SUMMARYFish use the lateral line system to sense the water flow created by a predator's strike. Despite its potential importance to the survival of a diversity of species, it is unclear whether this ability becomes compromised when a fish swims. Therefore, the present study compared the behavioral responsiveness of swimming and motionless zebrafish (Danio rerio) larvae when exposed to the flow of a suction-feeding predator. This flow was generated with an impulse chamber, which is a device that we developed to generate a repeatable stimulus with a computer-controlled servo motor. Using high-speed video recordings, we found that about three-quarters (0.76, N121) of motionless larvae responded to the stimulus with an escape response. These larvae were 66% more likely to respond to flow directed perpendicular than flow running parallel to the body. Swimming larvae exhibited a 0.40 response probability and were therefore nearly half as likely to respond to flow as motionless larvae. However, the latency between stimulus and response was unaffected by swimming or the direction of flow. Therefore, swimming creates changes in the hydrodynamics or neurophysiology of a larval fish that diminish the probability, but not the speed, of their response to a flow stimulus. These findings demonstrate a sensory benefit to the intermittent swimming behavior observed among a broad diversity of fishes.

The fluid forces that govern propulsion determine the speed and energetic cost of swimming. These hydrodynamics are scale dependent and it is unclear what forces matter to the tremendous diversity of aquatic animals that are between a millimeter and a centimeter in length. Animals at this scale generally operate within the regime of intermediate Reynolds numbers, where both viscous and inertial fluid forces have the potential to play a role in propulsion. The present study aimed to resolve which forces create thrust and drag in the paddling of the water boatman (Corixidae), an animal that spans much of the intermediate regime (10