Gametogenesis of one year-old induced triploid mussels, Mytilus galloprovincialis, was examined histologically and compared to sibling diploid mussels. Histological analysis revealed that triploid mussels developed a number of primary spermatocytes that were arrested at prophase I. Late in the reproductive season, triploid mussels produced an extremely small number of spermatozoa (9/10000 μm2 gonadal section) compared to diploid mussels (1072/10000 μm2 gonadal section). All triploid mussels were identified as males, whereas the sex ratio of diploid mussels was almost equal (1.12:1.0, male:female), indicating that sex determination for this species may follow a Z:W model. Sertoli cells in triploid mussels were prominent, had an enlarged cytoplasm, and were easily seen using light microscopy. In comparison, Sertoli cells in diploids were thin and could only be seen by electron microscopy. Sertoli cell hypertrophy in triploid mussels probably reflects their role in eliminating abnormal and degenerating germ cells.
