ABSTRACT
The effect of exogenous thyroid hormones (TH), thyroxine (T4) or triiodothyronine (T3), on spontaneous metamorphosis and serum T4 and T3 levels was examined in immediately premetamorphic sea lampreys (Petromyzon marinus) from two populations. Size (> or = 120 mm in length and 3.0 g in weight) and a condition factor (CF) of > or = 1.50 were used to predict the number of larvae that were expected to metamorphose. The smallest size and lowest CF found in metamorphosing animals of each population (i.e., the minimum length, weight, and CF) were also used in our assessment. Untreated larvae from Putnam Creek metamorphosed at a larger size (minimums; 134 mm, 4.12 g) than anticipated, out the minimum CF (1.59) and the incidence of metamorphosis (4/5 based on minimums) were consistent with results from the Salmon River population (minimums: 121 mm, 3.15 g, 1.54 CF, 8/9 metamorphosing). In the two experiments, T4-treated animals showed the predicted incidence of metamorphosis (2/2, 10/10), but significantly fewer larvae metamorphosed in the T3-treated groups (1/5, 5/11) than predicted. It was concluded that exogenous T3 administration affected the incidence of metamorphosis. In the treatment groups, serum TH levels in most nonmetamorphosing and metamorphosing animals were significantly higher than controls. Metamorphosing animals exposed to either TH had lower serum TH concentrations than nonmetamorphosing animals. A decline in serum TH levels is an early feature of metamorphosis in lampreys, but the artificial maintenance of elevated serum levels of TH cannot inhibit the decline in spontaneous metamorphosis. If the depression of serum TH levels contributes to the initiation of metamorphic change, the magnitude of the decline is not a contributing factor.
