National Research Program

Invasion of San Francisco Bay

Introduction of exotic species

Large numbers of sediment-dwelling invertebrates were probably first introduced into San Francisco Bay during latter half of 19th century with live oysters shipped by train from the East coast and transplanted in the bay. Now, exotic invertebrates arrive in ballast water of cargo ships (similar to zebra mussel invasion in the Great Lakes.) Within months of its first detection in October 1986, a clam from China became the most abundant benthic organism in northern part of the bay. An efficient feeder, it has nearly eliminated phytoplankton from the area, causing a decrease in abundance of other species that depend on phytoplankton for food. A new exotic species in San Francisco Bay, the Atlantic green shore crab (introduced to U.S. Atlantic coast from Europe in early 1800's) is a highly predatory species that destroyed the soft-shell clam industry in northern New England in 1950's. Its arrival in San Francisco Bay portends still more change, including possibly one of the West coast's commercially most important species, the Dungeness crab.

On a local scale, the functioning of large ecosystems can be greatly altered by species invasions. On a world-wide scale, modern coastal and estuarine faunas are being homogenized.


Carlton, J.T., Thompson, J.K., Schemel, L.E., and Nichols, F.H., 1990, The remarkable invasion of San Francisco Bay (California, U.S.A) by the Asian clam Potamocorbula amurensis, I. Introduction and dispersal: Marine Ecology Progress Series, v. 66, p. 81-94.

Nichols, F.H., Thompson, J.K., and Schemel, L.E., 1990, The remarkable invasion of San Francisco Bay (California, U.S.A) by the Asian clam Potamocorbula amurensis, II. Displacement of a former community: Marine Ecology Progress Series, v. 66, p. 95-101.

For additional information and references, see San Francisco Bay: Lessons Learned for Managing Coastal Water Resources. Also see USGS San Francisco Bay and Delta or contact Janet Thompson,

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