Thyroid hormones are necessary for the metamorphosis of tarpon Megalops cyprinoides leptocephali

Abstract

This study investigates the effects of thyroxine (T 4), triiodothyronine (T 3) and thiourea (TU) on the metamorphosis of tarpon Megalops cryprinoides leptocephali. TU is an anti-thyroid hormone drug that inhibits the production of T 4 and T 3 in the thyroid tissue. Fully grown tarpons leptocephali were collected at the river mouth and, in the laboratory, were immediately treated with 100 ppb T 4, 10 ppb T 3, or 300 ppm TU. The appropriate concentrations were validated in a preliminary dose response experiment. Morphological and physiological characteristics that indicate metamorphic processes were measured every 2 days. T 4 and T 3 slightly speeded up the metamorphosis of tarpons compared with the control group. The experimental treatments produced accelerated reductions in length, increases in head/body ratio, swimbladder development, and loss of body water and sodium. In contrast, TU treatment caused metamorphic stasis with complete inhibition of metamorphosis between days 6 and 8. Thyroid hormone treatment stimulated fast otolith growth while TU treatment stopped otolith growth between days 6 and 9. Leptocephali in T 4, T 3 and control groups completed metamorphosis in 10–14 days, but TU-treated tarpons remained in the metamorphic leptocephalus stage more than 22 days. In addition, the inhibition of leptocephalus metamorphosis by 300 ppm TU can be reversed in the presence of 10 ppb T 3. These results indicate that thyroid hormones are involved in regulating the metamorphosis of leptocephali.