Embodied postural dynamics underlie behavioral diversity in a benthic sessile chordate

Sessile benthic animals are thought to have limited behavioral repertoires due to immobility and simple nervous systems. Here, we characterize the spontaneous and stimulus-evoked behaviors of adult Ciona intestinalis, a basal chordate with a sessile adult stage. Using time-lapse imaging and markerless pose estimation, we extracted detailed body outlines and kinematic features and showed that Ciona organizes behavior into three postural engagement states distinguished by configuration, kinematics, temporal structure, and transitions. These states are strongly modulated by external cues: mechanical pokes across the body and water flow elicit distinct sensorimotor responses, while chemical cues drive compound-specific motor patterns. Dimensionality reduction revealed that five "eigencionas'' capture most postural variance, and spatiotemporal embedding uncovered 16 stereotyped behavioral modules deployed in a context-dependent manner. Hidden Markov modeling showed that mechanical, hydrodynamic, and chemical stimuli reorganize transitions within a shared state space. Together, these results reveal a rich, modular behavioral repertoire in adult Ciona.