Serum steroid hormones and the reproductive cycle of the female bonnethead shark, Sphyrna tiburo.

The bonnethead shark Sphyrna tiburo reproduces by placental viviparity with one of the shortest gestation periods (4.5-5 months) known in sharks. In southwest Florida, mating in this species occurs in November, sperm is stored until ovulation/fertilization the following March-April, and parturition occurs in August. Serum concentrations of four steroid hormones (17 beta-estradiol, progesterone, testosterone, and dihydrotestosterone) were determined by radioimmunoassay over a complete reproductive cycle in mature females from a wild population. Serum 17 beta-estradiol and testosterone levels are high during mating and preovulatory stages. Preovulatory concentrations of testosterone are greater in female S. tiburo than in any other female elasmobranch previously studied. Progesterone levels are significantly elevated during preovulatory, ovulatory, and postovulatory stages, while serum dihydrotestosterone levels increase significantly during the preovulatory stage. Our study is the first to demonstrate a sustained rise in progesterone during gestation in a placental shark and suggests a regulatory role for this hormone during the period prior to implantation of the embryos in the uterus.