Donax (Plebidonax) deltoides (Lamarck, 1818)

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 : 333

DONAX (PLEBIDONAX) DELTOIDES ( FIG. 6)

Acrosomal complex

The acrosomal vesicle measures approximately 1.16 ± 0.02 Mm (N = 4) in length, has a maximum diameter of 0.85 ± 0.05 Mm (N = 4) (about one-third of the distance from the vesicle base) and rests within a broad depression of the nuclear apex (depth 0.24 ± 0.03 Mm, N = 4) ( Fig. 6A–C, E). A narrow invagination runs almost the full length of the vesicle and is filled with longitudinally fibrous subacrosomal material. Contents of the acrosomal vesicle can be differentiated into a very electron-dense, basal ring (curved-cylindrical in longitudinal profile) and homogeneous, less dense material – the latter enclosing the basal ring and filling the anterior region of the vesicle. The basal ring shows an internal structure of fine, parallel layers (approximately 25–35 in number) ( Fig. 6A) that in transverse section ( Fig. 6B) are arranged concentrically.

Nucleus

The nucleus is short [length 1.4 ± 0.1 Mm (N = 4), inclusive of an overlap region with the acrosomal complex] and squat (maximum diameter 1.5 ± 0.04 Mm, N = 4), with highly electron-dense contents ( Fig. 6A, C–E). The apical surface is broadly indented to receive the basal portion (0.2 ± 0.05 Mm, N = 4) of the acrosomal vesicle, while posteriorly, shallow indentations act as contact surfaces for the midpiece mitochondria and the anterior edge of the proximal centriole. Irregularly shaped electron-lucent lacunae are present throughout, though largest posteriorly ( Fig. 6A, D).

Midpiece

The midpiece exhibits four, roughly spherical, mitochondria (showing prominent cristae) in addition to a pair of orthogonally arranged centrioles ( Fig. 6A, E, F). Each centriole is composed of microtubular triplets set in a dense matrix ( Fig. 6F). The proximal centriole is loosely connected via a diffuse deposit to a shallow indentation of the nuclear base, and is also attached to the distal centriole ( Fig. 6A, E). The distal centriole is anchored to the plasma membrane via nine satellite fibres and is continuous with the doublets of the flagellar axoneme ( Fig. 6A). The entire midpiece region has a length of approximately 0.6– 0.7 Mm and a maximum diameter (of mitochondrial cluster) of 1.6–1.8 Mm.

Flagellum

The flagellum consists of a 9 + 2 microtubular configuration axoneme, sheathed by the plasma membrane ( Fig. 6A). From light microscopic observations, flagellar length is 46 ± 3.0 Mm (N = 10).