The Mozambique tilapia (Oreochromis mossambicus) is a euryhaline cichlid which has been introduced from its native Africa to tropical freshwater (FW) and marine environments around the world [7]. This species has provided a good model for studying the mechanisms of osmoregulation in teleost fishes (e.g., 21 and 22), due to its euryhaline nature and hardiness in captivity [18].
There have been many studies on the physiological changes that occur during the seawater (SW) acclimation process in O. mossambicus. Following transfer from FW to SW there is a temporary elevation in plasma osmolarity and sodium ([Na+]) and chloride ([Cl−]) ion concentrations 2 and 26accompanied by a transient rise in plasma cortisol concentrations [4], and a more gradual increase in plasma growth hormone (GH) levels [49]. There is an alteration in branchial chloride cell morphology [25]and an increase in gill Na+,K+-ATPase activity 16, 17 and 26in SW. Furthermore, there is recent evidence to link the actions of cortisol and GH to these processes in O. mossambicus 11 and 31. Tilapia possess two forms of prolactin (tPRL177 and tPRL188) 44 and 50, which both have a sodium retention effect in FW and thus decrease to low levels in the plasma of fish transferred to SW 6 and 49. The result of these biochemical and hormonal responses is a net efflux of Na+ and Cl− to maintain ionic balance in a hyperosmotic environment