It is difficult to distinguish between any species within Mytilus on shell shape alone. Identification is compounded by the ability of many of the species to hybridize.
Mytilus galloprovincialis is often confused with M. edulis although M. galloprovincialis can grow larger than M. edulis. As recently as 1955, M. galloprovincialis was considered a subspecies and until 1968, it was considered a warm water derivative of M. edulis. Because of the confusion the introduction status of the Mediterranean mussel M. galloprovincialis was long overlooked in California due to its previous identification as the “native” Mytilus edulis (Carlton 1992).
Mytilus galloprovincialis is often confused with M. trossulus and it can be hard to distinguish them by shape as mussel shells vary by environment. These two species can also hybridize making identification of species almost impossible visually.