Variation in plasma oestrogens and androgens during the seasonal and semilunar spawning cycles of female gulf killifish, Fundulus grandis (Baird and Girard)

Variations in 17β‐oestradiol, oestrone, testosterone, and 11‐ketotestosterone were measured by radioimmunoassay in the plasma of female Gulf killifish, Fundulus grandis, during their seasonal and semilunar spawning cycles. Both 17β‐oestradiol and testosterone exhibited distinct seasonal variation, peaking very early in the breeding season during March and April, decreasing gradually thereafter the cessation of spawning in late August and the seasonal regression of ovaries in September, and eventually falling below the detectable limits of our assays during the very early stages of seasonal ovarian recrudescence in November. Both steroids also exhibited distinct semilunar variation within the breeding season, with highest plasma concentrations immediately prior to, and during, each spring tide spawning. Such results suggest that these steroids have physiological roles in the generation and regulation of both seasonal and semilunar reproductive cycles: 17β‐oestradiol by controlling development of vitellogenic oocytes; testosterone perhaps by acting as a precursor in the production of oestrogens and other steroids. In contrast, oestrone and 11‐ketotestosterone were only rarely detected, implying that these particular oestrogens and androgens are probably not physiologically active in female killifish.