Daily variations in plasma hormone concentrations during the semilunar spawning cycle of the gulf killifish, Fundulus grandis

Abstract

Male and female gulf killifish, <i>Fundulus grandis</i>, were kept in large cages submerged in their natural habitat in the Gulf of Mexico. Gonadosomatic indices (GSI: 100 × gonad weight/body weight) and blood samples were obtained from male and female fish every 4 hours (beginning at 1600) throughout a day on 3 separate days of the semilunar cycle: at the GSI and spawning peak (July 27–28), at midcycle (August 2–3), and on a day prior to the probable spawning peak (August 9–10). Blood samples of females were assayed for cortisol, estradiol-17β (E₂), progesterone, testosterone, thyroxine (T₄), and triiodothyronine (T₃). The daily rhythms of hormones at each of the three days during the cycle differed markedly from one another. At the spawning peak, E₂ and T₃ peaks occurred at 1200, testosterone and progesterone peaks were found at 2400 and the GSI and cortisol peaks were at 0400. The GSI decreased sharply at 0800 indicating a period of probable spawning. At midcycle, most hormone levels were low throughout the day and the amplitudes of the hormone rhythms were suppressed. One day prior to the spawning peak, testosterone and progesterone variations (peaks at 2400) were similiar to those found on the day of the first semilunar GSI and spawning peaks. Daily variations of other hormones, especially E₂, were prominent on the day prior to peak spawning but different from those found on the day of peak GSI/spawning. In males, plasma concentrations of cortisol, testosterone, T₃ and T₄ also underwent daily variations that differed from one another on the 3 days of the cycle. The results clearly demonstrate that there are dramatic changes in the daily variations of plasma hormone concentrations during the semilunar spawning cycle.