Sperm ultrastructure in Hemidonax pictus (Hemidonacidae, Bivalvia, Mollusca): comparison with other heterodonts, especially Cardiidae, Donacidae and Crassatelloidea Author Healy, John M. Author Mikkelsen, Paula M. Author Bieler, Rüdiger text Zoological Journal of the Linnean Society 2008 2008-06-30 153 2 325 347 https://academic.oup.com/zoolinnean/article-lookup/doi/10.1111/j.1096-3642.2008.00385.x journal article 10.1111/j.1096-3642.2008.00385.x 0024-4082 5447636 FRAGUM UNEDO ( FIG. 3 ) Acrosomal complex The acrosomal vesicle is short (0.28 ± 0.02 Mm, N = 3), broadly conical, with a maximum diameter of 0.7 ± 0.05 Mm ( N = 3) (at mid-length level) ( Fig. 3A– C ). It rests on the membranes of the flattened and slightly inclined nuclear apex. Contents of the vesicle are differentiated into a highly electron-dense basal ring (lower, outer portion of the vesicle) and less dense material (lower, inner portion of the vesicle and anteriorly) ( Fig. 3C ). Nucleus The nucleus is short (2.38 ± 0.05 Mm, N = 3), barrel-shaped (maximum diameter mid-nucleus of 1.35 ± 0.05 Mm, N = 3) but slightly curved or twisted ( Fig. 3A, B ). Contents are highly electron-dense with the exception of scattered, electron-lucent lacunae ( Fig. 3A, D ). Apically the nuclear surface is flat and inclined due, in part, to nuclear curvature, whereas · Figure 2. Ultrastructure (TEM) of spermatozoa of Vasticardium vertebratum . A, longitudinal section (LS) showing acrosomal complex, curved nucleus, portion of midpiece region. Note also several flagellae cut in transverse section (TS). B, LS of acrosomal complex (acrosomal vesicle + subacrosomal material) and nuclear apex. The degree of anterior dimpling is variable. C, TS at mid-level region of acrosomal complex. D, E, LS at base of nucleus, entire midpiece and proximal portion of flagellum. The proximal centriole shown in LS in D, in TS in E. Putative glycogen granules are also visible. F, TS of midpiece showing four mitochondria, distal centriole and putative glycogen granules. Abbreviations: a, acrosomal complex; av, acrosomal vesicle; ax, axoneme; br, basal ring (of acrosomal vesicle contents); dc, distal centriole; f, flagellum; g, putative glycogen granules; m, mitochondrion of midpiece; n, nucleus; nl, nuclear lacuna; pc, proximal centriole; pm, plasma membrane; sf, satellite fibres; sm, subacrosomal material. basally, very shallow indentations are associated with the midpiece mitochondria and proximal centriole ( Fig. 3A ). Midpiece The midpiece contains four spherical mitochondria grouped around the centriolar pair (proximal and distal centrioles) ( Fig. 3D, E ). The centrioles are attached to each other, with the distal one anchored to the plasma membrane via nine satellite fibres. The entire midpiece region has a length of 0.75 ± 0.05 Mm ( N = 3) and a maximum diameter (of mitochondrial cluster) of 1.75 ± 0.08 Mm ( N = 3). Flagellum The flagellum consists of a 9 + 2 microtubular configuration axoneme sheathed by the plasma membrane. From light microscopic observations, flagellar length is 48 ± 2.0 Mm ( N = 10).