Characterization of diurnal urea excretion in the mangrove killifish, Rivulus marmoratus

An unusual characteristic of nitrogen excretion in the ammoniotelic mangrove killifish Rivulus marmoratus is that urea is excreted (J(urea)) in a distinct diurnal pattern, whereas ammonia is excreted (J(amm)) at a steady rate. In this study we tested the hypothesis that the diurnal pattern in R. marmoratus is an endogenously generated pattern that is characterized as a circadian rhythm. This hypothesis was tested by measuring J(urea) and J(amm) following manipulation of feeding or lighting regimes. The diurnal J(urea) pattern in food-deprived R. marmoratus had a 24 h periodicity under normal conditions of 12 h:12 h light:dark (12:12 L:D) with 72% more urea excreted during 12:00 h and 18:00 h. In contrast, there was no significant pattern in J(amm). Fed fish (12:12 L:D) demonstrated a diurnal pattern in both J(urea) and J(amm) with up to an eightfold increase in excretion rates compared with rates obtained from food-deprived fish. Patterns of J(urea) were free running with a 24 h period under conditions of continuous darkness (0:24 L:D). Exposure to an inverse photoperiod (12:12 D:L) resulted in entrainment of the J(urea) pattern to the new photoperiod, with the highest rates of excretion occurring during midday of the new photoperiod. In contrast to R. marmoratus, nitrogen excretion rates in the zebrafish Danio rerio remained constant over time. The results of this study show that J(urea) in R. marmoratus demonstrates the characteristics of a circadian rhythm: a 24 h periodicity, a free-running rhythm in continuous conditions, and entrainment to new photoperiods.