FISH AND CYCLOSTOME MIGRATIONS BETWEEN FRESH-WATER AND SEA-WATER - OSMOREGULATORY MODIFICATIONS

Long reciprocal migrations from fresh water to the sea give its specif icity to the metamorphosis observed in salmons, eels and lampreys, whe re the most significant physiological changes concern the hydromineral equilibrium. These events have been more specifically studied in the transition towards a marine environment which accompanies smoltificati on in salmons, the acquisition of a silver livery in yellow eels and a mmocete transformation in lampreys. The osmoregulatory function is mod ified in various organs including the gills, tegument, kidneys and dig estive tract. In particular, the gill chloride cells furnish an import ant contribution by excreting Na+Cl- ions into the ambient medium agai nst a concentration gradient. An endocrine control of the hydromineral equilibrium has been demonstrated in salmons and eels. Hormones such as cortisol, thyroid hormones or the growth hormone have a hypoosmoreg ulatory effect which dilutes the body fluids, essentially by accelerat ing ion excretion through the gill chloride cells. On the other hand, prolactin has a hyperosmoregulatory effect. During salmon smoltificati on or yellow eel metamorphosis, these hormones induce a global hypoosm oregulatory effect. Such an effect could preadapt smelts and silver ee ls to sea water by reducing the deleterious effects of a high ionic st rength on the body fluids.