Developmental changes in melanophores and their asymmetrical responsiveness to melanin-concentrating hormone during metamorphosis in barfin flounder (Verasper moseri).

Barfin flounder larvae exhibit unique black coloration, as well as left-right asymmetry in juvenile stage as in other flatfish. In this study, we first assessed the changes in melanophores with development and then investigated their responsiveness to melanin-concentrating hormone (MCH) during metamorphosis. Larval-type melanophores appeared on both sides of the body before metamorphosis, whereas adult-type melanophores appeared only on the ocular side after metamorphosis. Even in the individuals of this species displaying black coloration, the density of larval-type melanophores was similar to that in transparent larvae of other species. However, unlike in transparent larvae, larval-type melanophores completely dispersed in the black larvae of this species. Therefore, the black coloration during larval stages was mainly due to dispersion, and not the density, of larval-type melanophores. In vitro MCH treatment revealed, for the first time, the responsiveness of melanophores in larval stages. On the ocular side, larval-type melanophores aggregated against MCH during larval stages, while, in the larvae at later metamorphic stages and in juveniles, larval-type melanophores did not aggregate, although aggregation of adult-type melanophores was noted. In contrast, on the blind side, the responsiveness of larval-type melanophores to MCH was consistently present from larval to juvenile stages. The metamorphic transition of MCH responsiveness from larval- to adult-type melanophores only on the ocular side suggests the larval (therefore, immature) nature of the blind side skin. We propose that the inhibited development, and thus the retention of the larval-type skin leads to the formation of the blind side characteristics and is the central mechanism for the flatfish asymmetry.