Histological analysis of oogenesis and ovarian development of the pharaoh cuttlefish, Sepia pharaonis

The pharaoh cuttlefish, Sepia pharaonis, has become a commercially cultured cephalopod in coastal southeast China. However, information on the reproductive histology of this species remains limited. To describe its reproductive development, this study investigated the ovarian structure and oogenesis of S. pharaonis from hatchlings to the postbreeding stage using anatomical methods and histological techniques. The results showed that oogenesis in the ovary was asynchronous and morphologically variable, with immature oocytes attached to the gonadal cord and encircling the ovarian stroma in sequence. After maturation, the egg was free in the ovarian cavity and was discharged through the fallopian tube to combine with the sperm. Furthermore, serial histological dissections of the ovaries demonstrated that oocyte development was asynchronous. Based on egg size and morphology, follicular cell morphology, and yolk formation, oogenesis in S. pharaonis was divided into five distinct stages: The oogonia stage, protoplasm growth stage, follicular penetration stage, vitellogenesis stage, and resorbing stage. Moreover, based on the appearance of follicular cells in the protoplasm growth stage and their disintegration and disappearance in the vitellogenesis stage, it can be inferred that follicular cells secrete yolk substances and participate in the formation of egg membranes. Through the dynamic observation and description of the ovary development and oogenesis, these results provide an important foundation for studies of the regulatory mechanisms of oogenesis in this species, enriching the theory of cephalopod reproductive biology and improving artificial reproduction technology.