Systemic Levels of Thyroid Hormones and Insulin in Landlocked Sea Lampreys, Petromyzon marinus, during the Juvenile Feeding Period

Abstract

Thyroxine (T 4), triiodothyronine (T 3), and insulin concentrations were measured by radioimmunoassay in the serum of feeding juvenile Petromyzon marinus in Lake Huron between May and August. There was no correlation of hormone concentrations with nutritional status, gender, and condition factor over a period of 4 months when there is active feeding and growth. Only serum insulin concentration varied significantly between two of the monthly samples; the overall mean for juveniles (2.80 ± 0.21 ng/ml) was significantly greater than the mean for upstream migrants (1.70 ± 0.24). Higher serum T 3 levels showed weak correlations with increasing length ( r = 0.32) and weight ( r = 0.33), but T 4 and insulin concentrations had no relationship with these parameters. The mean concentrations of T 4 and T 3 were 4.60 ± 0.18 and 0.354 ± 0.016 nmoles/liter, respectively, in the feeding juveniles. These data show a general trend for stabilized levels of T 4, T 3, and insulin in the serum during the juvenile period of P. marinus.