Serum concentrations of thyroid hormones in KClO 4-treated larval sea lampreys ( Petromyzon marinus L.)

Abstract

A description is provided of serum thyroxine (T 4) and triiodothyronine (T 3) concentrations in three size-groups of untreated (control) larval sea lampreys ( Petromyzon marinus) and similar size groups subjected for 117 days to two concentrations of KClO 4, a goitrogen which induces metamorphosis in some individuals (Holmes and Youson, 1993). Serum T 4 and T 3 concentrations in controls of the 65–95, 110–119, and the > 130 mm groups showed a general increase with increased animal size. In almost all cases following treatment with KClO 4, serum T 4 and T 3 levels were similar in unmetamorphosed and metamorphosing animals and were not influenced by the concentration of the goitrogen. Serum T 3 levels of the KClO 4-treated groups were significantly lower ( P < 0.05) than those of the controls in all size groups (91–95% decrease), whereas serum T 4 levels in only the smallest and intermediate KClO 4-treated groups were significantly lower than control values (30–32% and 27–31% decrease, respectively). In the > 130 mm group, in which all but one animal was metamorphosing, serum T 4 concentration declined by a statistically insignificant 18–22%, which is a less pronounced decline than has been described for spontaneous metamorphosis. The size-dependent incidence of metamorphosis in larval P. marinus following KClO 4 treatment cannot be explained by the absence, or variation in the degree, of decline in serum T 4 and T 3.