Fig. 4

The Pacific oyster Crassostrea gigas sperm by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A–D The low magnification of sperm sampled from C. gigas collected in Amursky Bay (A), Eastern Bosphorus Strait (B), Ussuriisky Bay (C) and Vostok Bay (D) by SEM; note the head (h) and flagellum (f) that normally constitute sperm cells. E–H The random projections of the sperm sampled from C. gigas collected in Amursky Bay (E), Eastern Bosphorus Strait (F), Ussuriisky Bay (G) and Vostok Bay (H) by TEM; note the flagellum (f) (E), nucleus (n), mitochondrion (m), acrosome (a) and periacrosomal area (pa) that normally constitute sperm cells (E–H); the arrowhead shows the axial rod that could be occasionally seen in the periacrosomal area at some projections (F). I–K The anterior–posterior sperm section projections showing typical intraspecific variations by TEM. Note the sperms having a two-level acrosome (a) with an apical acrosomal knob (large arrow) (I), the sperm having a two-level acrosome (a) without an apical knob (J), and the sperm with a platelet-like acrosome (a) (K); note the round (I, K) and elongated (J) mitochondria (m); note that the sperm features that are similar in the three images (I–K) include the barrel-shape nucleus (n), the axial rod (arrowheads) located in the periacrosomal areas, a proximal centriole (top black arrows) having a projection that contacts a nucleus in the area of posterior nuclear fossa (white arrows) and a distal centriole (bottom black arrows) acting as a basal body of the flagellum (f). Scale bar—10 µm (A–D), 1 µm (E–K)