Fragum unedo (Linnaeus, 1758)

Healy, John M., Mikkelsen, Paula M. & Bieler, Rüdiger, 2008, Sperm ultrastructure in Hemidonax pictus (Hemidonacidae, Bivalvia, Mollusca): comparison with other heterodonts, especially Cardiidae, Donacidae and Crassatelloidea, Zoological Journal of the Linnean Society 153 (2), pp. 325-347 : 329-331

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 ·

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).