Thyroxine content of eggs and larvae of coho salmon, Oncorhynchus kisutch

Abstract

The whole‐animal content of the thyroid hormone thyroxine (T₄) was measured in embryonic and larval coho salmon (Oncorhynchus kisutch) by radioimmunoassay following the validation of an extraction method. The embryonic yolk, at 20 days after fertilization and 10 days before hatching, was found to have 5.3 ± 0.3 ng T₄ (mean ± SEM). At hatching, the total T₄ content decreased to 4.4 ± 0.3 ng, of which 96% remained in the yolk and 4% was distributed in the larva. T₄ content decreased to 1.5 ± 0.1 ng during the 30 days of larval development. The first increases in T₄ were detected within 2 weeks following emergence from the substrate of the actively feeding juvenile; the newly emerged salmon responds significantly to bovine TSH. The presence and clearance of T₄ from the yolk suggest that T₄ may be critical to normal larval morphogenesis, that initially the T₄ is of maternal origin, and that the hypothalamo‐adenohypophysial‐thyroid axis begins to function independently at or near the time of emergence.