Distribution of native Mytilus trossulus and non-native M. galloprovincialis (Mytilidae: Bivalvia) along the coast of Hokkaido Island, Japan

publisher奈良The distribution of two species of mytilid mussels, native Mytilus trossulus Gould, 1850, and non-native M. galloprovincialis Lamarck, 1819, was studied at 120 sites (83 harbors and 37 shores) along the coast of Hokkaido Island and on three small islands (Okushiri, Rishiri and Rebun Islands) in the Sea of Japan. M. trossulus occurred at 28 harbors and on one shore, all of which were located in eastern and southern Hokkaido: coastal areas of the Sea of Okhotsk, the Nemuro Strait and the Pacific Ocean. It was never found on the Sea of Japan coast including the three islands. M. galloprovincialis was found at 70 harbors and on three rocky shores all around Hokkaido and the three islands, predominating on the Sea of Japan coast. Additionally, mussels with inner shell morphology intermediate between those of the two species were found at 26 harbors and on one rocky shore almost only in eastern and southern Hokkaido, the distribution being very similar to that of M. trossulus. The relative abundance of M. trossulus individuals among these three mussel types was greater on the Nemuro Strait coast and eastern Pacific coast and tended to decrease westward. The relative abundance of M. galloprovincialis individuals among the three mussel types was 100% at almost all harbors along the Sea of Japan coast and tended to decrease eastward. Salinity and the scale of shipping traffic at the harbors did not show any relation to the occurrence and relative abundance of the three mussel types