OSMOREGULATORY MECHANISMS DURING EARLY LIFE STAGES IN FISH AND THEIR HORMONAL CONTROL

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The aim of this study is to clarify osmoregulatory mechanisms during early life stages of euryhaline fishes, specifically in euryhaline tilapia (Oreochromis mossambicus), and their hormonal control. The results obtained during the last 2 years are as follows.1. Prolactin cells identified by immunocytochemistry and in situ hybridization were developed earlier in the tilapia embryos developed in fresh water (FW) than those in seawater (SW), suggesting its important role in FW adaptation as in adult fish.2. Chloride cells on the yolk-sac membrane of tilapia embryos and larvae reared in FW developed earlier than in those in SW,and cortisol, SW-adapting hormone in the adult fish, accelerated their development.3. Tilapia larvae started drinking seawater 2 days after hatching. Drinking rate, as measured by accumulation of ^<14>C-dextran in the gut, was still greater in the larvae in SW than those in FW,and the drinkig rate increased as the larvae grow.4. Turnover rate of tritiated water in tilapia embryos and larvae in FW were twice as great as those in SW,whereas the turnover rate of ^<36>Cl was 50-80 folds greater in SW than in FW.5. Field studies indicated the importance of thyroid hormones and cortisol in the development of digestive system during early embryonic stages and also during metamorphosis from larval to juvenile stages in several marine species such as flounders, tunas and seabreams.