Locomotor activity rhythms in laboratory-reared adults of the mangrove cricket, Apteronemobius asahinai

Adults of the mangrove cricket, Apteronemobius asahinai, collected in the field continuously show a circatidal rhythm in their locomotor activity. However, it remained unclear whether adult crickets reared from eggs in the laboratory, without cyclic tidal influence, show the circatidal rhythm. We show that under constant red dim light, slightly less than half of crickets showed a circatidal rhythm continuously for more than 10 days. On the other hands, Some others showed a unimodal daily rhythm either before or after the tenth day in the activity recording. Thus, some environmental factors other than the laboratory conditions should be required in order for all crickets to express a clear circatidal rhythm. Even under constant light, unimodal daily rhythm appeared. The unimodal daily rhythm was entrained to light–dark cycles in some crickets, but not in the others. Therefore, we cannot conclude whether the unimodal daily rhythm is a circadian rhythm or a rhythm derived from a circatidal rhythm with its period doubled.