Gonadal Morphology, Histology, and Endocrinological Characteristics of Immature Female Whale Sharks, Rhincodon typus

Captive breeding of whale sharks is one of the great challenges for aquariums. However, there is limited information available related to reproductive physiology due to the difficulty of sampling and long-term observation. In the present report, we provide information on the reproductive physiology of female whale sharks, which were incidentally captured as bycatch in a set-net off the coast of Okinawa, Japan. Total lengths of three captured female whale sharks were 403, 665, and 761 cm, respectively, at the time of their death. Collected paired ovaries differed in size between right and left. However, it seems not to determine which side of ovary becomes developed. Histological observations revealed that oocytes surrounded by follicle cell layers localized in the developed ovary, and most developed oocytes exhibited yolk vesicle stage. Additionally, in the largest specimen, there were low levels of three steroid hormones (Testosterone, Dihydrotestosterone, and Estradiol-17 beta) that did not show seasonal variation. The present results indicate that even the whale shark over 7 m in TL are still histologically and endocrinologically immature. We expect that the present data will provide fundamental information related to reproductive physiology of female whale sharks, and will contribute to protection activities and increased success in captive breeding of whale sharks.