Fish and cydostome migrations between fresh water and sea water: Osmoregulatory modifications

Long reciprocal migrations from fresh water to the sea give its specificity to the metamorphosis observed in salmons, eels and lampreys, where 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 smoltification in salmons, the acquisition of a silver livery in yellow eels and ammocete transformation in lampreys. The osmoregulatory function is modified in various organs including the gills, tegument, kidneys and digestive tract. In particular, the gill chloride cells furnish an important contribution by excreting Na*Cl‐ions into the ambient medium against 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 hypoosmoregulatory effect which dilutes the body fluids, essentially by accelerating ion excretion through the gill chloride cells. On the other hand, prolactin has a hyperosmoregulatory effect. During salmon smoltification or yellow eel metamorphosis, these hormones induce a global hypoosmoregulatory effect. Such an effect could preadapt smolts and silver eels to sea water by reducing the deleterious effects of a high ionic strength on the body fluids.