Mytilicola orentalis (known as 'oyster redworm' or 'red worm disease') is an intestinal copepod parasite of bivalves with a direct life cycle. It is native to the waters around Japan and was first described in the Sea of Japan and was introduced to Europe in the 1960s and 70's with oyster imports. Since then it has also been observed in the Wadden and the Baltic Sea. It has a wide range of host species in both its native range and in Europe.

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