Diurnal behavioral and neural rhythms in a solitary ascidian (Chordata, Ascidiacea) Styela plicata

Biological rhythms enable organisms to anticipate and adapt to regular environmental fluctuations, yet their presence and neural basis in invertebrate chordates remain poorly understood. The solitary ascidian Styela plicata , a sessile marine invertebrate represents an early divergent chordate, offers a valuable system to explore rhythmicity in a phylogenetically significant lineage. We monitored contractile frequency under a 12:12 light-dark cycle and then delivered a tactile stimulation at 2-hour intervals, while simultaneously acquiring extracellular recordings were acquired its cerebral ganglion over a continuous 24-hour period. Behavioral analyses revealed consistent day-night differences, with more frequent siphon contractions during the night. Neural recordings identified four distinct spike types, one of which showed higher activity during the day. These findings suggest the existence of: a high-activity and a low-activity diurnal phases in a sessile ascidian, providing a foundation for investigating the emergence and organization of rhythmic behavior and neural dynamics in sessile marine chordates.