The effects of exogenous thyroxine (T4) or triiodothyronine (T3), in the presence and absence of potassium perchlorate, on the incidence of metamorphosis and on serum T4 and T3 concentrations in larval sea lampreys (Petromyzon marinus L.).

Larval sea lampreys (Petromyzon marinus) measuring 100-119 mm in length were exposed to thyroxine (T4; 10 mg liter-1) or 3,5,3'-triiodothyronine (T3; 1 mg liter-1) in the presence and absence of the goitrogen potassium perchlorate (KClO4; 0.01%), for 4-24 weeks. Every 4 weeks, treated and untreated (control) groups of sea lampreys were examined for external signs of metamorphosis and serum was assayed for T4 and T3 concentrations. Precocious metamorphosis was observed following 8, 12, and 24 weeks of KClO4 treatment; however, metamorphosis was not observed in any control, or T4-, T3-, T4+KClO4-, and T3+KClO4-treated larvae. In addition, serum T4 and T3 concentrations were 62 and 72% lower in KClO4-treated individuals than in control animals, respectively. Treatment with exogenous thyroid hormones (TH), in the presence or absence of KClO4, resulted in serum T4 concentrations which were significantly greater (1.2- to 58-fold) than those of the controls in all sampling periods except one, but serum T3 concentrations were not significantly elevated in more than 50% of the cases. TH+KClO4 treatments produced serum T3 concentrations which were significantly greater than those of KClO4-treated animals and never less than those of controls. These data indicate that larval sea lampreys have a tremendous capacity to take up and store exogenous T4 in their serum, but the uptake and/or serum storage of T3 appears to be stringently regulated. Also, the absence of both metamorphosis and a decline in serum TH concentrations in TH+KClO4-treated animals suggests that a decline in serum TH concentrations may be an essential factor contributing to the induction of metamorphosis by KClO4.