Internal circadian clocks allow organisms to anticipate the regular environmental changes associated with the cycles of day and night. These circadian clocks are driven by a set of regulatory genes that comprise negative and positive transcriptional and translational feedback loops, oscillating in activity over a period of 24 h. Many of these genes are shared between insect and mammalian models, suggesting a common origin of circadian clocks among animals.

Many organisms also exhibit rhythms of longer or shorter period lengths. For example, species inhabiting coastal environments, from brown algae and corals to worms and vertebrates, synchronize their maturation and spawning not only to particular times of day, but also to a particular moon phase or specific seasons within a year, responding to moonlight and photoperiod cues, respectively. Two recent studies shed light on the relationship of the circadian clock to other endogenous clocks.

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A study led by Kristin Tessmar-Raible (University of Vienna, Austria) examined the interactions between the endogenous circadian and circalunar (monthly) clocks in the bristle worm (Platynereis dumerilii). In a distinct nucleus of the worm's forebrain, levels of core circadian genes oscillate not only over 24 h, but also across different phases of the lunar month. The period length and strength of locomotor behaviors that are controlled by the circadian clock, such as spawning, change over different phases of the circalunar clock as well (Cell Rep. doi:10.1016/j.celrep.2013.08.031; published online 26 September 2013 ). “Our results suggest that the bristle worm possesses independent, endogenous monthly and daily body clocks that interact,” says Tessmar-Raible.

Another study shows that the intertidal crustacean Eurydice pulchra, also known as the sea louse, is guided by not only circadian rhythms but also tidal rhythms (Curr. Biol. doi:10.1016/j.cub.2013.08.038; published online 26 September 2013). Metabolism, reproduction and swimming behavior show cycles of 12.4 h in parallel to their 24-h cycles. Charalambos Kyriacou (University of Leicester, UK) and his colleagues at MRC Laboratory of Molecular Biology (Cambridge, UK) and Aberystwyth University (UK) reported that the tidal rhythm meets the three criteria that define a biological clock: it is sustained in constant conditions, can be entrained by the appropriate stimuli and shows temperature compensation. They further showed that environmental and molecular signals that interfere with circadian timing do not affect tidal timing. Says Kyriacou, “The identification of the tidal clock as a largely separate mechanism now presents us with an exciting new perspective on how coastal organisms define biological time.”