Field studies on the ureogenic gulf toadfish, in a subtropical bay. II. 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 were measured in samples taken from freshly collected gulf toadfish from a subtidal population in Biscayne Bay, Florida, U.S.A. This indirect appoach was used, instead of direct measurements of nitrogen excretion, because nitrogen excretion patterns of gulf toadfish are altered markedly during the first 24 h of capture disturbance or laboratory confinement. The values obtained for plasma cortisol levels, and the activities of hepatic ornithine-urea cycle enzymes, including glutamine synthetase (and its partitioning between cytosolic and mitochondrial compartments), suggest that gulf toadfish in Biscayne Bay may excrete a substantial portion of their waste nitrogen as urea. Also conducted were correlation analyses of several biotic variables (plasma cortisol], enzyme activities, plasma urea], hepatosomatic index, and plasma Ca⁺⁺]) with several abiotic variables (temperature, salinity, depth and dissolved oxygen), and with collection site and season. Results of these analyses are discussed in the context of hypotheses to explain ureotely in this teleost fish.