DEVELOPMENTAL-CHANGES IN LOW-SALINITY TOLERANCE AND RESPONSES OF PROLACTIN, CORTISOL AND THYROID-HORMONES TO LOW-SALINITY ENVIRONMENT IN LARVAE AND JUVENILES OF JAPANESE FLOUNDER, PARALICHTHYS-OLIVACEUS

In Japanese flounder (Paralichthys olivaceus), metamorphic period invo lves not only transformation from larva to juvenile but also migration from offshore areas to estuaries. In the present study, the role of e ndocrine systems in low-salinity adaptation was examined during early development and metamorphosis of the flounder. Survival rate 48 hr aft er transfer to 1/8 SW was relatively high in yolk-sac larvae, decrease d gradually to 0% at premetamorphosis, and increased to 100% at metamo rphic climax. The ratio of prolactin (PRL)-immmunoreactive part to who le pituitary increased gradually during larval stages and reached a co nstant level during metamorphosis. When the larvae at premetamorphosis and metamorphic climax and the benthic juveniles were transferred fro m SW to 1/4 SW, PRL-immunoreactive part increased significantly 48 hr after the transfer at all stages examined. Whole-body concentration of cortisol was measured with a modified extraction method which is much robuster to lipid-rich sample than the ordinary method, but no signif icant difference was observed after the transfer. Whole-body concentra tions of thyroid hormones decreased slightly but significantly at prem etamorphosis and metamorphic climax. These results suggest possible in volvement of PRL and thyroid hormones in low-salinity adaptation of th e flounder during metamorphosis and inshore migration.