Cnidarian circadian clocks model how animals find predictability in a complex world.

Circadian clocks are a fundamental mechanism through which organisms anticipate and respond to daily cycles in light, temperature and other environmental features. Clocks regulate daily rhythms in behavior and metabolism, and in some animals, this includes strong causal relationships between daily activity cycles and metabolic demands. Several components of animal circadian clocks are uniquely shared between cnidarians and bilaterian animals, and the complexity of sensory systems and circadian regulation have increased in parallel within these groups. However, the specific drivers of this innovation, and potential evolutionary correlations between sensory and circadian systems, are poorly known. This review examines the diversity of circadian regulation in cnidarians in a comparative context with a specific emphasis on environmental entrainment. We discuss mechanisms of circadian photoentrainment across domains of life and within Metazoa, molecular components of circadian clocks that are shared between bilatarians and non-bilaterian animals, and the robustness of cnidarian clocks in comparison with other animals. We also suggest how emerging methods and creative approaches can be applied to improve our understanding of how environmental signals are perceived by cnidarians and integrated into the circadian regulatory machinery.