Effect of photoperiod and temperature on gonadal activity and plasma steroid levels in a reared strain of the mummichog (Fundulus heteroclitus) during different phases of its annual reproductive cycle.

Mummichog, a spring and summer spawning teleost, were exposed to various photoperiod and temperature conditions to investigate the environmental regulation of the annual reproductive cycle. In early spring, latter phases of gonadal development (vitellogenesis in females and active spermatogenesis in males) were effectively accelerated by warm temperature (16 degrees C) regardless of the photoperiod (11L or 16L), although internal factor(s) may be concerned with triggering the initiation of the development. In late summer, intense gonadal regression which leads to the termination of the spawning period was accelerated by a short day length (</=13L) in both yearlings and underyearlings. In underyearlings, however high water temperature and internal factor(s) may also be concerned. In early autumn, early phases of gonadal development (growth of cortical alveolus stage oocytes in females and basal spermatogenesis in males) were induced by moderate or lower temperatures (</=22 degrees C) even under a short day length (11L). From the middle of autumn to early winter, this fish is probably in the "refractory period," and did not progress to the latter phases even under adequate temperature and long day length conditions (22 degrees C-16L). Mummichog showed a probable circa-annual rhythm of gonadal activity under constant temperature and photoperiod conditions (22 degrees C-16L): this rhythm may be the basis of the trigger of the gonadal development prior to the spawning period, the termination of the spawning period in underyearlings, and the occurrence of the "refractory period" during autumn. Plasma concentrations of estradiol-17beta in females and testosterone in males correlated well with the gonadal development during early spring and the regression during late summer. However, there was no correlation between plasma steroid levels and the degree of progress during the early phases of gonadal development in autumn, suggesting other factor such as direct action of GtH(s) or mediating substance other than sex steroids for these phases.