Light-induced body color change in developing zebrafish.

In response to ambient light levels, many lower vertebrates darken or lighten their body colors by regulating dispersion or aggregation, respectively, of melanin granules (melanosomes) in the melanophore. This physiological reaction is mediated by photoreception in the eyes, the pineal gland, the deep brain and the melanophores themselves, depending on species and their developmental stages. In this study, we established a method for quantitative measurement of the light-induced body color change in zebrafish larvae. From 2 days post-fertilization (dpf), the dermal melanophores responded to light illumination, but the response patterns and temporal profiles changed across the developmental stages. At 2 dpf, light illumination on larvae induced a relatively fast dispersion of the pigments in the melanophores, whereas continuous illumination additionally caused a delayed pigment aggregation at 3 dpf or later stages. Removal of the eyes abolished the light-dependent pigment aggregation but not the pigment dispersion at 5 dpf, while the pigment dispersion at 2 dpf was retained even in the isolated tail. These results suggest that the pigment dispersion is triggered by photoreception intrinsic to the melanophores and that the pigment aggregation is mediated by photoreception in the eyes. The monitoring system developed in this study will be useful to understand the neural mechanisms underlying the body color change depending on the ocular system. We also discussed the putative role(s) of opsin-type photoreceptive molecules in the light-induced body color change of the larval zebrafish.