Developmental changes in tissue thyroid hormones and cortisol in Japanese sea bass Lateolabrax japonicus larvae and juveniles

Ontogeny of thyroid and interrenal systems were examined in laboratory-rear ed and wild Japanese sea bass Lateolabrax japonicus. In laboratory-reared fish, concentrations of 3,5,3'-triiodothyronine (T-3) several times higher than thyroxine (T-4) were found in newly hatched larva e. Both T-4 and T-3 decreased to low levels during yolk absorption. T-4 con centration started to increase coinciding with the first appearance of thyr oid follicles. In contrast, Tg did not increase until transformation stage. Transient increases were observed in both T-4 and T-3 during transformatio n and early juvenile stages. Cortisol concentration was close to non-detect able level at hatching but started increasing before the completion of yolk absorption and also before the first appearance of interrenal cells examin ed by histological sections. A transient increase in cortisol concentration was found during larval period, followed by a decrease at transformation a nd early juvenile stages, suggesting specific role(s) of cortisol during th e larval period in Japanese sea bass. Wild sea bass were caught in Chikugo river estuary, Ariake Sea. Since wild sea bass at transformation and early juvenile stages showed T-4 and T-3 con centrations similar to those of laboratory-reared fish, thyroid hormones ar e considered to be controlled mainly in a developmentally-programmed manner , not in an environment-related manner, suggesting an involvement of thyroi d hormones in events occurring during the early life history of sea bass. P robably because of the stress induced by sampling pressure, cortisol concen tration in wild sea bass was 10 to 30 times higher than that of laboratory- reared fish. However, the fish caught at the uppermost freshwater station s howed as low cortisol concentration as in laboratory-reared fish.