Cichlids of the genus Oreochromis are fish of economic importance in African countries. They tolerate brackish water, however, with great variations between species. In this work, two species, both from the Ivory Coast but of different origins, O. niloticus (field and laboratory strains) and O. aureus (field strain) were compared during osmotic challenges (10, 20 and 30%o salinity) in order to provide physiological support for their specific behaviour when confronted with natural hypertonic environments. Tolerance to salinity was assessed by correlated observations on gill structure, plasma sodium levels and gill Na+/K+ ATPase activity. In fresh water (FW), all fish presented a gill epithelium structure characteristic of FW stenohaline fish: no chloride cells (CC) on the lamellae and few CC on the filaments. An increase in external salinity induced the proliferation of CC on filaments, a feature typical of seawater teleosts. This change in gill structure was accompanied by an increase of gill Na+/K+ ATPase activity. In the most tolerant strains, plasma Na+ did not change, indicating successful ion regulation in the hypertonic media. With regard to potential interest of field strains in fish culture, O. aureus acclimated more easily to brackish water than O. niloticus. Interestingly, O. niloticus, kept for several generations in the laboratory, performed best in our challenge studies. Plasma Na+ levels and gill CC proliferation upon transfer to an isotonic medium may be the parameters of choice when testing these fish for their response to a salinity change.

Branchial sodium uptake was measured across secondary lamellae (SL) using the isolated-perfused head preparation. This study was correlated with histological examination.Freshwater trout, Sulmo guirdneri, were exposed to various artificial media:distilled water (DW), fresh water (FW: NaCl < 200 pMA; C a + + = 1.5 mMA), FW + 5 mM CaCl2 and FW + 10 mM CaC12. These environments altered the number and forms of "chloride cells" (CC) located in SL and were related to changes in maximal transport rate of Na+. An increase in a specific type of CC (round) was associated with a n increase in V, , , , and a decrease in round cells was associated with a decrease in V, , , .

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Tilapia species are generally characterized by a large tolerance to salinity; however, such capacity to adapt to brackish or seawater may be modulated by environmental factors. ï h e major osmoregulatory mechanisms involved in salinity adaptation are presented. Most of the available data concern the role of gills in salt or water exchange. The importance of different factors (environmental or endogenous) in such adaptation is also discussed. ï h e second part presents a survey of the endocrine control of osmoregulation in tilapia. Both fast-acting (e. g. glucagon, urotensins, catecholamines) and long-acting (e. g. prolactin, cortisol) hormones have been studied. In conclusion, several areas of osmoregulatory physiology potentially interesting for aquaculture are discussed.