Ultrastructure of the sperm and spermatogenesis of three species of Mytilidae (Mollusca, Bivalvia).
The mature spermatozoa of 3 species of the subfamily Mytilinae (Choromytilus meridionalis, Mytilus galloprovincialis and Aulacomya ater) are of the primitive type. The spermatozoa are 50-55 µm long, with a distinct head, midpiece of 4-6 mitochondria, and tail. In all 3 species the acrosome is in the form of an elongated hollow cone; yet each has its own characteristic morphology, indicating that sperm ultrastructure can be used as an aid to bivalve identification. A comparison of the acrosomes from 5 subclasses of bivalve show that each subclass has an acrosome with a unique form. The process of spermatogenesis is similar in all 3 species; early spermatogonia are located close to the wall of the germinal follicle, and successive stages are displaced toward the center. Observations support the theories of acrosome formation by coalescence of numerous proacrosomal vesicles and of reduction in mitochondrial number by mitochondrial fusion.