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Invasive species threaten oysters in Tomales Bay

July 20, 2009 |  8:00 am

The native Olympia oyster population in Tomales Bay is under siege by invasive snails, and it looks like the culprit is higher up the food chain.

Greencrab Invasive European crabs, brought to the Bay Area estuary accidentally many years ago, aren't effectively killing and eating invasive Atlantic Coast snails, according to a study published last month by scientists at UC Davis and the Bodega Marine Laboratory. Since snails feed on oysters, an increased snail population spells bad news for the bivalves.

While native crabs use a skillful can-opener-like maneuver to get at the snails, the invasive crabs can only kill them using brute force, a less effective tactic, researchers say. To make matters worse, the invasive snails aren't afraid of the crabs, since they have no vicious predators in their natural environment.

Native crabs, on the other hand, have kept the native snail population under control in part by "scaring the bejesus out of them," said David Kimbro, the study's lead researcher.

Invasive species, it seems, just don't quite know how to coexist.

"They just don't have the evolved history that the natives do," said Kimbro. "As a result, they're just sort of fumbling along."

Kimbro said that along the outer regions of the estuary, native crabs still control the native snail population, and the oyster population is "healthy." But in the rest of the estuary, where there are fewer native crabs, there are also fewer oysters.

Oysters are vital to the health of Tomales Bay, Kimbro said, because they filter water and create nooks and crannies in which other invertebrates can burrow.

The study was published in the journal Oecologia.

-- Amy Littlefield

Photo: A marine biologist holds a European green crab, an invasive species that is less effective at killing invasive snails than the native crabs in Tomales Bay. Credit: Robert Durell/Los Angeles Times