Semilunar changes in estradiol and cortisol coincident with gonadal maturation and spawning in the killifish Fundulus heteroclitus.

Radioimmunoassay techniques were used to measure estradiol-17 beta (E2) and cortisol in the serum and ovarian fluid of Fundulus heteroclitus throughout complete semilunar spawning cycles. The cyclic nature of spawning activity was confirmed by monitoring the gonadosomatic index and noting the times at which ovulated eggs were found within the ovarian lumen. Levels of serum E2, ovarian fluid E2, and serum cortisol all exhibited cycles which paralleled the semilunar cycle of gonadal maturity and spawning in this euryhaline killifish. Elevations of serum cortisol were temporally associated with ovulation, whereas ovarian fluid E2 levels peaked several days prior to ovulation. Serum E2 remained elevated after ovarian fluid E2 levels had returned to baseline. Cortisol concentrations in ovarian fluid did not cycle and no correlation was apparent between serum cortisol and glucose concentrations.