CONCENTRATIONS OF INSULIN AND THYROID-HORMONES IN THE SERUM OF LANDLOCKED SEA-LAMPREYS (PETROMYZON-MARINUS) OF 3 LARVAL YEAR CLASSES, IN LARVAE EXPOSED TO 2 TEMPERATURE REGIMES, AND IN INDIVIDUALS DURING AND AFTER METAMORPHOSIS

Serum concentrations of thyroxine (T-4) triiodothyronine (T-3), and in sulin were measured in landlocked sea lampreys (Petromyzon marinus) of three larval year classes (II to IV), larvae in two fixed temperature s (13 and 21 degrees), individuals during metamorphosis (stages 3-7), and juveniles immediately following metamorphosis. There was no differ ence in serum insulin levels in the larval year classes but the concen trations of both T-4 and T-3 increased during the protracted larval li fe to reach a maximum in individuals of metamorphosing size (age) with out initiating early (pro) metamorphic events. A lower temperature dep ressed serum insulin and elevated T-4 levels but had no effect on T-3. As expected, metamorphosis was marked by a sharp decrease in both ser um T-4 and T-3 by stage 3. Insulin did not change until stage 6 when a n elevated serum concentration probably corresponded with development of the endocrine pancreas. Serum T-4 and T-3, but not insulin, levels decreased during feeding in the early juvenile period. Serum concentra tions of these hormones during larval growth, varied temperature expos ure, and metamorphic development provide some new insight into the int ricacies of lamprey metamorphosis. (C) 1994 Academic Press, Inc.