Towards an Understanding of Circatidal Clocks

Abstract

Circadian clocks are an intrinsic element of life that orchestrate appropriately timed daily physiological and behavioural rhythms entrained to the solar cycle, thereby conferring increased fitness. However, it is thought that the first archaic ‘proto-clocks’ evolved in ancient cyanobacteria in a marine environment, where the dominant time cues (zeitgebers) probably would have been lunar-driven and included tidal cycles. To date, non-circadian ‘marine clocks’ have been described with circatidal (~12.4 h), circasemilunar (~14.8 days), and circalunar (~29.5 days) periodicity, mostly studied in accessible but temporally complex intertidal habitats. In contrast to the well-described circadian clock, their molecular machinery is poorly understood, and fundamental mechanisms remain unclear. We propose that a multi-species approach is the most apposite strategy to resolve the divergence that arose from non-circadian clockwork forged in an evolutionary environment with multiple zeitgebers. We review circatidal clock models with a focus on intertidal organisms, for which robust behavioural, physiological, or genetic underpinnings have been explicated, and discuss their relative experimental merits. Developing a comprehensive mechanistic understanding of circatidal clocks should be a priority because it will ultimately contribute to a more holistic understanding of the origins and evolution of chronobiology itself.

Endogenous rhythm variation and adaptation to the tidal environment in the freshwater snail, Semisulcospira reiniana

“…Revealing the molecular mechanisms of the circatidal clock is an ongoing challenge in chronobiology. Given that the first proto-clocks evolved in an aquatic environment, developing a comprehensive understanding of the circatidal clocks should contribute to a more holistic understanding of the evolution of the biological clock (Rock et al, 2022).…”

Network switches and their role in circadian clocks

“…Thus, switch-like nonlinearities are not only necessary for limit cycle generation but also provide additional rhythmicities. The physiological relevance of these harmonics are still debated ( 165 ) and might even link circadian rhythms to tidal oscillations ( 161 , 166 , 167 ).…”

Timely Questions Emerging in Chronobiology: The Circadian Clock Keeps on Ticking

“…While much has been learned about the molecular mechanisms and locations of the circadian clocks that control overall behavior and physiology, much less is known about "other clocks" that allow for anticipation and synchronization to periodic environmental effects with different fundamental frequencies including tidal, lunar, and annual periodicities. Although the existence of endogenous clock systems that help organisms to synchronize to these periodic environmental changes has been demonstrated in many species [102], little is known about the location or underlying mechanisms of the clocks that might control them [103,104]. Perhaps most is known about the clocks that control circatidal rhythms and it appears that the clock system that controls these rhythms appear to be controlled by two phase locked "circadian" clocks (better termed "circalunidian") each with a fundamental period of 24.8 hours yielding two bouts of activity phase locked at 180 degrees and producing behavioral rhythms with a period of 12.4 hours [102,105].…”