The circadian clock is a molecular network that translates predictable environmental signals, such as light levels, into organismal responses, including behavior and physiology. Regular oscillations of the molecular components of the clock enable individuals to anticipate regularly fluctuating environmental conditions. Cnidarians play important roles in benthic and pelagic marine environments and also occupy a key evolutionary position as the likely sister group to the bilaterians. Together, these attributes make members of this phylum attractive as models for testing hypotheses on roles for circadian clocks in regulating behavior, physiology, and reproduction as well as those regarding the deep evolutionary conservation of circadian regulatory pathways in animal evolution. Here, we review and synthesize the field of cnidarian circadian biology by discussing the diverse effects of daily light cycles on cnidarians, summarizing the molecular evidence for the conservation of a bilaterian-like circadian clock in anthozoan cnidarians, and presenting new empirical data supporting the presence of a conserved feed-forward loop in the starlet sea anemone, Nematostella vectensis. Furthermore, we discuss critical gaps in our current knowledge about the cnidarian clock, including the functions directly regulated by the clock and the precise molecular interactions that drive the oscillating gene-expression patterns. We conclude that the field of cnidarian circadian biology is moving rapidly toward linking molecular mechanisms with physiology and behavior.
Bibliographical noteFunding Information:
This work was supported by the United States – Israel Binational Science Foundation (award 2011187 to A.M.T. and O.L.), the National Institutes of Health/National Institute of Child Health and Human Development award (HD062178 to A.M.R.), and generous funding from the University of North Carolina at Charlotte (to A.M.R.). This article resulted from the symposium ‘‘Keeping Time During Animal Evolution: Conservation and Innovation of the Circadian Clock’’ presented at the 2013 Annual Meeting of the Society of Integrative and Comparative Biology. The symposium was supported by the Society of Integrative and Comparative Biology and award 1239607 from the Integrative Organismal Systems Program at the National Science Foundation.