Diurnal rhythms with sunrise-induced oscular contractions suggest photosensitivity in the demosponge Halichondria panicea

Despite their quiescent lifestyle, sponges display contraction-expansion behaviours that regulate water flow through their aquiferous system. These behaviours are intrinsically generated but can be modulated by environmental parameters, including light exposure. Some sponges exhibit diurnal contraction-expansion rhythms synchronized with illumination cycles. Since the often-used photopigment opsin is unknown in sponges, the light-induced behaviour is likely mediated by cryptochromes and extra-ocular photoreceptors, as shown in sponge larvae. Previous studies on demosponges reported higher contraction-expansion activity during daylight than at night. Understanding such diurnal behaviours in nerve- and muscle-less metazoans may provide insights into the early evolution of sleep-like states. However, systematic diurnal observations of sponge contraction-expansion rhythms remain scarce. Here, we document the contractile behaviour of single-osculum explants of the marine demosponge Halichondria panicea over six days, maintained under a 12:12 h dark:light cycle with simulated sunrise and sunset. We find a light-induced diurnal rhythm in the contraction-expansion behaviour of their osculum, suggesting photosensitivity. Furthermore, we observe that sunrise triggers the onset of oscular contractions and discuss these results in context with the characteristics of sleep-like states in other metazoans. Summary statementA diurnal rhythm of light-induced oscular contractions suggests photosensitivity in the nerveless demosponge Halichondria panicea. This may offer insights into the early evolution of sleep-like behaviours in metazoans.