Amidst the darkness: variations in locomotor activity rhythms in troglobitic harvestmen

Epigean organisms synchronize their rhythms with light-dark cycles, but hypogean species often lack these rhythms due to stable cave conditions. This study examined circadian rhythms in troglobitic harvestmen (genus) by analyzing their locomotor activity under constant darkness (DD), constant light (LL), and light-dark (LD 12:12) cycles. Specimens from three species were collected in Minas Gerais, Brazil. None synchronized with LD cycles, and their activity was aperiodic under DD and LL. Instead, mean main periods suggested an infradian rhythm. Adults were predominantly diurnal, but individual patterns varied, showing no inter-individual dependency. The absence of circadian rhythms may be an evolutionary adaptation to cave life, while variations in activity likely reflect differences in diet and resource competition. Non-photic cues (zeitgebers) may influence rhythm expression in these cave-dwelling organisms.