Biologists have discovered giant invasive oysters that could threaten efforts to restore native oyster species in San Francisco Bay.
Government staffers and volunteers removed 256 of the exotic mollusks last week after searching the mudflats between the Dumbarton Bridge and the San Leandro Marina, biologists said Thursday.
Scientists have not identified the species, which grow up to 9 inches long and in a variety of shapes. They don't know how the exotic oysters got here or how they could affect the bay if their population expands.
Biologists are concerned the monster oysters could take over the best habitat and form reefs unsuitable for local fish and invertebrates. They could also threaten the bay's native Olympia oyster, Ostrea conchaphila, which usually grow no more than 2 1/2 inches long.
"We're really concerned about these nonnative oysters out-competing the native oysters," said Abe Doherty at the California State Coastal Conservancy.
A big empty shell of an exotic oyster first turned up more than two years ago near the eastern end of the Dumbarton Bridge. The species wasn't found again until three weeks ago when five large live oysters were discovered in the same area.
Wildlife officials quickly organized an effort to remove the unwelcome mollusks and hope they can eliminate them before they harm the bay ecosystem.
"We have a chance," said Andrew Cohen, who directs an invasive-species program at the San Francisco Estuary Institute. "We don't know if they're actively reproducing in the bay yet.