| Summary: | It is well known that glomerular teleosts excrete a profuse dilute urine in fresh water and a scanty urine isotonic to plasma in seawater. ' The striking changes in renal function that ensue when euryhaline fish are transferred from seawater to freshwater or the reverse have been studied in five species: the European eel, Anguilla anguilla,2 the Japanese eel, Anguilla japonica, ' rainbow trout, Salmo gairdneri, ' European flounder, Platichthys flesus,5 and the plains killifish, Fundulus kansae.6 In all, the mechanism of adjustment involves changes in glomerular filtration and tubular reabsorption of sodium. Depending on the species, glomerular filtration rate is three to twenty times higher in fresh water than in salt water, while urine excreted in fresh water contains very little sodium. The quantity of sodium reabsorbed by the kidneys in freshwater must therefore exceed by a considerable amount that transported by the renal tubules in seawater. Sodium transport by the kidney thus changes in a direction opposite to that of sodium transport by the gill when the salinity of the medium is changed.
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