Zebra mussels, the infamous invader from the Caspian Sea that has infested the Great Lakes and other water bodies, have finally made their way to the fringes of the Chesapeake Bay.

Maryland environmental officials confirmed in December that a thumbnail-size mussel was found attached to a boat at Glen Cove Marina on the Susquehanna River in Harford County, less than 10 miles from the Bay.

In November, Pennsylvania environmental officials confirmed the discovery of a zebra mussel at the Conowingo Dam in Maryland; the first time Driessena polymorpha had been found in the lower Susquehanna River.

Shortly thereafter, they were also found in Muddy Run, a Susquehanna tributary in Pennsylvania, just north of the state line.

The mussels, named for the distinctive zig-zag pattern of their shells, have thrived in the New York headwaters of the Susquehanna River since being discovered in 2002, and have steadily increased their foothold in that area. Although they had been seen in Pennsylvania's portion of the watershed near the New York border, this was the first time they had been found downstream.

It's unclear whether downstream mussels stemmed from the New York population, or were part of a new introduction. Zebra mussels latch onto solid objects and are easily transported on boats and fishing equipment.

Tom Horvath, a scientist with the State University of New York's College at Oneonta, who has monitored zebra mussels since they were discovered in the Susquehanna headwaters, said populations in New York lakes have "really taken off."

The typical pattern, he said, is for the mussel to invade a lake and rapidly expand its population until it ultimately spills out into the river below the lake, where the creatures often "carpet the bottom" for several hundred yards.

"Then they peter out and you just find them hit-or-miss the rest of the way down," he said. But when the zebra mussels, or their larvae which float with the current, find another lake or slow patch of water, they can produce a new "seed" populations that help infest downstream areas.

"I think the hydroelectric dams in Pennsylvania will start creating new source populations for further seeding of the downstream sites, sort of the hopscotch model," Horvath said.

No one can predict whether the zebra mussels will find much suitable habitat other than in the reservoirs; they typically do not grow as well in fast-flowing streams, although they have proliferated in some larger rivers, such as the Hudson. Large populations could spell trouble for water users, river health and the Bay.

Since arriving in the Great Lakes in the ballast hold of a ship during the 1980s, they have become a major nuisance for industrial and municipal water plants because they attach to water intake pipes, forming dense populations, often thousands per square meter. The U.S. Army Corps of Engineers has estimated economic losses caused by zebra mussels at more than $5 billion from 1993 through 1999.

Zebra mussels were discovered in the tidally influenced areas of the Hudson River in 1991, and populations rapidly exploded in slow-moving parts of the river. They increased the filtering of water from 3 percent to 100 percent a day.

Filtering the water may sound like a good thing, but the problem with zebra mussels is they are too effective at what they do. An individual zebra mussel can filter up to a gallon of water per day while feeding on algae. When thousands of zebra mussels are clustered per square meter, they literally consume everything in sight. On the Hudson, the phytoplankton biomass decreased 80-90 percent.

That reduced food for other aquatic dwellers. Also, with almost no algae left to use nutrients in the water, phosphorus and nitrogen exports from the river increased, studies of the Hudson found. River water clarity increased sharply, but dissolved oxygen levels declined as respiration by the mussels consumed oxygen faster than it could be replaced.

But scientists say it is difficult to predict exactly what will happen here. "Zebra mussels could potentially make that river look like a mountain stream as far as clarity goes, but it would take large numbers of them," said Robert Morgan, a Conservation Planning Biologist with the Pennsylvania Fish and Boat Commission. "If they are just found in sporadic clumps here and there, I don't think they are going to have much impact at all."

Likewise, no one can predict the consequence for the Chesapeake.

Their ability to spread around the Bay may be limited for two reasons. Zebra mussels need to attach to solid substrates, something in short supply in many areas. Also, they are not tolerant of high salinities.

Ron Klauda, a biologist with the Maryland Department of Natural Resources, said their maximum salinity tolerance seems to be between three and six parts per thousand. "There are a lot of areas of the Bay that would not be so vulnerable based on salinity tolerances," he said, "but there are some areas that would be."

Officials say there is little they can do to contain zebra mussels except to educate the public to inspect and clean boats and other equipment that is transported from one water body to another.

"Education is absolutely paramount. That is our number one weapon right now, unfortunately," Morgan said. "People are going to have to adjust. It's a brave new world out there right now, and they're just going to have to get used to doing things differently. You are not going to have the freedom you once had moving between water bodies."