Sexual maturation in a male whale shark(rhincodon typus) based on observations made over 20 years of captivity

Abstract

A male whale shark (Rhincodon typus) has been kept in captivity at the Okinawa Churaumi Aquarium, Japan, for 23 years, providing a rare opportunity to observe sexual maturation of a male. In 2012, when the individual was 8.5 m in total length (TL), the claspers began to elongate, and the distal ends of the claspers changed into cauliflower-shaped forms. These morphological changes in the claspers were completed in 11 months. Plasma concentrations of sex hormones (testosterone and progesterone) increased in association with clasper elongation. After elongation, additional movements, including the adduction and crossing of claspers, were displayed, and a whitish fluid, which may have been semen, was discharged through the clasper grooves, indicating that the male was functionally mature after clasper elongation. These observations indicate that this captive whale shark reached maturity when it was 8.5 m TL and over 25 years in age. This size at maturity is consistent with observations in the wild. However, maturity in this individual captive animal may have occurred later than has been previously reported in nature.