Fig. 2: Biomechanics for the VBR-mediated postural recovery.

a A model of postural recovery from roll tilt by the VBR. Frontal and dorsal views and cross-section around the swim bladder. Gravity and buoyancy act at the center of mass (COM) and center of volume (COV), respectively. The COM and COV are located near the swim bladder, and the COM is located slightly rostral to the COV²⁴. In the dorso-ventral axis, the model assumes that the COM and COV are located in the same position. Left: the COM and COV are on the midline. Right: in the fish performing the VBR upon a roll tilt, the head and caudal body move toward the ear-up side, while the body around the swim bladder moves toward the ear-down side (top and middle). The COM and COV move toward the ear-up direction in the cross-section (bottom). The COM moves more laterally than the COV (see main text). This results in misalignment between gravity and buoyancy, generating a moment of force that counter-rotates the body. b A fish model with a deflated swim bladder. Positions of the COM and COV are the same even when fish perform the VBR (middle and bottom). Gravity and buoyancy are antiparallel on the same vertical axis (bottom). This does not generate a moment of force. c, d Behavioral experiments on fish with the swim bladder deflated. c Snapshots of the frontal and dorsal images of a fish during a left-down tilt. d Traces of the head roll and body bend angles of a fish in response to roll tilt. Six trials in one fish are shown. The magenta traces correspond to the trial shown in c. Scale bars, frontal images 200 μm; dorsal images 500 μm. Source data are provided as a Source Data file.