Reproductive Biology of the Shortspine Spurdog Squalus cf. Mitsukurii in the Southwest Waters of Japan

Reproductive characteristics of the shortspine spurdog, Squalus cf. mitsukurii Jordan and Snyder, 1903, were examined based on specimens collected around Ishigaki Island, Okinawa, Japan. Females reached sexual maturity at a larger size than males (total length at 50% sexual maturity: male, 560 mm; female, 728 mm). Oogenesis and embryonic development in gravid females occurred simultaneously: they are ready to ovulate and mate immediately after parturition in February–August. While there was some evidence of seasonality in oogenesis, year-round occurrence of a wide range of follicle sizes and developing embryos together with year-round presence of mature spermatozoa suggest that mating occurs year-round with a peak between February and August. It was not possible to accurately estimate the gestation period based on growth trajectories of uterine embryos and ovarian follicles. Ovarian and uterine fecundities ranged from two to four yolky follicles (mean: 3.4) and from one to five uterine eggs and embryos (mean: 3.1), respectively, and both were positively related to the total length of the pregnant females.