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Pacific oyster
Magallana gigas

Last edited: January 6th, 2012

Pacific oyster - Magallana gigas

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Short description of Magallana gigas, Pacific oyster

Variable and irregular in appearance.  Off-white to yellow or bluish grey in colour, often with deep purple patches.  Grows up to 30 cm in length with a teardrop shape and rough shell.  The right valve is deeply cupped with six or seven bold ribs; the left valve is flat or slightly convex.

Impact summary: Magallana gigas, Pacific oyster

Once established the Pacific oyster may out-compete and displace native species. It also has the potential to smother or exclude other marine life (including reef-building species) and alter habitat type.

Habitat summary: Magallana gigas, Pacific oyster

Lives permanently attached to any hard substrate in intertidal and shallow subtidal zones of estuaries and coastal waters.  In muddy or sandy areas Pacific oysters will settle on small rocks, shells or other oysters and can create reefs by cementing their shells to each other, forming dense layers.

Overview table

Environment Marine
Species status Non-Native
Native range Kazan-retto, Ogasawara-shoto
Functional type Filter-feeder
Status in England Non-Native
Status in Scotland Non-Native
Status in Wales Non-Native
Location of first record River Blackwater, Essex
Date of first record 1926


Native to Japan and North-East Asia.

First Record

Pacific oysters were deliberately introduced to GB from Canada during the 1960s for commercial purposes.  The first record from the wild was 1965.  

Pathway and Method

Imported into GB from Canada for commercial aquaculture. Natural spatfall now occurs in several GB sites. Some spat settlement in the southwest of England may have come from French stock, with possible vectors including transport by current systems, discarded food waste, transport on ship's hulls and intentional (illegal) introductions.

Species Status

Farmed populations occur throughout England, Scotland, Wales and Ireland, and are widespread in Europe.  It was initially presumed that temperatures in GB waters would not be suitable for Pacific oysters to successfully spread, settle and spawn in GB waters, but escapees have established feral populations in south-east and south-west England and Wales. There are extensive beds of naturally recruited Pacific oysters in some southern estuaries of England and sparse settlements are known from the north coast of Wales near Conwy.

Dispersal Mechanisms

Pacific oyster larvae are planktonic for three to four weeks, during which period they are dispersed by tidal currents; larvae are documented to travel up to 1000 km on ocean currents although such distances are unlikely in GB waters.  Dispersal of larvae may allow new populations to colonise areas too cold for successful reproduction.


Pacific oysters change sex during life, most commonly maturing first as males before subsequently transforming into females.  Spawning is temperature dependant and breeding occurs during summer months at temperatures of around 18 °C.  Each individual may release 50 to 60 million eggs up to a maximum of 100 million eggs although juvenile mortality is high.  Fertilisation takes place externally and larvae are planktonic for three to four weeks before settling; the lower shell valves are cemented onto hard substrate.

Known Predators/Herbivores

Pacific oyster larvae are consumed by filter feeding animals.  Juveniles are eaten by a variety of species including worms, snails, starfish, fishes, birds and crabs. Adults are less vulnerable to predation, but may be preyed upon by birds, starfish and large crustaceans.

Resistant Stages

None known.

Habitat Occupied in GB

Pacific oysters inhabit intertidal and shallow subtidal estuarine and coastal waters, settling on hard substrate.  Where rocky substrate is scarce the oysters settle on any available hard substrate including rocks and other shells, and can form dense reefs by cementing their shells to each other.

Native range Japan and Northeast Asia.  In GB the Pacific oyster is farmed at several locations around GB coasts and estuaries.  Escapees have established populations in estuaries in the south-west and south-east of England, and sparse settlements are known from the north coast of Wales near Conwy.

Environmental Impact

In North America the Pacific oyster is known to settle in dense aggregations, excluding other intertidal species.  In the Dutch Wadden Sea and more recently in the GB the oysters have started to form reefs consisting of dense layers which can alter the natural state of the ecosystem, posing a potential threat to native species and altering habitats, some of which are protected under European law. In the Wadden Sea it has been suggested that these reefs could cause major shifts in benthic filter feeding populations, which could have detrimental knock-on effects on bird populations.

Health and Social Impact

The sharp oyster shells pose a hazard to humans; the formation of reefs on mudflats may render the intertidal zone unsuitable for human leisure activities.

Economic Impact

The Pacific oyster is presently the most widely grown bivalve in aquaculture around the world.  In 2006 1400 tonnes were produced in the GB.  However, where oysters establish wild populations economic losses may occur through the loss of mussel and other bivalve fisheries. 


Miossec, L., Le Deuff, R‐M., and Goulletquer, P. (2009) Alien species alert: Crassostrea gigas (Pacific oyster). ICES Cooperative Research Report No. 299.

Biology, ecology, spread, vectors

Child, A.R., Papageorgiou, P., & Beaumont, A.R. (1995) Pacific oysters Crassostrea gigas (Thunberg) of possible French origin in natural spat in the British Isles. Aquatic Conservation: Marine and Freshwater Ecosystems, 5, 173-177.

Minchin, D. & Gollasch, S. (2008) Crassostrea gigas. Delivering Alien Invasive Species Inventories for Europe. [online] Available from: http://www.europe-aliens.org/pdf/Crassostrea_gigas.pdf

Management and impact

Couzens, G. (2006) The distribution and abundance of the non-native Pacific oyster, Crassostrea gigas, in Devon – a result of climate change? Shellfish News, 22, 5-7.

Guy, C. & Roberts, D. (2010) Can the spread of non-native oysters (Crassostrea gigas) at the early stages of population expansion be managed? Marine Pollution Bulletin, 60, (7), 1059-1064.

Maggs, C., Mineur, F., Bishop, J. & McCollin, T. (2010) Non-natives in MCCIP Annual Report Card 2010-11, MCCIP Science Review, 11pp. Available from: http://www.mccip.org.uk/arc

Miossec, L., and Goulletquer, P. (2007) The Pacific cupped oyster Crassostrea gigas: from an introduced species for aquaculture to an invasive species for the ecosystem. In 5th International Conference on Marine Bioinvasions, 21 – 24 May 2007, Cambridge, MA, Abstract Book. MIT Sea Grant College Program, Cambridge, MA.

Padilla, D.K. (2010) Context-dependant impacts of a non-native ecosystem engineer, the Pacific oyster Crassostrea gigas. Integrative and Comparative Biology, 50, (2), 213-225.

Sewell, J., Lindsley-Leake, S., Tyler-Walters, H. (2010) Non-native species Risk Assessment for the Pacific oyster Crassostrea gigas. A report to GB Non-Native Species Secretariat and Defra.


Minchin, D. & Gollasch, S. (2008) Crassostrea gigas. Delivering Alien Invasive Species Inventories for Europe. [online] Available from:http://www.europe-aliens.org/pdf/Crassostrea_gigas.pdf