Plausible link between circabidian activity rhythms and circadian-clock systems in the large black chafer Holotrichia parallela

Two-day rhythms referred to as circabidian rhythms were reported in humans and mosquitos. However, these rhythms only appear under constant conditions, and functional mechanisms of two-day rhythms have not been known. Here, we report clear circabidian rhythms of large black chafers (Holotrichia parallela, Coleoptera: Scarabaeidae) in both the laboratory and field. Under 12 h light and 12 h dark conditions (LD 12:12) at 25°C, H. parallela appeared on the ground at the beginning of the dark phase every two 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 LD 12:12 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 consisted of a less responsive and high responsive period, in a circabidian oscillation, and the circabidian rhythm is driven by the circadian clock. A mark and recapture study showed that beetles repeatedly appeared on the same tree approximately every two days. However, the periodicity was not as rigid as that observed under laboratory conditions in that individuals often switched appearance days. For instance, large precipitation made the two-day rhythm shift the phase by a half cycle of the rhythm at a time. Here, we propose a novel function of the circadian clock characterized by the release of an output signal every two cycles to produce the two-day rhythm.