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

Abstract

The thyroid hormone levels in female rabbitfish, Siganus guttatus, plasma, eggs, and yolk-sac larvae were measured before and after thyroxine (T 4) injection in female spawners at doses of 1, 10, and 100 μg T 4/g body weight (BW) fish. T 4 and triiodothyronine (T 3) levels in maternal plasma, eggs, and yolk-sac larvae were elevated following T 4 administration. Apparently, there is conversion of T 4 into T 3 in the broodfish which suggests the presence of the enzyme, 5′-monodeiodinase, in rabbitfish. T 4 and T 3 in maternal circulation were easily transferred into the oocytes and subsequently into the larvae. Larvae from spawners treated with 10 and 100 μg T 4/g BW tended to be longer and showed sornewhat better survival compared to the control and those injected with 1 μg T 4/g BW 7 days after hatching. These findings suggest that thyroid hormones may play an important role during early larval development of rabbitfish.