Thyroid hormones are necessary for teleostean otolith growth

Experiments were conducted to understand the effects of thyroid hormones and thiourea (TU) on otolith growth during metamorphosis of leptocephali. TU is an anti-thyroid hormone drug that inhibits the production of thyroxine (T4) and triiodothyronine (T3) in the thyroid tissue. Fully grown leptocephali of tarpon Megalops cyprinoides collected at the mouth of the Gong-Shy-Tyan Brook, N Taiwan, were immediately treated with 10 ppb T3 and 300 ppm TU for the experimental period of 13 d. The newly grown otolith lengths measured from the maximum radius were 77.0 ± 12.0, 91.9 ± 7.2 and 47.9 ± 6.3 μm in the control, T3 and TU groups, respectively. The newly grown otolith length of the T3 group was significantly longer than those of the control and TU groups, and the control group had significantly larger length values than the TU group. The T3 treatment stimulated otolith growth, increasing daily increments from 7 to 13 cm (control) to 9 to 16 μm (T3) during the growth peak of metamorphosis. Maximum daily increments were only 6 to 10 μm in TU-treated tarpons. The otoliths of the fish treated with T3 and TU had increments deposited daily. Otoliths of T3treated and control fish grew throughout the experimental period, but otoliths of TU-treated fish stopped growth beginning on Day 6 to Day 9. These results suggest that thyroid hormones are essential for otolith growth, probably by regulating the expressions and functions of the otolith protein matrix and ion transporters in the saccular epithelium.