The vertical migration of two Euglena species and several diatom species into and out of the sediment on the banks of the River Avon has been studied under natural conditions. All species have been shown to migrate vertically upwards when exposed during daylight. Tidal flooding of the sediment is generally preceded by re-burrowing of the algae beneath the surface. Methods have been devised to follow these migrations in both the field and laboratory. Laboratory experiments show that these migrations are rhythmic, continuing under constant illumination and temperature and removed from tidal influence. The effect of three different temperatures and three different light intensities has been investigated. Transfer from low to high temperatures has been shown to reset the phase of the rhythm. The results are discussed in relation to other work and to the ‘biological clock’ hypothesis.

No abstract

During daytime low tides on the River Avon at Bristol, England, the exposed river banks become a deep green colour owing to the presence of enormous numbers of Euglena obtusa which emerge out of the black mud. Cell densities on the surface surpass 105 cells/cm2. Before the tide returns to cover the area, the cells re-burrow back into the mud and remain there during high tide and throughout the night.The cells can be prevented from emerging on to the surface mud at low tide by artificially darkening the area with an opaque covering just as the tide recedes. Cells which are already on the surface can be made to re-burrow by similarly placing them in darkness.The vertical-migration rhythm will persist in the laboratory in constant illumi-nation, constant temperature, and away from the influence of the tide, for nearly one month. In these conditions the rhythm is diurnal, rather than tidal. The rhythm will not persist in constant darkness.Because of the excessive turbidity of Avon water, each high tide imposes a period of darkness on the surface mud. It is thought that these dark periods transform the fundamental diurnal rhythm into one of tidal frequency.Various intensities of constant illumination alter the form and amplitude of the rhythm, but not the period. The stable nature of the period, under different light intensities, is thought to be due to a unique, self-stabilizing feature inherent in this type of rhythm.The rhythm is inhibited at 2° C. Between 5° and 15° C, the period of the rhythm is virtually unaltered; it remains approximately 24 h in length.

No abstract

No abstract

No 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...

The living clock that governs tide-associated organismic rhythms has previously 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 results that could not be explained by the action of such a clock. Prime among these findings was the discovery that, occasionally, one of the two daily peaks in an organism's rhythm assumed a different period from its partner. Similar results have since been observed in a host of different organisms. These data led to the circalunidian-clock hypothesis that envisions two basic 24.8 h clocks, coupled together in antiphase, as the driving force for these rhythms. There is, however, only a slight difference (50 minutes) in running 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 display "circa" periods that overlap. Here, I postulate that the two clocks are fundamentally one and the same. BioEssays 22:32-37, 2000.