Résumé
The adaptation to salinity of a fish species is mainly based on the capacity of each of its stages to osmoregulate. Adult teleosts maintain their blood osmolality close to 300 mOsm center dot kg(-1) thanks to water and ion regulation effected by the tegument, digestive tract, gills and urinary system. Studies on the ontogeny of osmoregulation in fish larvae are less abundant. The sea-bass Dicentrarchus labrax is a euryhaline teleost that tolerates salinities from fresh water to hypersaline media. Upon hatching, sea-bass prelarvae are able to osmoregulate, a capacity which increases during ontogeny. In this and other species, osmoregulation in prelarvae is effected by tegumentary ionocytes. In larvae, and especially at the larva/juvenile transition, the urinary system, gills and digestive tract progressively develop and become involved in osmoregulation. In these organs, ionic transports are effected by ionocytes where transmembrane proteins and enzymes such as Na+/K+-ATPase are expressed. The ontogeny of euryhalinity, which permits adaptation to habitats with different salinity regimes, is thus based on the successive involvement of osmoregulatory sites.