THYROID-HORMONE SURGES DURING MILKFISH METAMORPHOSIS

Both thyroxine (T4) and triiodothyronine (T3) were detected in fertili zed eggs and larvae of milkfish. The concentration of T4 was lower tha n that of T3 only in fertilized eggs and newly-hatched larvae. The tot al body concentrations of both thyroid hormones decreased after hatchi ng. The T4 concentration fluctuated at relatively low levels during th e first week after hatching, increased gradually during the next 2 wee ks and was highest in 4-week-old larvae. On the other hand, T3 was und etectable in samples taken between days 3 and 11, showed a gradual inc rease beginning on day 13 and was highest in 27 day old larvae. The co ncentrations of both T4 and T3 declined again in 31-day-old juveniles. The peak corresponds with the development of silver coloration in hat chery-reared milkfish and the inshore migration of larvae in the wild, suggesting a role for thyroid hormones during early development of th is species.