Three-dimensional escape trajectories in larval fish

Fish execute a C start when they escape from a predator. Previous studies suggest that fish randomize their horizontal escape trajectories, but bias the response away from the stimulus. The fewstudies that looked at the vertical trajectory found that fish larvae respond to a horizontal stimulus with a downward escape trajectory. This study quantifies the escape trajectories of fish larvae in three dimensions. We use a vertical and a horizontal suction stimulus to explore the effect of stimulus direction on the escape trajectory. We found that zebrafish larvae (age 3 to 12 days post-fertilization) consistently responded to a horizontal stimulus with a downward trajectory. For the horizontal stimulus, out of 70 video recordings, 54 showed escape responses (77.1%). Fifty twoof those responses showed a downwards trajectory (96.3%). Thesedata suggestthat the same trend holds true when we use a vertical stimulus, simulating a benthic predator. The vertical stimulus, out of 131 video recordings, 131 showed escape responses (100%). From those responses, 129 showeda downwards trajectory (98.8%). Given the age range of the larvae, the downward trajectory cannot be explained by asymmetry of the body due to the presence of a yolk sac; the yolksac is absorbed usually at age 5 to 6 days. So the downward trajectory might be a hardwired response (zebrafish larvae are demersal) or indicate that fish have less control over their pitch than their yaw angle �the body movements during an escape response might be able to generate a wide range of yawing moments, but not pitching moments, leading to the observed bias in the trajectories.