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 involves not only transformation from larva to juvenile but also migration from offshore areas to estuaries. In the present study, the role of endocrine systems in low-salinity adaptation was examined during early development and metamorphosis of the flounder. Survival rate 48 hr after transfer to 1/8 SW was relatively high in yolk-sac larvae, decreased gradually to 0% at premetamorphosis, and increased to 100% at metamorphic climax. The ratio of prolactin (PRL)-immunoreactive part to whole pituitary increased gradually during larval stages and reached a constant level during metamorphosis. When the larvae at premetamorphosis and metamorphic climax and the benthic juveniles were transferred from 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 significant difference was observed after the transfer. Whole-body concentrations of thyroid hormones decreased slightly but significantly at premetamorphosis and metamorphic climax. These results suggest possible involvement of PRL and thyroid hormones in low-salinity adaptation of the flounder during metamorphosis and inshore migration.