STEROID PROFILES DURING MATURATION AND INDUCED SPAWNING OF THE STRIPED MULLET, MUGIL-CEPHALUS L

In Hawaiian waters, female mullet were first observed to possess early vitellogenic oocytes in November. Ten weeks later, vitellogenesis had progressed to the point where spawning could be induced (i.e., mean oocyte diameter > 600-mu-m). The model: Ln (egg diameter) = 4.96 + 0.014 (days) was found to best describe oocyte growth up to that stage (r2 = 0.83, P < 0.001). Both serum testosterone and estradiol-17-beta were highly correlated with oocyte growth. During hormonally induced spawning, mullet exhibit an initial increase in serum testosterone and estradiol-17-beta compared to saline-injected controls. Both steroids fell to almost undetectable levels 6 h after the second injection of a two-injection protocol. A surge in 17-alpha,20-beta-dihydroxyprogesterone was observed when estradiol-17-beta and testosterone levels decreased. These dramatic changes are coincident with oocytes undergoing hydration, germinal vesicle breakdown, and ovulation. The surge of 17-alpha,20-beta-dihydroxyprogesterone was short-lived and fell to almost undetectable levels at spawning. No changes in 17-alpha-hydroxyprogesterone were observed. The most effective steroid tested in inducing germinal vesicle breakdown, in vitro was found to be 17-alpha,20-beta-dihydroxyprogesterone. This suggests that this steroid is the maturation-inducing substance for this teleost. Results suggest that certain cytoplasmic events such as oil droplet coalescence and dissolving of yolk granules are gonadotropin(s) mediated and independent of the appearance of 17-alpha,20-beta-dihydroxyprogesterone.