Head stabilization behavior and underlying circuit mechanisms in larval zebrafish

Head stabilization is essential for animal survival, enabling stable sensory input and effective motor coordination. Animals stabilize their heads in response to vestibular stimuli through the vestibulo-collic reflex (VCR). While the VCR has been characterized in tetrapod vertebrates, it remains unknown whether fish, which lack an anatomical neck, employ a comparable behavior. Here, we demonstrate that larval zebrafish exhibit VCR-like behaviors: they adjust their head orientation relative to the body by rostral body flexion during pitch tilts. The rostral body flexed ventrally during head-up posture, whereas it flexed dorsally during head-down posture. These flexions partially compensated for the head pitch changes, thereby contributing to head stabilization. We also identified the muscles and neural circuits responsible for these two types of body flexions. Both the dorsal and ventral flexions were mediated by the same vestibular nucleus, but neural signals were transmitted through distinct pathways, either involving or bypassing a class of reticulospinal neurons. The dorsal and ventral flexions were ultimately produced by specialized dorsal and ventral muscles in the rostral body, respectively. The neural circuits underlying these body flexions in fish share similarities with those underlying the mammalian VCR. Together, our results demonstrate that fish exhibit a VCR-like behavior through comparable circuit mechanisms, suggesting that the VCR is evolutionarily conserved across vertebrates.