Role of thyroid hormones in tilapia larvae (Oreochromis mossambicus): I. Effects of the hormones and an antithyroid drug on yolk absorption, growth and development.

Treatment of one-day old yolksac larvae of tilapia (Oreochromis mossambicus) by immersion in 0.05 ppm T4 or 0.01 ppm T3 significantly accelerated the differentiation and growth of all the fins, particularly pectoral and tail fins. Both the treatments also significantly accelerated yolk absorption and transition to free-swimming activity in the larvae. The treatments also significantly accelerated the growth of the larvae, with T3 at 0.01 ppm having a greater effect than T4 at 0.05 ppm. The yolk conversion efficiency was found not to be significantly affected by the hormone treatments but the treated larvae exhibited an increased heart beat, suggesting metabolic stimulation by the hormones.On the other hand, yolk absorption and free-swimming activity were significantly delayed in tilapia larvae immersed in 25 ppm solution of an antithyroid drug, phenylthiocarbamide (PTC). PTC also retarded the growth of the larvae. T4 (0.05 and 0.10 ppm) or T3 (0.01 and 0.02 ppm) therapy removed the PTC-inhibition,albeit not completely, suggesting that thyroid hormones are involved in the larval growth and development of tilapia.