Neural circuits underlying divergent visuomotor strategies of zebrafish and Danionella cerebrum.

Many animals respond to sensory cues with species-specific coordinated movements. A universal visually guided behavior is the optomotor response (OMR), which stabilizes the body by following optic flow induced by displacements in currents. While the brain-wide OMR circuits in zebrafish (Danio rerio) have been characterized, the homologous neural functions across teleost species with different ecological niches, such as Danionella cerebrum, remain largely unexplored. Here, we directly compare larval zebrafish and D. cerebrum to uncover the neural mechanisms underlying the natural variation of visuomotor coordination. Closed-loop behavioral tracking during visual stimulation revealed that D. cerebrum follow optic flow by swimming continuously, punctuated with sharp directional turns, in contrast to the burst-and-glide locomotion of zebrafish. Although D. cerebrum swim at higher average speeds, they lack the direction-dependent velocity modulation observed in zebrafish. Two-photon calcium imaging and tail tracking showed that both species exhibit direction-selective encoding in putative homologous regions, with D. cerebrum containing more monocular neurons. D. cerebrum sustain significantly longer directed swims across all stimuli than zebrafish, with zebrafish reducing tail movement duration in response to oblique, turn-inducing stimuli. While locomotion-associated neurons in D. cerebrum display more prolonged activity than zebrafish, lateralized turn-associated neural activity in the hindbrain suggests a shared neural circuit architecture that independently controls movement vigor and direction. These findings highlight the diversity in visuomotor strategies among teleost species with shared circuit motifs, establishing a framework for unraveling the neural mechanisms driving continuous and discrete locomotion.