Tidal Rhythms: The Clock Control of the Rhythmic Physiology of Marine Organisms

Abstract

Summary 1. A great number of vital processes are rhythmic and the rhythms quite often persist in constant conditions. The best‐known rhythms are circadian; much less is known about circalunadian rhythms, and this review was prepared in an attempt to rectify this deficiency. All through the article comparisons are drawn between circalunadian and circacian rhythms. 2. Activity rhythms. (a) The activity patterns of 28 intertidal animals are discussed. All describe a periodicity with a basic component of 24.8 hours, and this approximate period persists in the laboratory in constant light and temperature and in the absence of the tides. The duration of persistence ranges from a few cycles to months, and is a function of the species studied, the conditions imposed, and individual tenacity. (b) In those few cases where relatively long‐term observations have been made, there is a trend for the period of the rhythm to become circatidal, or better, circalunadian. (c) The ‘desired’ phase relationship between rhythm and tidal cycle is species‐specific. Geographical translocation experiments have shown that the phase is set by the local tides. (d) In some cases the amplitude of the persistent rhythm mimics the semidiurnal inequality of the tides. (e) In about a third of the species discussed, a circadian component has been found combined with the tidal component. Many of the other studies were of such short duration that a low‐amplitude circadian component would have gone unnoticed. (f) The tidal rhythm is innate. However, the rhythm is (i) sometimes lacking in organisms living in non‐tidal habitats, or (ii) fades after a spell of incarceration in constant conditions. Various treatments — some aperiodic — can induce the expression of the missing tidal rhythm. (g) In the green crab, removal of the eyestalks destroys the activity rhythm. 3. Vertical migration rhythms. (a) A rather surprisingly large number of intertidal animals have been found to undergo migration rhythms between the upper layers of the substratum and its surface. The movements are synchronized with the tides in nature, but most species have either been shown to be diurnal in constant conditions, or in cases where adequate testing has not been done, suspected of being so. (b) In only one species has confirming work shown that the fundamental frequency is truly tidal. This finding is especially important as it shows that tidal rhythms need only the single‐cell level of organization for expression. Even at this level there appears to be a dictatorial override by a circadian clock. 4. Colour change. Low‐amplitude tidal rhythms in colour change — superimposed on a more dominant circadian change — have been reported to be intrinsic in four species and inducible in a fifth. 5. Oxygen consumption. Tidal rhythms in oxygen consumption have been described for seven invertebrates and one alga; six of the species have superimposed solar‐day rhythmic components also. 6. Translocation. A total of five geographical translocation experiments, in which the organisms were mainta...

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