Plausible link between circa'bi'dian activity rhythms and circadian clock systems in the large black chafer Holotrichia parallela.

Two-day rhythms, referred to as circa'bi'dian rhythms, have been reported in humans and mosquitos. However, these rhythms only appear under constant conditions, and the functional mechanisms of 2-day rhythms were unknown. Here, we report clear circabidian rhythms of large black chafers (Holotrichia parallela, Coleoptera: Scarabaeidae) in both the laboratory and field. Under 12 h:12 h light:dark (L:D) conditions at 25°C, H. parallela appeared on the ground at the beginning of the dark phase every 2 days. Under constant darkness, H. parallela exhibited free-running with a period of 47.9±0.3 h, suggesting the existence of a clear circabidian rhythm entrained to two 12 h:12 h L:D cycles. Phase responses of the circabidian rhythm to light pulses occurred under constant darkness in a phase-dependent manner. Phase responses suggest that there are two circadian cycles, each consisting of a less-responsive and more-responsive period, in a circabidian oscillation, and the circabidian rhythm is driven by the circadian clock. A mark-recapture study showed that beetles repeatedly appeared on the same tree approximately every 2 days. However, the periodicity was not as rigid as that observed under laboratory conditions in that individuals often switched appearance days. For instance, a large precipitation made the 2-day rhythm shift phase by half a cycle of the rhythm at a time. We propose a novel function of the circadian clock characterized by the release of an output signal every two cycles to produce the 2-day rhythm.