Production of symmetrical flatfish by controlling the timing of thyroid hormone treatment in spotted halibut Verasper variegatus.

The thyroid hormone plays an essential role in the metamorphosis in flatfish, during which external asymmetry (for example, eye relocation and pigmentation) is established. However, no information is available on the expression mechanisms of metamorphic asymmetry. Since the presence of malformed juveniles (having either ocular or blind side characteristics on both sides) was known in spotted halibut (Verasper variegatus), the effect of the timing of thyroid hormone treatment was investigated. When thyroxine (T4, 10 or 30 ng/ml) was administered to the hypothyroid larvae (continuously receiving 30 microg/ml thiourea) from 25 days after hatching (DAH), the occurrence of one type of symmetrical juvenile (symmetrical pseudoalbino having blind side characteristics on both sides) increased to more than three times, and the occurrence was significantly greater than that observed in the control group. In the fish in which T4 treatment was initiated prior to 15 DAH or after 60 DAH, the occurrence of another type of symmetrical juvenile (symmetrical ambicolorate having ocular side characteristics on both sides) became more than two times, and its occurrence was significantly greater than that observed in the control group. These results suggest that both sides of the larval body independently possess the potential to become either the ocular or the blind side of the juvenile body and that the timing of thyroid hormone increase is the determining factor of "which side to become." Based on previous studies, since thyroid hormone is expected to induce blind side characteristics, the differential responsiveness to thyroid hormone-longer in the left side (blind side in normal juvenile) than the right-is strongly suggested as the central mechanism for metamorphic asymmetry in spotted halibut.