Threat-induced behavioral plasticity in two invertebrate chordates integrates sensory and neuromodulatory inputs

Threat detection and evasion are fundamental challenges across the animal kingdom. Here, we examine threat detection and avoidance behaviors in two planktonic tunicates, Ciona intestinalis larvae and Oikopleura dioica adults, by quantifying locomotion and ecologically relevant behaviors in the presence of copepods. Both species exhibited marked increases in swimming speed, distance traveled, and roaming behavior upon predator exposure, whereas conditioned water alone had no effect. Despite this shared response, behavioral organization diverged: C. intestinalis primarily reallocated time toward roaming with few state transitions, while O. dioica increased behavioral switching, reduced dwelling, and prolonged roaming bouts. Pharmacological manipulations revealed species-specific effects of compounds targeting monoaminergic, glutamatergic, and GABAergic signaling: in C. intestinalis , bioamine, glutamatergic, and GABAergic perturbations disrupted threat-induced movement patterning, whereas in O. dioica they suppressed motility increases. Predator presence also accelerated metamorphosis and altered settlement in C. intestinalis , while in O. dioica it triggered rapid house escape, delayed expansion, and reduced turnover. Finally, we show that C. intestinalis larvae leverage a network of polymodal sensory neurons to detect chemical cues emanating from the copepods. Whole-brain calcium imaging suggested that these cues engage widespread yet structured neural activity across the larval brain, recruiting distributed neuronal populations with distinct temporal response motifs. Circuit-level analysis further indicated that sensory inputs propagate through a hierarchical sensorimotor network linking peripheral neurons to central interneurons and motor outputs, supporting a distributed transformation of threat-related information into behavior. These findings uncover conserved yet mechanistically distinct antipredator strategies in two evolutionarily distant tunicates.