Changes in tissue thyroid hormone levels of metamorphosing spotted halibut Verasper variegatus reared at different temperatures

In 1999, spotted halibut Verasper variegatus were experimentally reared at 9, 12, 15, 18, and 21°C from eggs through early juveniles at the Miyako Station of Japan Sea-Farming Association in Iwate Prefecture. Tissue concentrations of thyroxine (T4) increased to maximum levels during metamorphic climax (H-stage) at all the temperatures. The peak T4 levels were higher at lower temperatures, whereas no relationship was observed between tissue 3,5,3′-triiodothyronine levels and rearing temperatures. However, epithelial cell height of thyroid follicles and the number of vacuoles in colloid were not different among rearing temperatures. Although the peak T4 levels were different, thyroid activity to produce T4 might not be so different at all temperatures. Therefore, much T4 observed at low temperatures was possibly due to the accumulation of the unused hormone in blood and tissue.