Concentrations of insulin and thyroid hormones in the serum of landlocked sea lampreys (Petromyzon marinus) of three larval year classes, in larvae exposed to two temperature regimes, and in individuals during and after metamorphosis.

Serum concentrations of thyroxine (T4) triiodothyronine (T3), and insulin were measured in landlocked sea lampreys (Petromyzon marinus) of three larval year classes (II to IV), larvae in two fixed temperatures (13 and 21 degrees), individuals during metamorphosis (stages 3-7), and juveniles immediately following metamorphosis. There was no difference in serum insulin levels in the larval year classes but the concentrations of both T4 and T3 increased during the protracted larval life to reach a maximum in individuals of metamorphosing size (age) without initiating early (pro) metamorphic events. A lower temperature depressed serum insulin and elevated T4 levels but had no effect on T3. As expected, metamorphosis was marked by a sharp decrease in both serum T4 and T3 by stage 3. Insulin did not change until stage 6 when an elevated serum concentration probably corresponded with development of the endocrine pancreas. Serum T4 and T3, but not insulin, levels decreased during feeding in the early juvenile period. Serum concentrations of these hormones during larval growth, varied temperature exposure, and metamorphic development provide some new insight into the intricacies of lamprey metamorphosis.