Abstract
The past few decades have seen a burgeoning of the science of chronobiology with the characterization of circadian clocks and their molecular basis. During the same timeframe, understanding of the nature of circannual, circalunar, circasemilunar, and circatidal clocks has, however, lagged behind, as the editors of this book acknowledge in the subtitle by referring to them as “enigmatic.” Early chronobiologists, when suggesting that animals and plants may have evolved internal clockwork enabling them to anticipate cyclical environmental events, were often challenged. It was not until the 1970s that the notion of endogenous biological clocks came to be fully established. Subsequently, while there has been ready acceptance that organisms have adapted to daily environmental cycles related to the sun, there has been some reluctance to consider the possibility that there may have been adaptation to environmental cycles associated with the moon. Indeed, some models of circatidal rhythms postulate, as in the chapter by Chabot and Watson in the present volume, that they are driven by circadian oscillators operating in antiphase rather than by distinct clocks operating at circatidal periodicity. However, some recent studies have also begun to acknowledge that circatidal and circalunar clockwork can be distinguished from circadian systems, as reviewed in chapters by Last and Hendrick, as well as by Zantke, Oberlerchner and Tessmar-Raible. For example, molecular techniques have been used to stop the circadian clock of a marine crustacean without affecting its circatidal clock. Moreover, molecular characterization of circatidal and circalunar clockwork has begun, as reported by Last and Hendrick, who discuss an array of noncircadian biological rhythms in polychaete worms, considering the worms to be the “ultimate marine chronometers.” Polychaetes evolved in tidal seas before the great Cambrian explosion 600 million years ago, and it is argued that their biological clockwork predates or at least coevolved with the circadian molecular clockwork that is so well understood today.
As an authoritative and impressive review of our current knowledge of the noncircadian field of chronobiology, comprising 16 reviews by specialists in the field, the book provides fertile ground from which future studies can grow. Repeatedly, the authors emphasize new and stimulating questions for investigation by current and intending researchers in the field. Notably, there is recognition of the need for fuller characterization of circatidal and circalunar clockwork in relation to the known molecular basis of circadian rhythms. In parallel, referred to in the opening chapter by the late Dietrich Neumann, there is also a need for fuller understanding of the molecular basis of moon-related synchronization of tidal and lunar rhythms by indirect lunar effects of tides and by direct perception of nocturnal moonlight.
Even more challenging are questions concerning the nature of annual periodicity in many organisms. Several chapters address these questions, including that by Monecke, Wollnik, and Pévet, asking whether such rhythmicity is controlled simply by seasonal changes in day length or whether truly molecular clockwork is involved. Then, as discussed by Goymann and Helm, if molecular clockwork is involved, how is circannual clockwork synchronized by the environment, particularly in tropical regions where seasonal changes in day length are minimal? Since “the moon choreographs sex among many coral species at the same night once a year,” as Sorek and Levy assert, how is this timing controlled? This last question may not be “one of the earth’s biggest mysteries,” as Sorek and Levy proclaim, but the book as a whole demonstrates that lunar periodicity in organisms has indeed moved out of the realm of mysticism into a world of testable hypotheses, as Carl Hauenschild predicted during skeptical times in 1960 (Hauenschild, 1960). In pioneering studies of moon-related reproduction in polychaete worms, Hauenschild posed questions as to how such rhythmicity might be controlled, and this very welcome treatise on “enigmatic” biological clocks provides the necessary impetus for such questions to be answered.