Spermatogenesis and Taxonomic Value of Sperm Ultrastructure in Coecella chinensis (Mesodesmatidae), and Sperm Ultrastructural Differences between Mesodesmatidae and Mactridae (Bivalvia, Heterodonta)

Abstract

Spermatogenesis and taxonomic value of sperm ultrastructure in Coecella chinensis (Mesodesmatidae) were investigated by transmission and scanning electron microscopy, and sperm ultrastructural differences were compared between Mesodesmatidae and Mactridae species(Bivalvia, Heterodonta). Ultrastructure of the acrosomal vesicles and nuclei of Mesodesmatidae and Mactridae species are very similar. In C. chinensis, the basal rings of the acrosomal vesicle of spermatozoa are composed of an electron-dense opaque part at the lower portion and an electron-dense lucent part at the upper portion. In Mactridae species, acrosomal vesicles of spermatozoa are similar. However, two different heights of acrosomal vesicle basal rings were observed in spermatozoa between the two families in subclass Heterodonta. In the case of Mesodesmatidae (C. chinensis), basal ring height is high toward the upper direction, whereas those of Mactridae species is low toward outer directions. Thus, height basal rings of the acrosomal vesicles of spermatozoa can be used as a key characteristic for identification of Mesodesmatidae and Mactridae species. There are four or five mitochondria in the sperm midpiece of the observed Mesodesmatidae species, whereas there are only four in previously analyzed Mactridae species. Consequently, number of mitochondria may be useful for taxonomic analysis at the family or superfamily levels.