Mytilicola orentalis (known as 'oyster redworm'[3] or 'red worm disease') is an intestinal copepod parasite of bivalves with a direct life cycle.

[5] Mytilicola orentalis originates in the Japanese Sea[4] and is known to occur on the Pacific coast of the United States and Europe.

[8] Since its introduction in Europe it has been observed across European waters, including locations in France, Ireland, the United Kingdom and the Netherlands.

These include, but are not limited to, Barnea dilatata japonica,[9] Barnea manilensis,[9] the blue mussels,[4] the Mediterranean mussel,[2] the pacific oyster,[4] the European flat oyster,[2] and the Japanese carpet shell.

[4] Mytilicola orentalis infection in pacific oysters correlates with the co-infection with Polydora sp.