Lunulicardia hemicardium (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 : 331

LUNULICARDIA HEMICARDIUM ( FIG. 4)

Acrosomal complex

The acrosomal vesicle is short (0.24 ± 0.02 Mm, N = 2) and dome-shaped (maximum diameter, at base, of 0.66 ± 0.01 Mm, N = 2) ( Fig. 4A, B). It rests on a shallow, circular groove of the nuclear apex (forming a shallow nuclear hump at the nuclear apex) ( Fig. 4B). Contents of the vesicle are differentiated into a somewhat, dense, basal ring that has an angulate profile, and is sheathed by a less dense material that also fills the anterior half of the vesicle ( Fig. 4A, B). The basal invagination of the vesicle is deep in comparison with vesicle length and very broad (maximum diameter at base 0.4 ± 0.02 Mm, N = 2) and partially filled with a granular, but well-defined, deposit of subacrosomal material.

Nucleus

The nucleus is short (length 2.15 ± 0.05 Mm, N = 3), barrel shaped (maximum diameter, at mid-length, of 1.65 ± 0.05 Mm, N = 3) and slightly tapered anteriorly ( Fig. 4A, B). Contents are highly electron dense with the exception of electron-lucent lacunae (the latter more pronounced posteriorly) ( Fig. 4A). Apically, the nucleus projects slightly into the acrosomal vesicle invagination ( Fig. 4A, B). Basally, very shallow indentations contact the surfaces of the midpiece mitochondria and also form an attachment point for the proximal centriole ( Fig. 4A).

Midpiece

The midpiece consists of four or more rarely five spherical mitochondria and a pair of centrioles ( Fig. 4A, C, D). The centrioles (proximal and centriole) lie in contact with each other and are arranged orthogonally. A thin layer of dense material connects the proximal centriole to a shallow indentation of the nucleus. Satellite fibres connect the distal centriole to the plasma membrane ( Fig. 4A, C). The entire midpiece region has a length of 0.7 ± 0.05 Mm (N = 3) and a maximum diameter (of mitochondrial cluster) of 1.8 ± 0.2 Mm (N = 3).

Flagellum

The flagellum consists of a 9 + 2 microtubular configuration axoneme, sheathed by the plasma membrane ( Fig. 4C, D). From light microscopic observations, flagellar length is 46 ± 3.0 Mm (N = 10).