Fig. 7

Spermatozoa of the bivalve mollusc Mactra chinensis by scanning electron microscopy (A–C) and transmission electron microscopy (D–I). Spermatozoa of M. chinensis collected in the Sea of Japan, Russia (A), Sea of Japan, South Korea (B), Yellow Sea, China (C) with a small magnification of an electron scanning microscope; note the head (h) and flagellum (f) that normally constitute sperm cells. The anterior–posterior sperm section projections showing typical intraspecific variations by transmission electron microscopy (D–I). Note the spermatozoa, which has an acrosome, consisting of two layers, one of which (anterior) is electron-light (arrowheads), and the other (basal) is electron-dense (black arrows), that was found in samples collected in The Sea of Japan, Russia (D), The Sea of Japan, South Korea (E), Yellow Sea, China (F). Note the spermatozoa having an acrosomes with central electron-dense rod that have intracellular part inside of the acrosome (circles), and external part protruding above the acrosome (arrowheads), that were also found in samples collected in The Sea of Japan, Russia (G), The Sea of Japan, South Korea (H), Yellow Sea, China (I). m mitochondrion; n nucleus; f flagellum; the centrioles are showed by white arrows. Scale bar—5 µm (A–C), 0.5 µm (D–I)