Ecological physiology of the circadian pigmentation rhythm in the fiddler crab Uca panacea

► I examined circadian pigmentation rhythms in the fiddler crab Uca panacea. ► Both cellular (melanophore) and organismal rhythms were examined. ► Pigment dispersion was maximal (and carapace reflectance minimal) during the day. ► The rhythm was entrained by LD cycles and, to a lesser extent, temperature cycles. ► The functional significance of the rhythm may be for UV‐protection.