Kinetics of water and chloride exchanges during adaptation of the European eel to sea water.

Since Smith (1930) and Keys (1933) studied water and electrolyte exchanges in the eel much work has been done on osmoregulatory problems in teleosts, and the fundamental principles of osmoregulation in this group have now been established. The freshwater teleost drinks little and urinates abundantly to compensate for osmotic water uptake through the gill; the marine teleost drinks abundantly ('drinks like a fish' in fact) to make good water loss through the gill. The eel, euryhaline and migratory in varying salinities during its life-cycle, is an excellent choice for the study of osmoregulatory problems. Transferring an eel from fresh water to sea water results in an increase of plasmatic electrolytes (Oide & Utida, 1968; Mayer & Nibelle, 1970; Kirsch, 1972). This increase could be caused by all or any of three factors factors which it is difficult to dissociate experimentally or to estimate quantitatively. They are: (a) dehydration due to the reversal of the osmotic gradient between internal and external media; (b) an influx of electrolytes temporarily higher than the outflux; (c) a considerable uptake of water from the digestive tract. The present work is concerned with the last of these phenomena, with regard to which previous data are contradictory. Oide & Utida (1968) report a gradually increasing ingestion of sea water in vivo with a maximum on the 5th day after transfer, followed by a reduction and stabilization. Kirsch (19726) isolated the head region of the eel and observed a considerable ingestion of water at the moment of osmotic shock. We have therefore undertaken complete and detailed study of the kinetics of the drinking rate during sea-water adaptation, with a parallel study of the little-known chloride exchanges. Dehydration was also followed indirectly by measurements of body weight.