Experimental Analysis of the Mechanism of the Circatidal Clock in Eurylana cookii

Abstract

Although the circatidal rhythm has been studied for more than 50 years, the mechanism of the clock has not yet been identified. In the last two decades, two competing hypotheses were proposed for the mechanism of the circatidal clock. Naylor's hypothesis for the mechanism of the circatidal clock is a 12.4 h oscillator. Palmer proposed that the two active phases per lunar day arise from the cycling of two relatively independent oscillators which each have a 24.8 h period, and are coupled in antiphase (half a lunar day (12.4 hours) or 180° apart). In this thesis, my experiments set out to distinguish between these hypotheses. In the experiments, I demonstrated that the two active phases per lunar day shown by the intertidal isopod Eurylana cookii can be entrained to produce active phases that are separated by time intervals which are different from 12.4 hours. Both phases recur at 24.8 hour (a lunar day, circalunadian cycle) intervals and continued to free-run at the same phase angle when the zeitgeber was removed. Based on my results, Palmer's hypothesis thus explains the circatidal clock better than Naylor's hypothesis does. The aim of this thesis was to test the hypothesis that the two circatidal active phases per lunar day in Eurylana cookii are produced by separate endogenous oscillators. I used three hour pulses of bubbling as artificial zeitgebers to entrain the two active phases produced by Eurylana cookii each lunar day. One of the artificial zeitgebers was shifted to a period of 24.4 h while the other was kept at 24.8 h. While the period remained at 24.4 h, half of the animals entrained to the shifted zeitgebers eventually, although the active phases that were being shifted disappeared at the beginning of the shorter tidal signal. When the shifted active phase had moved about five hours always away from antiphase, the zeitgeber was shifted back to 24.8 hours. The animals then entrained to the new phase relationship and subsequently retained that phase relationship in free run after the tidal signals were removed [...].