Serum concentrations of steroid hormones in the mature male bonnethead shark, Sphyrna tiburo.

The male bonnethead shark, Sphyrna tiburo, undergoes an annual cycle of spermatogenesis and testicular regression and recrudescence. In southwestern Florida populations, testicular development and spermatogenesis begin in late spring and peak in late summer, with mating not taking place until about 2 months after the peak. Steroid hormones, some of which are known to play influential, essential roles in spermatogenesis, were measured in the serum during a full annual cycle in mature males from a wild population. Serum 17beta-estradiol (E2), progesterone (P4), testosterone (T), and dihydrotestosterone (DHT) concentrations were determined by radioimmunoassay. Serum T, DHT, and P4 concentrations followed the pattern of testicular development and regression, with all three steroid hormones attaining the highest significant levels (P < 0. 001) in late summer and falling to the lowest levels in mid-winter. Serum E2 concentrations showed significant changes (P < 0.01) with a peak in early spring. Male bonnethead sharks had significantly lower concentrations of E2 (P < 0.001) and significantly higher concentrations of T, DHT (P < 0.001), and P4 (P < 0.05) than those published for females. When steroid concentrations were compared to a behavioral rating of stress, only P4 showed a significant correlation to the stress behavior (P = 0.0447), but the significance was much lower than the significant correlation of P4 by month (P < 0.001).