Gonad and Germ Cell Development and Maturation Characteristics of the Pot-Bellied Seahorse (Hippocampus abdominalis) under Captive Breeding Conditions in Northern China

Ovoviviparity and male pregnancy represent distinctive reproductive strategies in seahorses. However, the detailed process of gonadal development in seahorses, particularly in the pot-bellied seahorse (Hippocampus abdominalis), remains largely unknown. In this study, we investigated the complete gonadal development process of the pot-bellied seahorse under captive breeding conditions (18 +/- 1 degree celsius). Immediately after birth, primordial germ cells (PGCs) were found within the genital ridge, enclosed by a single layer of somatic cells. Around 7-9 days after birth (DAB), the ovary begins to differentiate. By 30 DAB, two germinal ridges had formed along the edge of the follicular lamina in the ovary. The primary oocytes, resulting from this differentiation process, gradually migrated from the dorsal sides to the mid-ventral area of the ovary, eventually maturing into eggs. In the testis, the primary and secondary spermatocytes appeared at 15 and 30 DAB, respectively, preceding the formation of the testicular lumen (50 DAB). The testis was observed to consist of a single large germinal compartment. Under captive breeding conditions in Northern China, the pot-bellied seahorse demonstrated year-round breeding capability, with each male producing approximately 100-150 larvae. The findings from this study contribute valuable insights into seahorse aquaculture and enhance understanding of the unique reproductive strategy employed by seahorses.