Circadian Variation in Circulating Levels of Melatonin, Testosteroneand Estradiol-17β in a Tropical Freshwater Catfish, Clarias batrachus, during Different Phases of the Annual Reproductive Cycle.

In the present investigation, daily variations in plasma levels of melatonin,testosterone and estradiol-17β were analysed in the tropical freshwatercatfish, Clarias batrachus during early and late-preparatory, pre-spawning,late-spawning and post-spawning phases of its annual reproductive cycle. Plasmamelatonin levels exhibited a significant circadian rhythm during all the reproductivephases but peaks were invariably at night. The amplitude of melatonin washigh during the early-preparatory phase and low in the pre-spawning phase.Testosterone also displayed significant rhythm during all the studied phasesexcept post spawning phase. Estradiol-17β revealed diurnal rhythm onlyduring preparatory and pre-spawning phases. The peak in the levels of plasmatestosterone and estradiol-17β were recorded during the photophase. Duringthe late-preparatory phase, the rhythm of testosterone coincided with therhythm of estradiol-17β. [ABSTRACT FROM AUTHOR]