Thyroxine content of eggs and larvae of coho salmon, <i>Oncorhynchus kisutch</i>

Abstract

Abstract The whole‐animal content of the thyroid hormone thyroxine (T 4 ) 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 T 4 (mean ± SEM). At hatching, the total T 4 content decreased to 4.4 ± 0.3 ng, of which 96% remained in the yolk and 4% was distributed in the larva. T 4 content decreased to 1.5 ± 0.1 ng during the 30 days of larval development. The first increases in T 4 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 T 4 from the yolk suggest that T 4 may be critical to normal larval morphogenesis, that initially the T 4 is of maternal origin, and that the hypothalamo‐adenohypophysial‐thyroid axis begins to function independently at or near the time of emergence.