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ABSTRACTThe present study is devoted to a comparison of the structure and quantitative ratio of heterogeneous spermatozoa of native wild mussels Mytilus trossulusGould, 1850, collected in four areas of the Peter the Great Bay of the Sea of Japan (the northwestern part of the Pacific Ocean). The mussels were identified as M. trossulusby shell appearance, and the species was confirmed by sequencing the mitochondrial cytochrome coxidase subunit I gene (COI). Heteromorphic spermatozoa were found; namely, a total of eight morphs (SPERM1–SPERM8) in M. trossulus. Surprisingly, some of the detected sperm morphs overlap morphologically with the sperms of other mytilids such as M. edulis, Crenomytilus(M.) grayanusand M. coruscus. Possible reasons for this phenomenon are discussed. In each geographic area, the ‘quantitative proportions of heterogeneous spermatozoa’ (QPHS) were unique. It has been suggested that the QPHS score can be considered in the context of its applicability as a biological marker for finding optimal mussel rearing sites.