THYROXINE INJECTION OF FEMALE RABBITFISH (SIGANUS-GUTTATUS) BROODSTOCK - CHANGES IN THYROID-HORMONE LEVELS IN PLASMA, EGGS, AND YOLK-SAC LARVAE, AND ITS EFFECT ON LARVAL GROWTH AND SURVIVAL

The thyroid hormone levels in female rabbitfish, Siganus guttatus, plasma, eggs, and yolk-sac larvae were measured before and after thyroxine (T4) injection in female spawners at doses of 1, 10, and 100 mug T4/g body weight (BW) fish. T4 and triiodothyronine (T3) levels in maternal plasma, eggs, and yolk-sac larvae were elevated following T4 administration. Apparently, there is conversion of T4 into T3 in the broodfish which suggests the presence of the enzyme, 5'-monodeiodinase, in rabbitfish. T4 and T3 in maternal circulation were easily transferred into the oocytes and subsequently into the larvae. Larvae from spawners treated with 10 and 100 mug T4/g BW tended to be longer and showed somewhat better survival compared to the control and those injected with 1 mug T4/g BW 7 days after hatching. These findings suggest that thyroid hormones may play an important role during early larval development of rabbitfish.