Diurnal nitrogen excretion rhythm of the functionally ureogenic gobiid fish Mugilogobius abei

This study was performed to determine the daily periodicity of urea excretion in the ureogenic gobiid fish Mugilogobius abei. In 20% seawater, urea excretion of all the fish examined showed daily periodic changes under a 12-h light-dark cycle, and some showed a free-running rhythm under constant darkness. This is the first report of a circadian rhythm in urea excretion in fishes. Daily variations in urea excretion under light-dark cycles were also observed under various conditions, i.e. exposure to water ammonia, confinement/non-confinement and solitary/group. Due to the daily variations in urea excretion, urea contents in tissues changed periodically, whereas enzyme activities related to urea synthesis did not change significantly. The index of urea permeability as determined by changes in body urea contents after 2-h immersion of 25 mM urea solution was high during the peak of daily variation in urea excretion. Locomotor activity and urea excretion showed clear daily variations under light-dark cycles, both of which were diurnal. Furthermore, daily variations in urea excretion were maintained even when the diurnal pattern in the locomotor activity was disturbed. These results suggest that periodic urea excretion was mediated by periodic enhancement of permeability for urea at excretion sites.