FIELD STUDIES ON THE UREOGENIC GULF TOADFISH, IN A SUBTROPICAL BAY .2. NITROGEN-EXCRETION PHYSIOLOGY

In order to examine the in situ nitrogen excretion physiology of gulf toadfish (Opsanus beta) (Fam. Batrachoididae), several biochemical and physiological measurements relating to urea synthesis and excretion w ere measured in samples taken from freshly collected gulf toadfish fro m a subtidal population in Biscayne Bay, Florida, U.S.A. This indirect appoach was used, instead of direct measurements of nitrogen excretio n, because nitrogen excretion patterns of gulf toadfish are altered ma rkedly during the first 24 h of capture disturbance or laboratory cofi nement. The values obtained fbr plasma cortisol levels, and the activi ties of hepatic ornithine-urea cycle enzymes, including glutamine synt hetase (and its partitioning between cytosolic and mitochondrial compa rtments), suggest that gulf toadfish in Biscayne Bay may excrete a sub stantial portion of their waste nitrogen as urea. Also conducted were correlation analyses of several biotic variables (plasma [cortisol], e nzyme activities, plasma [urea], hepatosomatic index, and plasma [Ca+]) with several abiotic variables (temperature, salinity, depth and di ssolved oxygen), and with collection site and season. Results of these analyses are discussed in the context of hypotheses to explain ureote ly in this teleost fish. (C) 1997 The Fisheries Society of the British Isles.