VARSAMOS, S.; NEBEL, C.; CHARMANTIER, G.*; Univ. of Montpellier, France: Fitted to fit: Ontogeny of Osmoregulation in Fish

Salinity and its variations are among the key factors that affect survival, metabolism and distribution during the fish development. The successful establishment of a fish species in a given habitat depends on the ability of each developmental stage to cope with salinity through osmoregulation. It is well established that adult teleosts maintain their blood osmolality close to 300 mosm/kg due to ion and water regulation effected at several sites: tegument, gut, branchial chambers, excretory organs (kidney, urinary bladder). But fewer data are available in developing fish. We propose a review on the ontogeny of osmoregulation based on studies conducted in the European sea bass Dicentrarchus labrax and other species. The prelarvae are able to hyper/hypo-osmoregulate at hatch, and the ability to osmoregulate increases in later stages. The drinking rate normalised to body weight tends to decrease throughout development and is differently affected by salinity according to the stage. Before the occurrence of gills, the prelarval tegument where a high density of ionocytes (displaying high contents of Na/K-ATPase) is located, appears temporarily as the main osmoregulatory site. Gills develop gradually during the prelarval stage; at mouth opening, gill arches are present and support numerous ionocytes. The tegument and gill Na/K-ATPase activity varies ontogenetically and in post-yolk sac larvae, the gills become the main osmoregulatory site. The capacity to produce hypotonic urine at low salinity increases during ontogeny, in relation to the progressive development of the kidney and urinary bladder. In summary, the ability of ontogenetical stages of fish to tolerate salinity through osmoregulation relies on integumental ionocytes, digestive tract development and drinking rate, developing branchial chambers and excretory organs.