Thyroid hormones promote early metamorphosis in grouper (Epinephelus coioides) larvae

The response of grouper larvae to the thyroid hormones, thyroxine (T-4) and triiodothyronine (T-3), was examined. Two-, 3-, and 4-week-old grouper lan ae were reared in seawater containing either T-4 or T-3 at 0.01, 0.1, and 1 ppm. T-4 or T-3 induced metamorphosis in all age groups in a dose-dependent manner. Regardless of the size of the larvae, metamorphosis was completed in 2 days in larvae treated with 1 ppm of either T-4 or T-3; 3-4 days in larvae exposed to 0.1 ppm; and 5-6 days in larvae immersed in 0.01 ppm. None of the fish in the control group completed metamorphosis during this period. Compared with the control fish, survival rates were higher in groups exposed to 0.01 ppm and lower in those exposed to 1 ppm of T-3. In 4-week-old larvae, T-4 treatment (0.01 to 1.0 ppm) resulted in higher survival compared to the control. These results suggest that a dose of 0.01 ppm is appropriate for acceleration of metamorphosis and improvement of survival in 3- and 4-week-old grouper larvae. A lower dose may be apropriate for earlier stages. (C) 1998 Academic Press.