Crassostrea gigas
Phylum: Mollusca
Class: Pelecypoda
Order: Ostreoida
Species name: Crassostrea gigas (Thunberg 1793)
Synonyms: Crassostrea angulata (Lamarck 1819)
Common name: Pacific oyster, Portuguese oyster
 
Date of introduction and origin
Crassostrea gigas was first introduced from Portugal into the River Blackwater, Essex, in 1926 (Utting & Spencer 1992). This colony was thought to have died out in 1965 because importations had ceased in 1962, but there was still a substantial population in the River Blackwater in 1970 (R. Mitchell pers. comm.). It was re-introduced in 1965 to Conwy, North Wales (MAFF quarantine) from the USA and British Columbia (Walne 1971; Walne & Helm 1979). This species occurs naturally in Japan and south-east Asia.
 
Method of introduction
It arrived through deliberate commercial introduction.
 
Reasons for success
It is most successful in commercial cultivation of hatchery-produced seed; over 30 years of extensive cultivation has provided opportunities for establishment in the wild.
 
Rate of spread and methods involved
It spreads through placement of hatchery-produced seed. Furthermore, local spatfall occurred in the River Blackwater, Essex, and light spatfalls registered in some estuaries of south-west England (Dart, Teign and Exe) and the Menai Strait following unusually warm summers in 1989 and 1990 (Spencer et al. 1994). Genetic evidence shows that spatfall in the River Teign originated from French stock (Child, Papageorgiou & Beaumont 1995), although it is unclear whether this was from adult specimens discarded at English sites or from larvae that crossed from the French side of the channel. Should the latter be the case, it would appear that, under favourable conditions for larval development, Crassostrea gigas has the capacity to spread substantial distances.
 
Distribution
It is distributed throughout England, Scotland, Wales and Ireland, and widely in Europe (France, Belgium, The Netherlands, Germany, Spain, Portugal, Denmark and Norway). Fisheries are sustained by natural spatfalls in The Netherlands and France.
 
Factors likely to influence spread and distribution
High temperatures are required for spawning and for larval development. Food required for developing larvae, and the presence of predators, especially shore crabs, can influence its spread (S.D. Utting pers. comm.).
 
Effects on the environment
No effects are recognised in Europe. In North America it has been known to settle in dense aggregations, excluding other intertidal species.
 
Effects on commercial interests
This species is cultivated widely as it is eaten.
 
Control methods used and effectiveness
None used.
 
Beneficial effects
Its presence benefits commercial oyster farming interests.
 
Comments
Crassostrea gigas and Crassostrea angulata are thought to be the same species and have been treated as such here (see e.g. Smith, Heppell & Picton in prep.). The only remaining population referred to as 'angulata' in Britain is a brood stock kept by MAFF in the Menai Strait. Populations of adult Pacific oysters may persist for years. Crassostrea gigas from a disused oyster farm at Tighavullin, Scotland, were observed in 1993, nine years after the farm was shut down, though no young were observed (Smith 1994).
 
References
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.
 
Hayward, P.J., & Ryland, J.S. eds.,1990. The marine fauna of the British Isles and north-west Europe. 2 vols. Oxford, Clarendon Press.
 
Smith, S.M. 1994. Conchological Society expedition to the Oban area, 18-22 August 1993. Conchologists' Newsletter, No. 7: 361-375.
 
Smith, S.M., Heppell, D., & Picton B.E. In prep. Mollusca. In: Marine species directory, ed. by C.M. Howson & B.E. Picton, 2nd ed.
 
Spencer, B.E., Edwards, D.B., Kaiser, M.J., & Richardson C.A. 1994. Spatfalls of the non-native Pacific oyster (Crassostrea gigas) in British waters. Aquatic conservation: Marine and Freshwater Ecosystems, 4: 203-217.
 
Utting, S.D., & Spencer, B.E. 1992. Introductions of bivalve molluscs into the United Kingdom for commercial culture - case histories. ICES Marine Science Symposium, 194: 84-91.
 
Walne, P.R. 1971. Introduction of Crassostrea gigas into the United Kingdom. Lowestoft, Ministry of Agriculture, Fisheries, and Food. (Shellfish Information Leaflet, No 21.)
 
Walne, P.R., & Helm, M.M. 1979. Introduction of Crassostrea gigas into the United Kingdom. In: Exotic species in mariculture, ed. by R. Mann, 83-105, Cambridge, Massachusetts, MIT Press.
 
Acknowledgements (contributions from questionnaire)
Dr S.D. Utting, Ministry of Agriculture, Fisheries and Food, Conwy.