Twenty-Four Hour Cycles in Marine Organisms

Abstract

Periodic variations in the behavior of organisms have been observed and recorded as a conspicuous feature of our biological environment for a very long time. Indeed, such variations are often so striking that it is extremely difficult to ignore them. Thus folklore and myth abound with examples of cosmological speculation which purport to explain astronomical cycles on the basis of biological rhythms. As plant and animal physiology emerged as experimental disciplines, some of the more striking of these cycles were studied and described. This early literature has been carefully reviewed by several authors including Welsh (1938), Kleitman (1949), Webb (1950), and Brown, Fingerman, Sandeen and Webb (1953). It quickly became apparent that there was a group of such periodic physiological variations which persisted for more or less considerable periods in conditions which were loosely described as constant. These persistent rhythms may 'be contrasted with other biological cycles which are merely direct responses on the part of the organism to some environmental factor which changes in a cyclical fashion. If we temporarily restrict our attention to cycles of 24-hour frequency as exhibited by marine organisms, we find that such cycles are very conspicuous and various. Thus persistent 24-hour cycles have been reported in light production, position of retinal pigments, color change, oxygen consumption and a variety of other parameters. Some of these cycles seem to be very broadly distributed. Thus it would appear to be the case that no marine organism which has been suitably studied has failed to manifest variations in oxygen consumption with a diurnal frequency component (see, for example, Brown, Webb, Bennett, and Sandeen, 1955). In view of the existence of a number of recent reviews of this literature, it does not seem profitable to attempt an exhaustive listing of 24.hour cycles in marine organisms. Furthermore, the limitation with respect to frequency and habitat suggested by the title of this paper is rather arbitrary and would be an undesirable and artificial restriction to a review. Consequently, it seems preferable to review some recent contributions which have been made to the analysis of one of the most intensively studied biological cycles, the 24-hour color change rhythm of the fiddler crab, Uca.