ROLE OF THYROID HORMONES IN FISH LARVAL GROWTH AND DEVELOPMENT : SOME STUDIES

Abstract

In the present study, the effects of thyroid hormones, L-thyroxine (T4) and triiodo-L-thyronine (T3) on larval growth and development were studied in three species of freshwater teleosts namely, tilapia (Oreochromis mossambicus), dwarf gourami (Colisa lalia) and exophthalmic black goldfish, popularly known as telescopic-eye black moor goldfish (Carassius auratus). In tilapia, treatment of one-day-old larvae by immersion in 0.05 ppm T4 or 0.01 ppm T3 significantly elevated the levels of T4 and T3 in the larvae compared to the control. These hormones 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 al so significantly accelerated the growth of the larvae, with T3 at 0.01 ppm having a greater effect than T4 at 0.05 ppm. However, prolonged treatment of T3 up to 40 days resulted in fin abnormalities. A higher dose of T4 (0.50 ppm) caused skeletal abnormalities and retarded growth in the larvae. 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) and T3 (0.01 and 0.02 ppm) replacement removed the PTC-inhibition (albeit not completely), suggesting that thyroid hormones are indeed involved in the larval growth and development of tilapia, O. mossambicus. T4 and T3 have also been found to delay hatching in tilapia eggs and this was found to be mediated through inhibition of the secretion of the hatching enzyme. In goldfish, T4 (0.05 ppm) and T3 (0.01 ppm) treatments significantly accelerated the differentiation of fins in larvae as well as formation of scales, and development of black coloration and exophthalmia (telescoping of eyes) in fry. They were also found to accelerate the growth of both larvae and fry. Similarly, in dwarf gourami, T4 (0.05 ppm) significantly promoted larval growth. The gourami larvae, unlike those of tilapia and goldfish, suffered heavy mortalities, and thyroid hormones were able to enhance their survival. Thyroid hormones profiles were determined during the early development of tilapia. Both T4 and T3 were present in significant amounts in fertilized eggs. There was a steady decrease in both T4 and T3 levels during embryonic development. The levels continued to decline after hatching until around 7 days later when most of the yolk had been absorbed. The T4 level started to rise after around 7 days, suggesting that the larval thyroid had begun to secrete T4 at this time, and this coincided with the period of faster growth of the larvae. The T3 level remained fairly constant until around 20 days after which there was a significant rise in the level. In vitro determination of 5'-monodeiodinase activity (5'-D activity) in the whole-body homogenates of larvae at different stages of early development showed that the enzymatic conversion of T4 to T3 was not detectable in eggs and 3-day-old larvae but detected in 5-day-old and older larvae. There was a gradual increase in the Vmax as development proceeded indicating increasing 5'-D activity during larval development. The Km values did not differ significantly in the different stages- of development. These results are discussed in relation to the T4 and T3 profiles and to the growth and development of the larvae.