S By Jean Marbella

TILLWATER, 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.

“What makes the St. Croix special is there are 42 species of native mussels here, and those 42 are what have been here since European settlement,” Karns said. “So, we have an obligation to maintain the diversity.”

Karns was among a group of divers from the park service and the U.S. Fish and Wildlife Service who spent a week gauging the spread of the zebra mussel in the St. Croix. In two boats, they cruised down the river to look for the tiny mussels.

They found that the mussels are spreading slowly northward. The divers found the invasive mussels in a bay about five miles south of Stillwater, the furthest north that they have been detected in the three years since the monitoring program began. The zebra mussel density in the St. Croix, however, remains substantially below that of the Mississippi, where some parts have tens of thousands of zebra mussels per square meter.

“Sometimes, in the Mississippi River, you’ll have four or five layers of zebra mussels,” said Scott Yess, a biologist with the Fish and Wildlife Service.

While the divers are concerned when they find zebra mussels, they are particularly worried when they find tiny, young mussels. Finding only one or two adult mussels could mean that they simply fell off a passing boat, but finding younger mussels as well could be a sign that a colony is establishing itself in the area.

“The main thing we’re looking for are signs of reproduction,” said Nick Rowse, a biologist with the Fish and Wildlife Service. “They produce so many offspring, they’ll blanket everything. They’ll suffocate the native mussels.”

Once a colony begins, it can grow rapidly – one female zebra mussel can lay more than a million eggs in a single spawning season. Power plants around the world have spent millions of dollars unclogging pipes where the zebra mussels have settled.

Unlike native freshwater species, zebra mussels have Velcro-like threads that enable them to attach themselves to seemingly anything hard in a river – rocks, boats, pipes, mussels. This ability has allowed the zebra mussels to spread far and wide, hitching barnacle-like onto boats and moving greater distances than they could on their own.

For two years, the park service has restricted boats from traveling up the St. Croix beyond a point near Stillwater to prevent them from transporting zebra mussels. While the restriction has angered boaters, the park service says it has been necessary to contain the spread of the mussels.

Containment is crucial, because once zebra mussels get established in a river or lake, their effect on the ecosystem can be devastating. They compete with native aquatic species for food, upset the food chain and even change oxygen levels.

Zebra mussels, and a related species, quagga mussels, are possible culprits in a mysterious “dead zone” of depleted oxygen that has formed in Lake Erie. The dead zone is caused by excessive phosphorus, which contributes to the growth of algae that in turn drains oxygen from the water. Researchers suspect the mussels because, as part of their feeding process, they take in organic material and expel phosphorus.

While zebra mussels have no major predators in North America, some fish and ducks are known to eat them. A 50 percent decline in one species of duck, the blue bill, may be caused by its consumption of zebra mussels. The mussels are filter feeders, meaning they take in and retain toxins from the water, which are then passed on to ducks that eat them, scientists say.

Researchers are looking into natural predators that could help control the zebra mussel population. Federal and state wildlife agencies in Wisconsin are looking at whether fish in the Lake Winnebago system, particularly the freshwater drum, will eat enough zebra mussels to serve as a natural control.

While zebra mussels can be vulnerable to predators – their tiny size makes them a more manageable meal than larger native mussels – their capacity to breed often more than makes up the difference.

“They reproduce in such great numbers, it’s hard to make a dent in the population,” Rowse said.

“The zebra mussel has brought home how devastating an invasive species can be,” said Dan Stinnett, a field supervisor with the Fish and Wildlife Service. “Everything is connected in a river. That’s what makes the river dynamic. That’s the beauty of the river.”