shoreline. Every one had a striped shell growing on it. "I thought `What is this?' '' Mazur said Monday. "At first I thought, `Are they snails?' Then I saw what they were.'' This fall, the future everyone feared for Lake Lillinonah -- a full-bore infestation of zebra mussels -- has begun to play out. These mussels -- a non-native invasive species -- can multiply by the millions. They can cover docks and boats and rocks with their tiny sharp shells. They can clog filtration pipes. And they can severely alter a lake's ecosystem. They act as a powerful filter that takes out phytoplankton, making the water clear, and deprives other species of food. They also crowd out native shellfish. "I think in one to five years, we'll see them expand very rapidly," said Greg Bollard , another leader of Friends of the Lake. "They're here." Ray Hoesten of Newtown, chairman of the Lake Zoar Authority , said people are finding them in Zoar as well. "Not as many as in Lillinonah," Hoesten said. "But we're finding some." Ethan Nadeau , the owner of Biodrawversity, the Massachusetts-based research company that found the first zebra mussels in Lillinonah and Zoar two years ago, said he's now found adult zebra mussels along the Housatonic River, as far north in Connecticut at Canaan, and as far south as Lake Housatonic, the impoundment behind the Derby Dam. So far, however, they have not been found in Candlewood Lake, the largest of the three manmade lakes