Larval zebrafish maintain elevation with multisensory control of posture and locomotion.

Fish actively control posture in the pitch axis (nose-up/nose-down) to counter instability and regulate their elevation in the water column. To test the hypothesis that environmental cues shape the strategies fish use to control posture, we leveraged a serendipitous finding: larval zebrafish (Danio rerio) lose swim bladder volume and sink mildly after acute loss of lateral line hair cells. Using long-term (48 h) recordings of unrestrained swimming, we discovered that sinking larvae compensated differently depending on light conditions. In the dark, they swam more frequently, with an increased nose-up posture. In contrast, larvae in the light did not swim more frequently, but did climb more often. Finally, after lateral line regeneration, larvae returned to normal buoyancy and swam comparably to control siblings. We conclude that larvae can switch postural control strategies depending on the availability of visual information. Our findings complement and extend morphological and kinematic analyses of locomotion. More broadly, by quantifying the variation in strategies, our work speaks to the evolutionary basis of different balance behaviors.