Entrainment of the circatidal activity rhythm of the mangrove cricket, Apteronemobius asahinai, to periodic inundations

Adults of the mangrove cricket show a clear and persistent circatidal rhythm in their locomotor activities, with a free-running period of about 12.6 h. Individuals are active during the subjective low tide and inactive during the subjective high tide, under constant darkness. When we provided a 30 min inundation stimulus four times, at intervals of 12.4 h, the circatidal rhythm entrained to the given inundation cycles under constant darkness. After the treatment, crickets became inactive around the times when the inundation stimulus was expected. The circatidal rhythm responded to periodic inundations in a phase-dependent manner: when periodic inundations started during the first half of the active phase (i.e. the subjective low tide), a large phase delay resulted, whereas periodic stimuli that started during the second half of the subjective low tide caused a large phase advance. The shape of the phase response curve (PRC) resembled that of the Type 1 PRC of circadian rhythms, with continuous transitions between delays and advances. The similarity between the PRC patterns provides evidence of a circatidal oscillator, which has characteristics similar to a circadian oscillator.