Circadian Phase Dependent Thermal Stimulation of Ovarian Recrudescence in Indian Catfish, Clarias batrachus

Female Clarias batrachus acclimated to long photoperiod (13L:11D), were subjected to 30º ± 1ºC thermopulses of either 6-hour or 12-hour duration at different phases of the LD cycle during the late resting phase (first week of January) of their annual reproductive cycle. Six-hour pulses were given either at 0600 or 1200 or 1800 or 0000. Twelve-hour thermopulses were given at 0600 or 1800. The long photoperiods were started at 0530 and that of ambient at 0630 coinciding with the average timing of sunrise that prevailed during the period of the study. The results indicate that exposure to long photoperiod or constant high temperature induced gonadal growth (GSI) and elevated testosterone and oestradiol levels in plasma. The high temperature was significantly more effective in its action. Further, combination of long photoperiod and high temperature produced the strongest gonadal stimulation as gauged from GSI and the levels of steroid hormones. Interestingly, 30ºC thermopulses of 12-h duration when given at 0600 to fish held under long photoperiod induced gonadal development of comparable magnitude as observed in response to constant high temperature under long photoperiod. Thermopulses (30ºC) of 6-h duration given at 0600 or 1200 also induced significant gonadal recrudescence but of much lesser magnitude. Thermopulses either of 6-h or 12-h duration at 1800 failed to elicit any change in the variables under study. The results of cosinor analysis performed on the responses to 6-h thermopulses also substantiate that there is a rhythm in the sensitivity of C. batrachus to thermopulses. Thus it appears that in this species temperature-induced gonadal recrudescence would occur only following coincidence of high temperature with the thermoinducible phase. The underlying mechanism of this phenomenon might be circadian in nature.