Abstract
How animals precisely time behaviour over the lunar cycle is a decades-old mystery. Experiments on diverse species show this behaviour to be endogenous and under clock control but the mechanism has remained elusive. We present new experimental and analytical techniques to test the hypotheses for the semilunar clock and show that the rhythm of foraging behaviour in the intertidal isopod, Scyphax ornatus, can be precisely shifted by manipulating the lengths of the light/dark and tidal cycles. Using light T-cycles (Tcd) the resultant semilunar beat period undergoes shifts from 14.79 days to 6.47 days under T = 23 hours (h), or to 23.29 days under T = 24.3 h. In tidal T-cycles (Tt) of natural length Tt = 12.42 h, the semilunar rhythm is shifted to 24.5 days under Tt = 12.25 h and to 9.7 days under Tt = 12.65 h. The implications of this finding go beyond our model species and illustrate that longer period rhythms can be generated by shorter period clocks. Our novel analysis, in which periodic spline models are embedded within randomization tests, creates a new methodology for assessing long-period rhythms in chronobiology. Applications are far-reaching and extend to other species and rhythms, potentially including the human-ovarian cycle.
Highlights
Persistent lunar and semilunar rhythms have been observed and described in plants, algae and animals for decades[1,2,3]
Field studies have described the accurate timing of lunar reproduction in marine organisms such as the Palolo worm (Eunice viridis)[5,6,7], and laboratory studies of reproduction in the marine insects Clunio and Pontomyia have shown that the endogenous lunar clock is resistant to manipulations in period because it is temperature compensated[10, 11]
Even the most recent molecular techniques, which inhibit the Casein kinase 1δ/ε enzyme to disrupt the circadian clock while leaving the circatidal and circalunar timing systems in place[7, 19], would not naturally resolve the semilunar clock mechanism, because abolition of the circadian clock would remove the semilunar rhythm under at least two of the proposed semilunar mechanisms
Summary
Persistent lunar (circa 29.5 day) and semilunar (circa 14.7 day) rhythms have been observed and described in plants, algae and animals for decades[1,2,3]. Laboratory experiments on diverse species have shown these behaviours to be endogenous[4,5,6,7], under clock control, and able to be entrained[8], but the mechanisms, physical location and molecular nature of the controlling clocks remain almost totally unknown[9] This is in stark contrast to the study of the daily circadian clock which is well characterized in terms of behaviour, physiology and molecular clock gene machinery. The critical contemporary question is the neurobiological and molecular-genetic identity of these non-circadian clocks Progress in this direction has included using circadian clock gene knockouts which disrupt circadian behaviour but not tidal behaviour and suggest the existence of an independent 12.4 hour oscillator[17,18,19], and identifying potential metabolic markers of the tidal clock[20]. We sought to use foraging behaviour to distinguish among the three hypotheses for the mechanism of the semilunar rhythm in a series of experiments using artificially long and short circadian and circatidal T-cycles (see Fig. 1)
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