in the war against the invading mussel, and where those fighting its spread hope to draw the line.
"At this point, it's impossible to eradicate them," said Byron Karns, a biological science technician with the National Park Service. "All we can hope to do is contain them."
In the St. Croix, zebra mussels are largely found in the southernmost stretch, close to where it meets the Mississippi River, which has long been plagued by the invasive species.
Wildlife agencies are desperate to protect the St. Croix - the first to be named a Wild and Scenic River under a 1968 congressional act designed to preserve the nation's most beautiful waterways - from a similar fate. The river is the last stand of an endangered native mussel, the winged mapleleaf, that once thrived in the waters of a dozen states but now has vanished everywhere but in the St. Croix.
