ultrastructural characteristics of spermatogenesis in Onchidium struma (Pulmonata: Onchidiidae) and its functional adaptation

The ultrastructural characteristics of spermatogenesis of the mollusc Onchidium struma (Pulmonata: Onchidiidae) were studied by transmission electron microscopy. The spermatogenesis of O. struma is divided into five stages based on morphological changes of the nucleus, mitochondria and other organelles that include the spermatogonium, primary spermatocyte, secondary spermatocyte, spermatid and sperm. During sperm cell differentiation, the nucleus content is condensed from asymmetrical granules to flocculent-shape, filament-shape, long-filament-shape, uniform and finally dense granules. The shape of the nucleus is transformed from round to olive shaped, wing shaped and finally long cone shaped. There is a posterior nuclear fossa in the nucleus of the mature spermatozoon. During spermatogenesis, there are rich Golgi vesicles, endoplasmic reticula, endoplasmic reticulum vesicles and a great deal of mitochondria. The number of mitochondria increases at first and then decreases while the volume of mitochondria increases. The mitochondria crista increase in number and are fused. The distribution of mitochondria develops from random to polar, and the mitochondrial complex encloses around the axoneme at the final stage. The changes of nucleus and organelles during spermatogenesis are compared between O. struma and other gastropods, and its peculiarities for gastropod reproduction as well as physiological adaptations are discussed.