| Summary: | The flounder, Paralichthys orbignyanus, is found in coastal and estuarine waters of the Western South Atlantic Ocean. It is being considered for aquaculture due to its high market price and wide tolerance to environmental factors such as salinity, pH, and nitrogenous compounds. The objective of this study was to characterize the ionic and osmotic regulation of P. orbignyanus over the range of its tolerated ambient salinities (0 – 40x) and to evaluate the survival and growth in freshwater (0x) and seawater (30x) over 90 days. After 15 days of exposure to different salinities (0x, 10x, 20x, 30xand 40x), plasma osmolality and ionic (Na + , Cl , K + and Ca2 + ) concentrations slightly increased with salinity. The isosmotic point was estimated as 328.6 mOsm kg 1 H2O and corresponded to 10.9xsalinity. After 90 days, survival was similar in freshwater and seawater, but osmo- and ionoregulation was significantly affected in freshwater and flounders reared in this medium showed a lower growth rate than those reared in seawater. Based on the results from this study, P. orbignyanus can be characterized as a marine/estuarine euryhaline teleost capable of hyper/ hypo iono- and osmoregulation over the fluctuating salinity regime faced by this species in the environment. Furthermore, results suggest that the lower growth rate exhibited by P. orbignyanus in freshwater could be due, at least partially, to a higher energy expenditure associated to a higher branchial Na + , K + -ATPase activity in this environment. D 2002 Elsevier Science B.V. All rights reserved.
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