Increased body-axial stiffness reduces escape performance in larval zebrafish

Fish larvae swim by undulating their body. The shape of their body wave depends on the interaction between their body and the surrounding water. This study focuses on zebrafish, in particular the comparison between wild-type and a mutant called stocksteif. This mutant shows severe over-ossification of the entire notochord, causing the vertebrae to fuse into a stiff rod over the first 15 days of larval development. By comparing wild-type and stocksteif morphs, we studied how stiffening the vertebral column affects the shape of the body wave, and how this change in body wave kinematics in turn affects escape performance in zebrafish larvae. We recorded escape responses from a top view with a high-speed camera at 1500 frames per second to determine swimming kinematics. At age 5 days, that is before the vertebral column shows significant ossification, the two morphs body wave kinematics and escape performance are not significantly different. At age 15 days, the vertebral column of the stocksteifmutant is largely fused. We found that as ossification of the axial skeleton progresses, body wave speed and peak curvature decrease in the stocksteifmutant, but not the wildtype with unfused vertebrae. Both body wave parameters affect the larvas escape performance: stocksteif larvae achieve lower translational accelerations. The increasing stiffness of the vertebral column seems to seriously limit the axial muscles ability to bend and undulate the fishs body.