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Andy Cohen previously knew Asian clams were a important factor in the varying ecosystem of San Francisco Bay when he headed out to gather invertebrates on an exceptionally low tide at Port Sonoma a few years ago.

But he didn't have a primitive grasp of the issue until he put on his rubber boots and trudge out on the mudflats, following the receding water.

"I began digging, and the clams were really thick, packed together like walnuts," said Cohen, an environmental scientist with the San Francisco Estuary Institute who specializes in enveloping species.