Plasma and ocular melatonin rhythms in Amazonian fish.

The daily melatonin rhythms and the effect of light pulse on this hormone were evaluated for Amazonian fish, Astronotus ocellatus and Brycon amazonicus. In experiment 1, fish were exposed to a 12h light:12h dark, and blood was sampled every 4h. In experiment 2, a light pulse was provided in the middle of the dark period for 1h. Melatonin peaked in the middle of the dark period in both species, and both showed an inverted ocular rhythm. The light pulse reduced plasma melatonin to daytime levels, while ocular melatonin remained unchanged. Thus, photic stimulation can be used as a physiological time cue.