Preadaptive Role of Thyroid Hormones in Larval and Juvenile Salmon: Growth, the Gut and Evolutionary Considerations

Abstract

SYNOPSIS. The argument is advanced that a fundamental role of thyroid hormones is to preadapt animals to exploit new habitats by inducing developmental changes in the gut and other structures associated with the acquisition and transport of nutrients. Two early life transitions of migratory salmonids are examined: first, larval development and the transition from dependence on yolk reserves to active feeding; and second, smoltification and adaptation to seawater. These developmental phases are similar in that thyroid hormone levels are elevated for a period immediately preceding changes in the animal's gut and habitat. In turn, another period of elevated thyroid hormone levels accompanies adaptation to the new environment where growth rate accelerates. Salmon that stop growing after entering seawater before smoltification occurs show reduced thyroid function and nutrient transport capabilities. These considerations might provide a point of departure for re-examining the functions of thyroid hormones, if any, in jawless fishes and lower chordates. The metamorphosing lamprey, amphioxus, and tunicates are discussed as excellent model systems in which to test whether thyroid hormones induce important developmentally specific events in the gut and associated structures.