Effects of thyroxine on metamorphosis of orange-spotted grouper (Epinephelus coioides) larvae

The response of orange-spotted grouper (Epinepnelus coioides) larvae to thyroxine (T4) was studied. At pre-metamorphosis (Pre-M) and metamorphosis climax (MC), metamorphosis was completed in 7 days for larvae treated with 0.01mg·L -1 of T4, in 4 days for larvae exposed to 0.10mg·L -1 and in 3 days for larvae immersed in 1.00mg·L -1, but none of the fish in the control groups completed metamorphosis during this period. The survival rates of the fish were significantly higher but the total lengths obviously shorter in the T4 treatment group of 0.01mg·L -1 than in the control group at MC,while there was no difference at Pre-M. The survival rates of the fish were significantly lower and the total lengths obviously shorter in the T4 treatment groups of 0.10mg·L -1 and 1.00mg·L -1 than in the control groups both at Pre-M and MC. These results suggested that 0.01mg·L -1 of T4 could accelerate the metamorphosis and increase the survival rate of orange-spotted grouper larvae at metamorphosis climax.