Light-dependent conversion of body color change pattern during zebrafish development

Zebrafish (Danio rerio) shows body color change in response to light: when light comes, it whitens body color. Our previous study has shown that larval zebrafish as early as 5 days post-fertilization (dpf) have this capacity fully developed whereas 2-dpf larvae exhibit light-induced darkening in sharp contrast (Shiraki et al. 2010). Here, we discovered that the conversion in body color change pattern between 2 dpf and 5 dpf is light-dependent: larvae reared in darkness exhibited light-induced darkening of body color, reminiscent of 2-dpf larvae. We also found that a 14-h light exposure on 3 or 4 dpf is sufficient to trigger this conversion, and that the conversion by light on 4 dpf still occurred after ablation of most exorh-expressing pineal photoreceptor cells. The dark-reared larvae had significantly lower transcription levels of pmch and pmchl, which encode melanin-concentrating hormone precursors, than the LD-reared larvae. However, the levels in larvae that had undergone the conversion (3-L and 4-L) were not significantly different from those in dark-reared larvae, indicating that the reduced pmch/pmchl expression alone does not account for the conversion. In conclusion, light exposure in rearing is indispensable for the maturation of body color change ability in larval zebrafish through a regulatory mechanism independent of exorh-expressing pineal photoreceptors.