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

Abstract Biological rhythms enable organisms to anticipate and adapt to regular environmental fluctuations, yet their presence and neural basis in basal chordates remain poorly understood. The solitary ascidian Styela plicata , a sessile marine invertebrate closely related to vertebrates, offers a valuable opportunity to explore rhythmicity in an evolutionarily pivotal group. We monitored siphon contractions under light-dark conditions and recorded extracellular activity from the cerebral ganglion over 24-hour period. Behavioral analyses revealed consistent day-night differences, with more frequent siphon contractions during the night. Neural recordings identified multiple spike types, one of which showed higher activity during the day. These findings suggest the existence of two: high-activity and a low-activity diurnal phases in a sessile ascidian, establishing an experimental framework for investigating the emergence and organization of rhythmic behavior and neural dynamics in basal chordates and other sessile marine animals.