Spontaneous and induced metamorphosis in the American brook lamprey, Lampetra appendix

We described the seven stages of spontaneous metamorphosis in the American brook lamprey (Lampetra appendix) and assessed the importance of size as a determinant of spontaneous and induced metamorphosis. Serum concentrations of the thyroid hormones (TH) thyroxine (T-4) and triiodothyronine (T-3) wer e measured in metamorphosing and nonmetamorphosing L. appendix. The sequenc e of stages in metamorphosis and changes in the relative lengths of most bo dy regions were consistent with data reported for other lamprey species. Ho wever, premetamorphic and metamorphosing L. appendix in the early stages of metamorphosis (1-3) were much larger in size (at least 155 mm and 5.40 g) than has been observed for other lamprey species. Serum concentrations of T -4 and T-3 were high by the end of the larval period and declined significa ntly by stage 2 of metamorphosis. Larvae >=130 mm in length were treated wi th either potassium perchlorate (KClO4; 0.01 and 0.05%) or 10 mg/L propylth iouracil (PTU; 0.0001%) for 117 days from September to January to determine if metamorphosis could be induced by these goitrogens. Both concentrations of KClO4 successfully induced metamorphosis in L. appendix, but the incide nce of metamorphosis (62%) was much lower than reported in sea lampreys (98 %) of comparable size. Serum concentrations of T-4 and T-3 declined by 64-7 6 and 93-96% relative to control values, respectively, in metamorphosing an d nonmetamorphosing L. appendix treated with KClO4. PTU elicited declines o f 55% for T-4 and 80% for T-3, but only one animal metamorphosed. Based on these data, we conclude that a decline in serum TH levels is necessary for metamorphosis in L. appendix, but not sufficient by itself to trigger the p rocess.