Natural and artificial rephasing of a tidal rhythm

Abstract

1. Populations of the sand-beach isopod,Excirolana chiltoni, show rhythmic swimming activity, under non-tidal conditions in the laboratory. The underlying period of this rhythm approximates the double-tidal interval of 24.8 hours rather than the tidal interval of 12.4 hours. 2. Three patterns in swimming activity were observed: 1) diurnal activity, in which one peak of activity was observed each day; 2) semidiurnal equal activity, in which two peaks of equally intense activity were observed, and; 3) semidiurnal unequal activity in which a high level activity peak alternating with a low level activity peak was observed. The ability to perform each of these patterns resides in single individuals. 3. The pattern of activity which was observed in any particular experiment reflected the wave form of the tide on the day preceding confinement of the population in the laboratory (Figs. 1 and 2). 4. The application of turbulence cycles, in the laboratory, which simulate wave action on the beach can phase shift the ongoing rhythm of a population and can initiate rhythmic activity in previously arhythmic populations. One stimulus application per day initiates diurnal activity while two stimuli evoke semidiurnal activity. The effect of turbulent stimuli persists for several days after cessation of treatment indicating a long lasting conditioning effect of these stimuli on the pattern of endogenous activity (Figs. 6–10).