The kinematics of directional control in the fast start of zebrafish larvae.

Larval fish use the 'fast start' escape response to rapidly evade the strike of a predator with a three-dimensional (3D) maneuver. Although this behavior is essential for the survival of fishes, it is not clear how its motion is controlled by the motor system of a larval fish. As a basis for understanding this control, we measured the high-speed kinematics of the body of zebrafish (Danio rerio) larvae when executing the fast start in a variety of directions. We found that the angular excursion in the lateral direction is correlated with the yaw angle in the initial stage of bending (stage 1). In this way, larvae moved in a manner similar to that reported for adult fish. However, larvae also have the ability to control the elevation of a fast start. We found that escapes directed downwards or upwards were achieved by pitching the body throughout the stages of the fast start. Changes in the pitching angle in each stage were significantly correlated with the elevation angle of the trajectory. Therefore, as a larva performs rapid oscillations in yaw that contribute to undulatory motion, the elevation of an escape is generated by more gradual and sustained changes in pitch. These observations are consistent with a model of motor control where elevation is directed through the differential activation of the epaxial and hypaxial musculature. This 3D motion could serve to enhance evasiveness by varying elevation without slowing the escape from a predator.