LAKE TEXOMA, TEX. - In a cove on this massive lake on the Texas-Oklahoma border, it doesn't take long to find zebra mussels.

Just a few feet from shore, a large rock is covered with them. A quick scan of the shoreline turns up many more.

This invasive species, an import from Eastern Europe, has long been spreading across the United States. The Great Lakes have been dealing with it for years, and now it has reached Texas.

So far, it has been found only in Lake Texoma and Sister Grove Creek, a tributary that the North Texas Municipal Water District uses to send water from Texoma downstream to Lake Lavon.

"The best way to avoid introduction is to prevent it from ever getting there in the first place - of course, the consequences once it gets established are pretty great," said Robert McMahon, a professor emeritus in the University of Texas at Arlington department of biology who has studied zebra mussels since 1990.

The fear is the resilient mollusks and their larvae will be washed downstream into Lake Lavon, where they could spread throughout the Trinity River basin, creating havoc with pipelines and intake valves and threatening native species.

While scientists have tried to kill them off in Sister Grove Creek, the North Texas water district has stopped pumping for the past 18 months from Lake Texoma. Biologists have been successful in killing off most of them.

What makes the task more daunting is that there is another, perhaps more likely way for zebra mussels to spread: via boats that are hauled from infested lakes. That is what is thought to have happened at Lake Texoma.

"My feeling is it's inevitable we're going to see them here," said David Marshall, engineering services director of the Tarrant Regional Water District. "The only question is, will it be in two years or 10 years?"

Scientists can't say for sure what will happen if they get established in Texas, but the evidence in other states isn't pretty. Zebra mussels have changed the ecosystems of lakes and ruined beaches for swimmers by depositing razor-sharp shells along the shoreline.

For those who maintain reservoirs, such as the U.S. Army of Corps of Engineers and the Tarrant Regional Water District, there is concern that they could choke intake valves and screens. That could force agencies to spend untold dollars retrofitting old systems with copper screens, creating regular scraping programs and pumping higher doses of chlorine through pipelines to prevent clogging by the mussels.

"It is a difficult challenge because they are so resilient," said Randy Cephus of the Corps of Engineers' Fort Worth District. "That's why preventive measures are so vitally important. Once they get a toehold, they can multiply very rapidly. They're very hard to get rid of."

The corps has been conducting boater education at Texas lakes and fishing tournaments about the need to thoroughly clean vessels when moving from lakes.

The mussels, which first showed up in the Great Lakes region in the 1980s, are believed to have been brought to North America from the release of ballast water from a single ship that came from the Black Sea, according to the U.S. Geological Service.

They are in the Mississippi River basin all the way to the Gulf Coast as well as 611 lakes in 26 states, including Arkansas and Louisiana, according to the service. In Oklahoma, they have been found in 10 lakes in addition to Lake Texoma.

One North Texas lake has already had a close call. At Eagle Mountain Lake last year, a pontoon boat covered in zebra mussels that was hauled from Lake Texoma was discovered before it got into the water. It was scrubbed clean, but it still had old rubber tires, used as bumpers, that could provide the perfect habitat for mussel larvae, said Bruce Hysmith, a Texas Parks and Wildlife inland fisheries biologist who worked on the eradication efforts at Sister Grove Creek.

"I'm telling you, despite all of the efforts, it only takes one infested boat to get them into a lake," Hysmith said, although there is some debate how many larvae it takes to get a colony established.

The mussels do improve water clarity, but that robs lakes of nutrients, experts say.

If they become numerous, they could compete with the fry of sport fish, really all fish, and be a competitor for all of that plankton, Hysmith said. Tarrant Regional officials echo those concerns.

"My biggest concern would be for the native species," Marshall said. "They outcompete everything. For native clams and mussels, this is just going to provide so much competition. We may see a loss of species in our Texas lakes."

- Fort Worth Star-Telegram