Effedts of confinement Volume on Ureogenesis in the Gulf Toadfish Opsanus Beta
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The gulf toadfish, Opsanus beta, is a facultative ureotele that, when stressed, can excrete nitrogenous waste as urea instead of ammonia. This study focused on the relationship between cortisol, hepatic glutamine synthetase (GNS) activity, and the form of nitrogen excreted (urea or ammonia) in toadfish that were confined to variable water volumes and fish that were unconfined. Fish were placed individually in tubs containing 2, 4, 6 or 8 I of sea water for 48 h during which time the ureogemc indicators (nitrogen excretion rates, cortisol and GNS levels) were measured. Throughout the first 24 h, fish excreted predominantly ammomia with low total nitrogen excretion rates. A transition to urea excretion was seen at the end of the 48 h, with an overall higher rate of total nitrogen excretion. In addition, plasma cortisol levels were elevated, while GNS levels showed only a slight but non-significant Increase. Findings from this study show that confinement elicits ureogenesis. However, results indicate that differences in water volume within the range tested did not affect the degree of ureogenesis expressed by fish. Results of this study support the hypothesis that when toadfish are stressed they begin producing urea. Additionally, it demonstrates that the threshold confinement volume eliciting the stress response is greater than 8 I. However, the ecological relevance of this phenomenon remains to be definitively identified.