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

The aim of this work was to determine the effects of supplemental dietary s odium chloride on salt water acclimation of tilapia Oreochromis niloticus. Fish were fed a basal diet supplemented with NaCl (8%) during three weeks i n fresh water (FW) and then transferred to salt water (SW) at 15 and 20 par ts per thousand. Changes in plasma osmolality, chloride ion concentration ( Cl-), plasma level of cortisol and gill Na+, K+-ATPase activity were measur ed at 6, 12, 24, 48, 72 and 168 h after transfer to 15 parts per thousand S W, while the higher strength SW group (20 parts per thousand) was only moni tored up to 24 h. Morphological changes in the gill mitochondria-rich (MR) cells were examined in relation to environmental salinity. The changes asso ciated with dietary NaCl were sporadic and of small magnitude. The plasma o smolality and Cl- increased immediately after transfer up to 12-24 h, but f ish fed dietary salt (S) showed lower values than the control group (C). Th e S group showed higher plasma levels of cortisol than the control, which m aintained its initial levels during the experiment. Gill Na+, K+-ATPase act ivity of the S group began to increase in the first hours after transfer, r eaching maximum at 12 h and returned to basal level at 24 h, while the cont rol group maintained basal levels. The differences between gill Na+, K+-ATP ase activity of S and C fish were significant (p < 0.05) at 12 h. Transmiss ion electron microscopy (TEM) revealed that MR cells in SW show more mitoch ondria and a more developed tubular system arising from the basolateral mem brane. The MR cells of both groups frequently formed a multicellular comple x in SW, consisting of a main MR and one or more accessory cells. Such comp lexes are rarely observed in FW. Some MR cells of fish fed supplemented die tary salt displayed convex apical membrane in FW.