The Living Clocks of Marine Organisms

Abstract

Many facets of organismic physiology and behavior repeat themselves with such beat-like regularity, that the patterns are referred to as being rhythmic. The peaks and troughs of a major category of these rhythms are synchronized to prominent events in the earth’s cyclic geophysical environment, e.g., day or night. However — and this is their most fascinating property — the persistence of the rhythms is not dependent on periodic changes in the environment, it is controlled instead by a “living clock” amalgamated with the physiochemical components of protoplasm. Almost all plants and animals, including man, possess these horologes.

The biorhythm most commonly studied in the past is the one whose period approximates the interval of the rotation of the earth on its axis. This rhythm has come to be called a circadian one, and is discussed in other sections of this volume.

A category of biorhythm much less studied than the circadian one, but which shares many properties, has a period length equal to the duration of a lunar day (the 24.8-hour interval between successive moonrises). As might be expected, this rhythm is characteristic of marine plants and animals inhabiting the intertidal zone along seacosts, and therefore usually displays a bimodal waveform with the peaks being locked to specific epochs of the local tidal cycle. When these rhythms are studied in organisms removed from their seaside habitat, and placed in the nontidal setting of the laboratory, they persist, often with a slightly altered period. They are therefore referred to as being circalunadian. It is this eategory of rhythm that is discussed here. The properties of tidal rhythms are described, as are the means by which the environment sets the “hands” of the living tidal clock. Because both solar-day and lunar-day rhythms have several properties in common, it is speculated that a single clock may drive both rhythms. Nothing is as yet known about the actual clockworks of this organismic horologe.