San Diego water officials said the small, fast-growing clam, first discovered in the Southern California county during the summer, was found in "significant" numbers at Maramar Reservoir in Scripps Ranch, The San Diego Union-Tribune reported Tuesday. Quaggas apparently were transported from Europe to Lake Erie in the United States in the ballast water of oceangoing ships, the newspaper said.

The mussels grow virtually unmolested because they have no natural predators. They clog pipes and screens at power stations, water treatment plants and agricultural irrigation lines.

San Diego officials will trying to slow their spread by increasing inspections of watercraft used at Miramar Reservoir, said Arian Collins of the city's water department. Other reservoirs in the county also have imposed tighter restrictions.