Crassostrea gigas - Pacific oyster, Japanese oyster, Giant Pacific oyster
Phylum- Mollusca 

Class- Bivalvia

Order- Ostreoida Family- Ostreidae
Description- up to 10” (25cm) long; grayish-white to brown in color sometimes with purple markings, narrow and irregularly shaped shells, highly textured with fluted edges; right shell flattened, left shell rounded; found low intertidally and subtidally in wave sheltered areas, usually attached to a firm substrata such as rock, debris, or other oyster shells, sometimes found in the mud.

Possible vectors- Deliberate transplantation and cultivation as a food product, Ship fouling, Ballast water,

Ecology & life history- Crassostrea inhabits estuaries between 10-35 psu salinity and -1.8-35°C water temperature (FAO 2009). Oysters are filter feeders that attach to firm substrates between the mid-intertidal to shallow subtidal. Pacific oysters are protandrous hermaphrodites; they begin life as males and change to females later in life. Female Pacific Oysters can produce 50 to 200 million eggs in a single season (Quayle 1988, FAO 2009). Larvae develop in estuarine waters and preferentially select substrates to adhere to.

Invasion history- Crassostrea is an important commercial food product and has been deliberately introduced to many bays on the Pacific coast and in many other areas of the world. Pacific oysters were first introduced to the Pacific coast in 1900's (Perry and Cirino 2000) and are cultivated in several Oregon bays. Populations cultivated in Pacific coast bays are sterile (triploid) and are not supposed to be capable of reproduction and spread. However, some juvenile individuals have been found adjacent to Oyster aquaculture facilities (per. obs.); it is possible these juveniles came from cultivated populations or perhaps through another vector.

Threats/implications- May compete with native species for food and space, high densities may disrupt eelgrass communities, transplantations of Pacific oysters resulted in numerous species introductions (Cartlon 1979). Pacific oysters may also cause ecosystem level impacts through the formation of extensive reefs that can reduce oxygen levels in the sediment, increase organic matter, and provide a hard complex substrate that other species including other non-native species may exploit (Buhle 2006).

Management suggestions- Oysters are private property and an important commercial fishery. Control of adult populations is not desired or encouraged in Oregon bays. Unintentional spread and establishment of Crassostrea can be prevented by proper treatment of ballast water.

Page created by Timothy M. Davidson, Questions:, Pictures courtesy of Timothy M. Davidson

Crassostrea gigas
Crassostrea gigas- note the slight purple coloration
Oysters colonizing rocks on a mudflat

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Literature cited:

Food and Agriculture Organization of the United Nations (2009) Fisheries and Aquaculture Department homepage. In: FAO Fisheries and Aquaculture Department [online]. Rome. Updated 6 October 2003. [Cited 26 March 2009].

Perry HM and Cirino JD. (2000). Biology of certain commercial mollusk species: Oysters. In: Marine & freshwater products handbook. Ed: RE Martin, E Paine Carter, GJ Flick, Jr., LM Davis. Technomic Publishing Inc., Lancaster, Pennsylvania. 963 pp

Quayle DB (1988) Pacific oyster culture in British Columbia. Canadian Bull. of Fish. and Aquatic Sciences No. 218: 241 pp