A review of plasma hormone changes during ovulation in cyprinid fishes

Abstract

Plasma hormone changes were studied during the natural spawning cycle of three cyprinid fishes: the kanehira bitterling, Acheilognathus rhombea, the common carp, Cyprinus carpio, and the goldfish, Carassius auratus. In these fishes, a preovulatory gonadotropin (GTH) surge occurred in the latter part of the light phase. The highest values of GTH were observed at midnight and were coincident with ovulation of the respective females. Changes in the plasma 17α-hydroxyprogesterone and 17α,20β-dihydroxy-4-pregnen-3-one showed a similar progression to that of GTH. With the commencement of the GTH surge, plasma testosterone (T) level decreased in the bitterling, while, with the carp and goldfish, an increase in T level was observed during this period of time. Plasma estradiol-17β (E2) level increased slightly during ovulation in the kanehira bitterling and goldfish which are both multiple spawners. In the common carp, a one-time spawner, E2 level remained relatively low and unchanged. In response to increasing water temperature, the preovulatory GTH surge of carp and goldfish was initiated in the latter part of the light phase under a variety of photoperiods irrespective of the time when the elevation in temperature started. The GTH surge began at the clock time corresponding to the latter part of the light phase to which the fish had been acclimated before induction with the temperature increase. Likewise, ovulation was usually observed during the dark phase. This took place even in the extreme situation of a 24 L or 24 D “day”. In male goldfish, a small increase in GTH could be induced in the absence of females by raising the water temperature. The level was significantly enhanced, however, by the presence of ovulatory females. Olfactory-tract sectioned males failed to show sexual behavior or the GTH surge while sham-operated males spawned and exhibited the GTH surge. These results indicate that cyprinid fishes have a reproductive circadian rhythm and the onset of the preovulatory GTH surge is determined by a photoperiodic cue. Pheromone (s) from ovulatory females functions as a releaser for stimulating sexual behavior and as a primer for inducing the GTH surge in males. The synchrony of the GTH surge in both sexes allows ovulation and milt preparation to occur at the same time, optimizing the chances of successful fertilization.