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

Abstract

Color change can serve a number of functions, and can be a singular aperiodic event or a rhythmic process driven by responses to environmental cues or endogenous rhythms. Endogenous pigmentation rhythms have been identified in a number of taxa, with fiddler crabs being the subject of much of this research. Despite a long history of experimental studies of chromatophore-level rhythmicity in fiddler crabs, few studies have considered the entrainment cues for these rhythms or extended findings to whole-organism coloration, information important for understanding the biological properties of circadian systems and understanding the functional significance of these rhythms. This study examined the circadian pigmentation rhythm in the subtropical fiddler crab Uca panacea at both the cellular (melanophore) and organismal levels, including expression in artificial light/dark cycles and constant darkness, entrainment by light/dark and temperature cycles, and relationships between melanophore rhythms and the spectral reflectance of the carapace. On the melanophore level, crabs exhibited a circadian rhythm in pigment dispersion, with maximum dispersion occurring during the day and maximum concentration occurring during the night. This rhythm persisted under ambient or reversed light/dark cycles, with maximum pigment dispersion occurring during the light phase, or under constant darkness. Both light/dark and temperature cycles entrained the rhythm, although light/dark cycles resulted in greater phase shift. The circadian rhythm in melanin dispersion within melanophores is associated with a circadian rhythm in organismal coloration, with carapace reflectance low during the day and high at night. Because of the high absorption of UV radiation by melanin, the functional significance of this rhythm may be as a mechanism of UV‐protection during the day when crabs are exposed to high levels of UV radiation while foraging on open sand flats of the intertidal zone.