First discovery of Perkinsus beihaiensis in Mediterranean mussels (Mytilus galloprovincialis) in Tokyo Bay, Japan.

During analyses of the invasive Mediterranean mussel Mytilus galloprovincialis for pathologies in Tokyo Bay, infection by the protozoan parasite Perkinsus beihaiensis was found through histological examination, Ray's Fluid Thioglycollate Medium assays, and molecular analyses. Specific PCR assays for each Perkinsus species also revealed the presence of an indigenous congeneric species, Perkinsus olseni, but P. beihaiensis was dominant in M. galloprovincialis. Sequences of the ribosomal internal transcribed spacer region I of P. beihaiensis found in Japan were genetically more similar to those found in South American countries (Panama and Brazil) than in Asian countries (China and India). Though Mediterranean mussels have become widespread in Japanese waters since their invasion in the 1930s, epidemiological surveys show that mussels collected outside Tokyo Bay are free of any Perkinsus infections. Based on these results, it was strongly suggested that P. beihaiensis invaded Tokyo Bay by transportation of bivalves originating from South America but has not yet spread to other parts of Japan. The possibility is not ruled out, however, that the parasite is indigenous in Japan but the environment in Tokyo Bay favors its transmission to Mediterranean mussels.