Mytilus trossulus, the Pacific blue mussel, bay mussel or foolish mussel, is a medium-sized edible marine bivalve mollusc in the family Mytilidae.

Mytilus trossulus is one of the three principal, closely related taxa in the Mytilus edulis complex of blue mussels, which collectively are widely distributed on the temperate to subarctic coasts the Northern Hemisphere, and often are dominant inhabitants on hard substrates of the intertidal and nearshore habitats.

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