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 PAPYRIDEA SEMISULCATA ( FIG. 5 ) Acrosomal complex The acrosomal vesicle is dome-shaped, with a flat or slightly domed apex, and measures approximately 0.55 ± 0.05 Mm ( N = 5) in length and 0.7 ± 0.05 Mm ( N = 5) in maximum diameter ( Fig. 5A–D ). Contents of the vesicle are differentiated into an extensive, very electron-dense basal ring, and a less dense (and granular) anterior component ( Fig. 5B, D ). A broad basal invagination extends for approximately half the length of the acrosomal vesicle and is filled with granular subacrosomal material ( Fig. 5A–C ). Discernible within the basal ring are two curved, very electron-dense layers and an innermost, electronlucent layer ( Fig. 5B, D ). Nucleus The nucleus is short (length 1.45 ± 0.05 Mm, N = 3) and barrel shaped, with a maximum diameter of · Figure 3. Ultrastructure (TEM) of spermatozoa of Fragum unedo . A, B, longitudinal sections (LS) showing acrosomal complex, nucleus (showing slight curvature) and portion of midpiece region. C, LS of acrosomal complex (acrosomal vesicle + subacrosomal material) and nuclear apex. Note narrow basal invagination of acrosomal vesicle. D, LS of flagellum projecting from distal centriole of midpiece. Note also nuclear lacunae and also satellite fibres attached to distal centriole and plasma membrane. E, transverse section (TS) of midpiece showing four mitochondria and distal centriole. Abbreviations: a, acrosomal complex; av, acrosomal vesicle; ax, axoneme; br, basal ring (of acrosomal vesicle contents); dc, distal centriole; f, flagellum; m, mitochondrion of midpiece; n, nucleus; nl, nuclear lacuna; pc, proximal centriole; pm, plasma membrane; sf, satellite fibres; sm, subacrosomal material. 1.45 ± 0.05 Mm ( N = 3) (at mid-length) ( Fig. 5A ). Nuclear contents are highly electron-dense, granulate, with numerous irregular lacunae ( Fig. 5A, E ). Apically the nuclear surface is straight or slightly concave, while posteriorly shallow indentations contact the anterior surfaces of each midpiece mitochondrion ( Fig. 5A ). Midpiece The midpiece consists of four spherical mitochondria (with well-developed cristae) surrounding a pair of centrioles ( Fig. 5A, F ). The entire midpiece region has a length of 0.7 ± 0.04 Mm ( N = 3) and a maximum diameter (of mitochondrial cluster) of 1.7 ± 0.1 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 54 ± 2.0 Mm ( N = 10).