Circadian phase dependent thermal stimulation of ovarian recrudescence in Indian catfish, Clarias batrachus

Female Clarias batrachus acclimated to long photoperiod (13L:11D), were sub jected to 30 degrees +/- 1 degrees C thermopulses of either 6-hour or 12-ho ur duration at different phases of the LD cycle during the late resting pha se (first week of January) of their annual reproductive cycle. Six-hour pul ses were given either at 0600 or 1200 or 1800 or 0000. Twelve-hour thermopu lses 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 ex posure to long photoperiod or constant high temperature induced gonadal gro wth (GSI) and elevated testosterone and oestradiol levels in plasma. The hi gh temperature was significantly more effective in its action. Further, com bination of long photoperiod and high temperature produced the strongest go nadal stimulation as gauged from GSI and the levels of steroid hormones. In terestingly, 30 degrees C thermopulses of 12-h duration when given at 0600 to fish held under long photoperiod induced gonadal development of comparab le magnitude as observed in response to constant high temperature under lon g photoperiod. Thermopulses (30 degrees C) of 6-h duration given at 0600 or 1200 also induced significant gonadal recrudescence but of much lesser mag nitude. Thermopulses either of 6-h or 12-h duration at 1800 failed to elici t 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 ap pears that in this species temperature-induced gonadal recrudescence would occur only following coincidence of high temperature with the thermoinducib le phase. The underlying mechanism of this phenomenon might be circadian in nature.