Long-term acclimation of the teleost Oreochromis mossambicus to various salinities: two different strategies in mastering hypertonic stress

Laboratory-reared tilapia (Oreochromis mossambicus) were long-term acclimated to freshwater (FW), brackish water (BW, 10 salinity), seawater (SW, 35 salinity) and two hypersaline media (45 and 60 salinity). We examined the influence of these ambient salinities on the density (D _cc) and diameter (d _cc) of DASPMI-stained chloride cells and on the capacity for electrogenic Cl^- secretion of the in vitro opercular epithelium. To provide a characterisation of Cl^- secretion, transepithelial potential difference (PD _te), conductance (G _te) and short-circuit current (I _sc) were measured after mounting the respective epithelium in an Ussing-chamber. The cellular electromotive forces (E _c) and conductances (G _c) as well as the leak conductances (G _l) were obtained from G _te: I _sc plots. In the salinity range between FW and SW both D _cc and d _cc increased. All electrophysiological parameters recorded increased in parallel, indicating a strong enhancement of the capacity for Cl^- secretion on the cellular and epithelial level. In the salinity range above SW a further increase of D _cc was observed. However, despite a higher concentration gradient across the body surface of the tilapia during acclimation to hypersaline media, the short-circuit current (I _sc) was not significantly different compared to SW preparations. This reflects proportional decreases of G _c and increases of E _c, respectively. Of particular interest, we found a strong decrease of the leak conductance (G _l) in preparations from tilapia acclimated to hypersaline media compared to those from SW fish, indicating that the tight junctions become less permeable.