The effect of dietary sodium chloride on some osmoregulatory parameters of the teleost, Oreochromis niloticus, after transfer from freshwater to seawater

The aim of this work was to determine the effects of supplemental dietary sodium chloride on salt water acclimation of tilapia Oreochromis niloticus. Fish were fed a basal diet supplemented with NaCl (8%) during three weeks in fresh water (FW) and then transferred to salt water (SW) at 15 and 20[per thousand]. Changes in plasma osmolality, chloride ion concentration (Cl^sup -^), plasma level of cortisol and gill Na^sup +^, K^sup +^-ATPase activity were measured at 6, 12, 24, 48, 72 and 168 h after transfer to 15[per thousand]SW, while the higher strength SW group (20[per thousand]) was only monitored up to 24 h. Morphological changes in the gill mitochondria-rich (MR) cells were examined in relation to environmental salinity. The changes associated with dietary NaCl were sporadic and of small magnitude. The plasma osmolality and Cl^sup -^ increased immediately after transfer up to 12-24 h, but fish fed dietary salt (S) showed lower values than the control group (C). The S group showed higher plasma levels of cortisol than the control, which maintained its initial levels during the experiment. Gill Na^sup +^, K^sup +^-ATPase activity of the S group began to increase in the first hours after transfer, reaching maximum at 12 h and returned to basal level at 24 h, while the control group maintained basal levels. The differences between gill Na^sup +^, K^sup +^-ATPase activity of S and C fish were significant (p < 0.05) at 12 h. Transmission electron microscopy (TEM) revealed that MR cells in SW show more mitochondria and a more developed tubular system arising from the basolateral membrane. The MR cells of both groups frequently formed a multicellular complex in SW, consisting of a main MR and one or more accessory cells. Such complexes are rarely observed in FW. Some MR cells of fish fed supplemented dietary salt displayed convex apical membrane in FW.[PUBLICATION ABSTRACT]