Nitrogen metabolism and excretion in an intertidal population of the gulf toadfish (Opsanus beta)

The present study was undertaken to assess the ability of gulf toadfish (Opsanus beta) from an intertidal population in northern Florida to make and excrete urea as a nitrogenous waste product. Using an electronic tide gauge and data logger, we were able to demonstrate that toadfish in the intertidal mudflats at Panacea. Florida were air-exposed approximately 9.3% of the over 2000 h monitored, and that these periods could last as long as 9 h. Toadfish were collected from this site during several seasons in 1993-1994, and were subjected to experiments which measured their nitrogenous waste production (as ammonia and urea). Samples were also analyzed for several biochemical proxies of urea excretion (including liver ornithine-urea cycle (O-UC) enzyme activities, and activities of nitrogen feeder enzymes, including the key enzyme glutamine synthetase. G Sase), as well as plasma cortisol, urea and other variables. Although freshly collected toadfish from Panacea exhibited relatively high activities of the O-UC enzymes, they showed rather low activities of GSase, indicating a low potential for immediate urea production. Furthermore, urea excretion rates were extremely low during the first 24-36 h in captivity, even when toadfish were subjected to an experimental emersion period of 8 h intended to simulate air-exposure in nature. When compared to responses and values of subtidal toadfish from Biscayne Bay. Florida published in a prior report, and from a single sample of subtidal toadfish at Panacea, it is apparent that urea synthesis and excretion in the gult toadfish are likely not a direct consequence of air-exposure during the tidal cycle. Our data are discussed in the context of an ammonia-exposure hypothesis explaining urea synthesis and excretion in this species.