Snails Threaten Native Oysters in Tomales Bay

Researchers:

Edwin D. Grosholz
Department of Environmental Science and Policy
University of California, Davis
E.: tedgrosholz@ucdavis.edu
T.: (530) 752-9151

David Kimbro
Florida State University Coastal and Marine Laboratory
St. Teresa, Florida
E.: dkimbro@bio.fsu.edu
T.: (850) 697-4092

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Revised:

August 25, 2009

Atlantic oyster drill. Credit: Andrew N. Cohen/SFEI

August 25, 2009

Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334

TOMALES BAY – Voracious alien snails are devouring California’s only native oyster in Tomales Bay along Point Reyes National Seashore.

Half of the pristine coastal estuary’s Olympia oysters have fallen prey to an exotic whelk snail, known as the Atlantic oyster drill, according to a recently published California Sea Grant-funded study in the journal Oecologia.

Olympia oysters on a recruitment collector in Tomales Bay. Credit: Chela Zabin/UC Davis

The predator, introduced decades ago with Atlantic oysters, drills through the top of an oyster’s shell, digesting the soft tissue inside.

Native crabs keep the exotic snail’s numbers in check in saline parts of the estuary. The enforcer red rock crabs, however, cannot hold the line against the invasive snail in prime oyster habitat near the bay’s head, where water is much fresher.

“For Tomales Bay, this means that nearly half the habitat is inhospitable to re-establishing native oysters,” says the article's lead author David Kimbro, a postdoctoral associate at Florida State University in Tallahassee and a Sea Grant trainee at UC Davis at the project's onset. UC Davis researcher Ted Grosholz is a co-author of the article.

To avoid oyster predation by the snails, scientists recommend rebuilding oyster beds in the estuary’s center, where there is also an abundance of phytoplankton (food).

Tomales Bay field site. Credit: Anna Deck/UC Davis

“The center of the bay receives optimal tidal currents for phytoplankton growth,” explains Grosholz, who is a co-author with Kimbro and physical oceanographer John Largier of UC Davis of an article on the subject to appear in Limnology and Oceanography. “Currents are strong enough to inject nutrients but not so strong they sweep away phytoplankton.”

UC Davis graduate student Anna Deck has also found that "space competitors" from fouling organisms such as tunicates and barnacles are likely not affecting oyster survival rates in the bay.

There is also mounting evidence that hard substrate may not be a factor limiting native oyster recruitment, meaning that future re-establishment efforts may require captive rearing of native oysters for release in the wild.