The energetics of osmotic regulation in ureotelic and hypoosmotic fishes

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Abstract
The major mechanisms of urea turnover and of active ion transport in osmoregula‐ tory organs are reviewed. Using published values for the molecular flow through these systems it is possible to estimate the energetic cost of osmotic regulation in hyporegulating marine teleosts and in ureotelic elasmobranchs. Oxygen consumption dedicated to osmotic regulation in the flounder (Platichthys flesus) is about 7 ml kg−1 hr−1 and in the rainbow trout (Salmo gairdneri) 6 ml kg−1 hr−1 For a dogfish (Scyliorhynus canicular) it was 5 ml kg−1 hr−1 and for the skate (Raja erinacea) 5 or 11 ml kg−1 hr−1 based on two values for urea loss by the fish. Both groups of fish used 10–15% of their standard metabolism for osmotic regulation. There are a number of uncertainties that may require modification of these values. It is not known whether TMAO lost by ureotelic regulators is replaced by ingestion or by synthesis. If by synthesis, the cost of regulation would be higher than calculated. Even more serious is the uncertainty regarding the use of organic osmolytes to maintain equilibrium between cells and blood. This strategy almost certainly requires some energy input, but there is insufficient information to permit its quantitative estimation, so the factor was omitted from the calculation. Among hyporegulators the sole unknown is whether the esophagus may absorb some of the ingested salt by diffusion. If so, it would decrease the energy expended in the intestinal tract and lower the total cost of regulation. The conclusion, based on known factors for the species examined, is that both modes are about equally efficient. If it becomes possible to factor in the unknowns it may be that hyporegulation is slightly more economical than ureosmotic.