Changes in iodide and thyroid hormone levels of hatchery-reared orange-spotted rabbitfish Siganus guttatus (Bloch 1787) during early larval development
This paper elucidates the developmental profiles of iodide and thyroid hormones, thyroxine (T<sub>4</sub>) and triiodothyronine (T<sub>3</sub>) in hatchery-reared rabbitfish (Siganus guttatus) during early larval development. The study evaluated the levels of iodide, T<sub>4</sub> and T<sub>3</sub> tissue content in rabbitfish larvae at 0, 10, 20, 30, 40 and 50 days after hatching (DAH). Under normal rearing conditions, iodide (11.98 ± 4.3 nmol g<sup>−1</sup>) and thyroid hormones (T<sub>4</sub>: 0.09 ± 0.01 nmol g<sup>−1</sup>; T<sub>3</sub>: 0.01 ± 8.5E-05 nmol g<sup>−1</sup> were already detected from newly-hatched larvae and their presence has been attributed from maternal origin. Iodide level sharply peaked at 10 DAH (1416.43 ± 149.6 nmol g<sup>−1</sup>) which is significantly higher compared to the iodide levels in the larvae at 20, 30, 40 and 50 DAH. The levels of THs gradually increased as the larvae developed with T<sub>4</sub> (0.96 ± 0.05 nmol g<sup>−1</sup>) and T<sub>3</sub> (0.03 ± 0.004 nmol g<sup>−1</sup>) exhibiting a peak on 20 and 30 DAH, respectively, which coincided with the onset of metamorphosis. Thyroid hormone levels gradually decreased which coincided also with the completion of metamorphosis. The present findings indicated a pattern of the rise and fall in iodide and thyroid hormone levels during larval development and its role in metamorphosis of rabbitfish larvae. The study has reported for the first time, the changes in iodide and thyroid hormone levels during the early developmental stage in hatchery-reared rabbitfish and the results are in agreement with those vital actions of thyroid hormones in other fish species.
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