Abstract
Ontogeny of thyroid and interrenal systems were examined in laboratory-reared and wild Japanese sea bass Lateolabrax japonicus.
In laboratory-reared fish, concentrations of 3, 5, 3'-triiodothyronine (T3) several times higher than thyroxine (T4) were found in newly hatched larvae. Both T4 and T3 decreased to low levels during yolk absorption. T4 concentration started to increase coinciding with the first appearance of thyroid follicles. In contrast, T3 did not increase until transformation stage. Transient increases were observed in both T4 and T3 during transformation and early juvenile stages. Cortisol concentration was close to non-detectable level at hatching but started increasing before the completion of yolk absorption and also before the first appearance of interrenal cells examined by histological sections. A transient increase in cortisol concentration was found during larval period, followed by a decrease at transformation and early juvenile stages, suggesting specific role (s) of cortisol during the 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 T4 and T3 concentrations similar to those of laboratory-reared fish, thyroid hormones are considered to be controlled mainly in a developmentally-programmed manner, not in an environment-related manner, suggesting an involvement of thyroid hormones in events occurring during the early life history of sea bass. Probably because of the stress induced by sampling pressure, cortisol concentration in wild sea bass was 10 to 30 times higher than that of laboratoryreared fish. However, the fish caught at the uppermost freshwater station showed as low cortisol concentrtion as in laboratory-reared fish.
