Resetting a circalunar reproduction rhythm with artificial moonlight signals: Phase-response curve and ?moon-off? effect

Abstract

Lunar-rhythmic reproduction in the polychaeteTyposyllis prolifera is controlled by an endogenous circalunar timing system which compares well with circadian oscillators but lacks the degree of precision characteristic of the latters. Artificial moonlight cycles (some successive ‘moonlit’ nights every 30 days) act as zeitgeber. Some details of how ‘moonlight’ resets the reproduction rhythm have been studied: 1. ‘Moonlight’ applied continuously every night does not affect the period of the freerunning rhythm (31 days at 20° C). 2. Two series of pulse experiments were conducted: In the first series, worms (otherwise kept with absolutely dark nights) were perturbed at various circalunar phases by a complex signal consisting of ‘moonlight’ on four successive nights. In the second series, worms (previously kept with constantly ‘moonlit’ nights) were pulsed with simple ‘moon-off’ signals. Both types of experiment resulted in a phase-response curve (pulse-induced phase shifts as function of circalunar phase) that shows the general properties of circadian response curves. Phase shifts (advances and delays) caused by the complex 4-day ‘moonlight’ signals were equal to those effected by simple ‘moon-off’ stimuli given at circalunar phases coinciding with the ends of the complex pulses. 3. It is concluded that the decisive resetting signal for entrainment to a ‘moonlight’ cycle is afforded by the transition from ‘moonlit’ to dark nights (discrete entrainment by ‘moon-off’). The phase-response curves strongly support the idea that the circalunar timing system displays an essentially oscillating nature. The curves allow prediction of the observed phase relation between the entrained reproduction rhythm and the zeitgeber cycle. 4. The significance of the results for interpreting entrainment under natural conditions in a field population is discussed.