Fig. 1

Hydras exhibit behavioral day-night variation a Behavioral recording system. Hydras were placed into rectangular chambers and illuminated with infrared light. The movements of the hydras were captured using an infrared camera. b Experimental scheme of behavior analysis under LD 12:12 cycles, DD and LL conditions. Hydras fasted for 24 h were placed into the chamber at ZT10 (setting). Light blue indicates subjective day under DD condition and dark yellow indicates subjective night under LL condition. c Frame subtraction analysis. Upper panel: movement of a hydra between two frames was detected as red (t = t’ + 5 and t = t’). Lower: no movement was detected when a fixed hydra was captured and analyzed. d, e Percentage of frames in which movement was detected during daytime and nighttime under LD 12:12 cycles (n = 27). f, g Percentage of frames in which movement was detected under DD condition (n = 26). h, i Percentage of frames in which movement was detected under LL condition (n = 27). The orange lines and shaded areas around the lines show the mean and the SEM, respectively. The bars indicate the SEM. *P < 0.05, **P < 0.01, ***P < 0.001, one-way ANOVA, Tukey post-hoc test. P > 0.05 for all comparisons in g (under DD) using one-way ANOVA. CT means circadian time