Genetic relatedness of a new Japanese isolates of Alexandrium ostenfeldii bloom population with global isolates.

•Genetic relatedness of Japanese and global A. ostefeldii isolates was investigated.•Western Japanese isolates group with isolates from low-saline and shallow areas.•Northern Japanese isolates form a separate group explained by geographic isolation.•Low genetic diversity of western Japanese population implies a recent introduction.•Bloom in a shallow coastal lagoon in western Japan after a change in salinity.