Photoperiodic and intrinsic circadian regulation of heart rate in Daphnia pulex.

Daphnia species are well-established models for drug testing. These crustaceans possess a unique myogenic heart similar to mammals and a transparent body akin to zebrafish, making them particularly suitable for cardiac studies. Circadian rhythms have long been identified in Daphnia species, with recent reports highlighting photoperiodic responses and circadian gene expression patterns specifically in Daphnia pulex. Using biological rhythm analysis and simple automated infrared microscopy, we investigated the heart rate of Daphnia pulex to determine whether a diurnal rhythm was present, whether this rhythm was entrained by photoperiod length, and if its control was intrinsic to the heart itself. Our results confirmed the presence of circadian rhythms in the heart rate of D. pulex, demonstrating clear photoperiod modulation of entrainment during both diurnal and circadian phases, with these rhythms intrinsic to the heart.