Rhythmic, diurnal, neural, and behavioral activity in the solitary ascidian (Chordata, Ascidiacea) Styela plicata

Background The ability to anticipate environmental changes through rhythmic activity is a hallmark of life across taxa. However, the existence of such rhythms and their neurophysiological basis in basal chordates remains poorly understood. The solitary ascidian Styela plicata (Phylum: Chordata, Class: Ascidiacea) is a sessile marine invertebrate closely related to vertebrates, making it a suitable model for investigating neural and behavioral rhythms in early chordates. Here we conducted behavioral tracking of siphon contractions under controlled fully dark or light-dark cycles and recorded extracellular neuro-potentials from the cerebral ganglion of S. plicata over 24- and 48-hour periods and. Results Our findings revealed distinct diurnal rhythmic patterns in behavioral activity. Siphon activity was more intensive during night and less during day. The physiological recordings revealed that many neurons exhibited a significant circadian rhythm. One type of neuron that fired more during daytime than nighttime consistently across individuals. Free-running recordings of the neural system suggested a decrease in synchronized firing over time. These results suggest that S. plicata is mostly active during nighttime and possesses an internal clock that coordinates physiological and behavioral rhythms. Conclusions Our study sheds light on the evolution of circadian regulation in early chordates and suggests a potential basic sleep-like state in these basal chordates. Furthermore, we provide a methodological framework for investigating sleep and rhythmicity in the context of neural dynamics in ascidians and other marine invertebrates.