Correlation between environmental iodide concentrations and larval growth, survival, and whole body concentrations of thyroid hormones and cortisol in Pacific threadfin (Polydactylus sexfilis)

Pacific threadfin (Polydactylus sexfilis), known locally by its Polynesian name “moi”, is rapidly becoming a premier aquaculture species in Hawaii and throughout the Indo-Pacific. Nevertheless, threadfin culture is suffering from extraordinary loss of seed stock during the pre-metamorphic and metamorphic periods, and from dramatic size variation between animals after metamorphosis that leads to cannibalism. The objective of the present study was to examine effects of iodide concentration of rearing tanks on larval growth, survival, metamorphosis, and whole body concentrations of thyroid hormones and cortisol. Threadfin larvae raised in ocean water grew significantly larger, and showed increased survival compared with larvae raised in water from a seawater-injected well that had lower iodide concentrations. Larvae raised in ocean water developed more rapidly than in well seawater. At 14 days post-hatch, 50% of the larvae raised in ocean water reached flexion-stage, compared to 15% in well seawater. In ocean water-raised larvae, whole body thyroxine (T4) concentrations increased sharply from 0.4–0.5 ng/g at hatching through day 13 post-hatch to 1.9 ng/g on day 15, and decreased gradually to 0.5 ng/g by day 23. Larvae raised in well seawater did not show a peak in T4, as levels increased gradually from 0.2 ng/g on day 1 to 2.4 ng/g by day 25. Profiles of triiodothyronine (T3) were similar between the two groups, decreasing from 0.17 ng/g at 8 h before hatching to 0.02–0.04 ng/g by day 7 post-hatch, and then gradually increasing to 0.45 ng/g by day 23. Whole body concentration of cortisol was 0.5 ng/g prior to hatching for both groups, increasing to 27–30 ng/g by day 5. Cortisol levels fluctuated during days 7 to 25 post-hatch between 12 and 26 ng/g for ocean water-raised larvae, and between 14 and 32 ng/g for larvae raised in well seawater. Absence of a peak in the T4 profile in well seawater-raised larvae may indicate incomplete synchronization of development or metamorphosis. These results suggest the importance of environmental iodide in larval metamorphosis, and subsequent survival and growth, possibly through the synthesis of thyroid hormones.