The clocks controlling the tide-associated rhythms of intertidal animals

The living clock that governs tide-associated organismic rhythms has previo usly been assumed to have a fundamental period of approximately 12.4 h, an interval that reflects the average period of the ebb and flow of the tide. But, in 1986, marine chronobiologists began to accumulate laboratory result s that could not be explained by the action of such a clock. Prime among th ese findings was the discovery that, occasionally, one of the two daily pea ks in an organism's rhythm assumed a different period from its partner. Sim ilar results have since been observed in a host of different organisms. The se data led to the circalunidian-clock hypothesis that envisions two basic 24.8 h clocks, coupled together in antiphase, as the driving force for thes e rhythms. There is, however, only a slight difference (50 minutes) in runn ing times between a solar-day clock with a period of approximately 24 h and a lunar-day clock with a period of approximately 24.8 h, both of which dis play "circa" periods that overlap. Here, I postulate that the two clocks ar e fundamentally one and the same. (C) 2000 John Wiley & Sons, Inc.