The paramecium (Paramecium caudatum) uses cilia for swimming, and its locomotion is known to vary diurnally. In this study, we examined diurnal and circadian changes in paramecium swimming behavior and the effects of melatonin on these changes, using the speed of swimming, the frequency of directional changes, and the periodicity of ciliary beating as indicators. Analysis of diurnal changes in the behavior of the paramecium showed that swimming speed significantly decreased during the dark period, at which time the frequency of change of direction significantly increased and the periodicity of ciliary beating lengthened. On the other hand, analysis of the behavior of the paramecium under constant-dark conditions revealed a significant decrease in swimming speed, a significant increase in the frequency of changes of direction, and a lengthening of the ciliary cycle during the subjective dark period. When melatonin, a known internal clock synchronizing factor in mammals, was added, the melatonin treatment group showed a decrease in swimming speed, an increase in change of direction, and a prolongation of the ciliary beating cycle, which were similar to those observed at night. These experimental results suggest that a circadian clock may exist in the paramecium, which may be affected by melatonin.
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