Topic: Pacific oyster

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    Left: Pacific oyster (Crassosstrea gigas) – Right: European flat oyster (Ostrea edulis). 

    Photo: Torjan Bodvin / Institute of Marine Research
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    Mapping the Pacific oyster (Crassostrea gigas) in Aust-Agder, Norway.

    Photo: Espen Bierud / IMR
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    Field work at Hui, Norway.

    Photo: Espen Bierud / IMR

The Pacific oyster is easily differentiated from the indigenous European flat oyster by its shape and shell structure. The European flat oyster has a rounded flat shell with a fine, “flaky” surface. The Pacific oyster is usually longer, has a cupped lower shell and a rough surface, often with radial brownish or violet lines.

The Pacific oyster originates from South-East Asia. It has been introduced to many areas outside its native range as an aquaculture animal. The Pacific oyster is robust and adaptive and has established self sustaining populations in several areas where it has been introduced. It settles mainly in the littoral zone and tolerates periods out of the water at low tide. Dense populations can form oyster banks or reefs, changing the ecosystems. Such dense populations have become established in the Netherlands, Germany and the Danish Wadden Sea. The Pacific oyster is considered invasive, and further spread is not desired.

The Pacific oyster in Norway

The pacific oyster is spreading in Scandinavia. Scientists in Norway, Sweden and Denmark are collaborating to describe the spreading and the effects on the environment. It has been present in Denmark for some time. Since 2006, Pacific oysters have been found at numerous coastal sites along the Swedish west coast, the south coast of Norway and at some sites north to Hordaland County. The Institute of Marine Research needs information from the public, in order to map the distribution, density of populations and the dynamic of the spreading.