Some characteristics of germ cell differntiations during spermiogenesis and mature sperm ultrastructure in male Phacosoma japonicus were investigated by transmission electron microscope observations. The morphology of the spermatozoon of this species has a primitive type and is similar to those of other species in the subclass Heterodonta. Morphologies of the sperm nucleus and the acrosome of this species are the cylindrical type and cap shape, respectively. The spermatozoon is approximately 45-50 μm in length, including a long curved sperm nucleus (about 3.70 μm long with 45° of the angle of the nucleus, an acrosome (about 0.55 μm in length), and tail flagellum (about 42-47 μm). The axoneme of the sperm tail shows a 9+2 structure. As some characteristics of the acrosomal vesicle structures, the basal and lateral parts of basal rings show electron opaque part (region), while the anterior apex part of the acrosomal vesicle shows electron lucent part (region). These characteristics of the acrosomal vesicle were found in the family Veneridae and other several families in the subclass Heterodonta. These common characteristics of the acrosomal vesicle in the subclass Heterodonta can be used for phylogenetic and systematic analysis as a taxonomic key or a significant tool. The number of mitochondria in the sperm midpiece of this species are four, as one of common characteristics appear in most species in the family Veneridae and other families in the subclass Heterodonta. However, exceptionally, only three species in Veneridae of the subclass Heterodonta contain 5 mitochondria. The number of mitochondria in the sperm midpiece can be used for the taxonomic analysis of the family or superfamily levels as a systematic key or tools.

Ultrstructural studies of germ cell differentiation and vitellogenesis in the oocytes of the female Rapana venosa in the brackish water area of Seomjin River were investigated by transmission electron microscope observations. In the early vitellogenic oocytes, the Golgi complex and mitochondria were involved in the formation of glycogen particle, lipid droplets, and yolk granules. In the late vitellogenic oocytes, the rough endoplasmic reticulum and multivesicular bodies were involved in the formation of proteid yolk granules in the cytoplasm. However, heterosynthetic vitellogenesis in this species were not observed in vitellogenic oocytes during oogenesis. A mature yolk granule was composed of three components: crystalline core, electron lucent cortex and the limiting membrane. As shown in some large gastropods, vitellogenesis in R. venosa occurred by way of endogenous autosynthesis without heterosythetic vitellogenesis (exogeneous endocytosis), which are found in the oocytes in bivalves. The mating period and spawning activity were related with the increases of seawater temperatures and salinities.