BARIMO, J.F. **; MCDONALD, M.D.; WASLH, P.J.; University of Miami; University of Miami; University of Miami: The influence of photoperiod on pulsatile urea excretion in toadfish (Opsanus beta)

The gulf toadfish Opsanus beta (Batrachoididae) is one of the few teleosts to maintain a functional ornithine-urea cycle (O-UC) during adult life and possess the capability to change from ammonotely to ureotely within 24 hours in the laboratory. In the laboratory, O. beta excrete most nitrogenous waste across the gill membrane and urea is generally excreted in daily pulses of 1.5 hrs in duration while ammonia is eliminated continually. At present, the mechanism of O-UC activation is due to an elevation of plasma cortisol (stress response) which promotes the up regulation of the key OU-C enzyme glutamine synthetase. Furthermore, plasma cortisol levels drop prior to pulsatile urea elimination. This study examines the diel pattern of nitrogen excretion with individual toadfish in the laboratory and outdoor mesocosms. Under both experimental conditions toadfish were exposed to natural photoperiod, samples were collected hourly from toadfish in shelters fabricated with PVC pipe, and assayed for urea and ammonia with standard chemical techniques. In laboratory, conducted in 2 L containers with static seawater changed daily, urea pulses occurred at random with no apparent correlation to light or dark cycles. In mesocosm experiments, toadfish were unrestrained in 8000 L tanks with the seagrass Thalassia testudinum planted on carbonate substrate effectively simulating their natural habitat. Shelters were outfitted with an underwater IR camera connected to a time-lapse video recorder to document toadfish behavior. In preliminary results, urea and ammonia excretion in mesocosms occurred predominately during daylight hours with peak levels near dawn or dusk. Differing results between experimental regimes are believed to reflect the degree of stress encountered by toadfish.