Larval zebrafish rapidly sense the water flow of a predator's strike

Abstract

Larval fishes have a remarkable ability to sense and evade the feeding strike of a predator fish with a rapid escape manoeuvre. Although the neuromuscular control of this behaviour is well studied, it is not clear what stimulus allows a larva to sense a predator. Here we show that this escape response is triggered by the water flow created during a predator's strike. Using a novel device, the impulse chamber, zebrafish (Danio rerio) larvae were exposed to this accelerating flow with high repeatability. Larvae responded to this stimulus with an escape response having a latency (mode=13-15 ms) that was fast enough to respond to predators. This flow was detected by the lateral line system, which includes mechanosensory hair cells within the skin. Pharmacologically ablating these cells caused the escape response to diminish, but then recover as the hair cells regenerated. These findings demonstrate that the lateral line system plays a role in predator evasion at this vulnerable stage of growth in fishes.

Figure 1

Water flow stimulates an escape response in zebrafish larvae. (a) The impulse chamber used to generate flow includes a computer-controlled linear motor that actuates a hydraulic piston. The motion of the hydraulic piston (black arrow) creates flow through the working section. A high-speed video camera (250 frames s⁻¹) recorded the responses of larvae that were backlit with an array of infrared LEDs in a darkened room. (b) The speed of the flow stimulus is shown for a representative flow visualization measurement (black curve) and the 95% confidence intervals (dashed area, n=15 trials) from repeated measurements. The frequency of larvae that responded to this stimulus (grey bars) is overlaid in 2 ms intervals. (c) Video stills of a representative fast start response for a single larva (5.90 dpf) from a dorsal view with velocity vectors from the representative flow stimulus in (b) ((i) 1 ms, (ii) 3 ms, (iii) 5 ms, (iv) 7 ms, (v) 9 ms, (vi) 11 ms, (vii) 13 ms and (viii) 15 ms).

Figure 2

The effects of lateral line hair cells on startle response probability. (a) The lateral line hair cells stained with DASPEI are visible as yellow points along the body of a control larva (5 dpf) with a single neuromast highlighted (white box; scale bar, 1 mm). (b) The individual soma of hair cells within a neuromast are visible at high magnification (scale bar, 5 μm). (c) A larva treated with neomycin stained with DASPEI. (d) The mean number of hair cells per neuromast among three loci (see §2 for details) for larvae after treatment with neomycin (blue curve) and an untreated control group (green curve) of the same age. At each age, 95% confidence intervals (shaded areas) were calculated from variation among individuals (n=3–5). (e) The probability of an escape response after recovery for treated (blue curve, n=77–93 at each age) and control (green curve, n=66–107 at each age) larvae with 95% confidence intervals (shaded areas). (f) Measurements of response probability (from (e)) versus the number of hair cells (from (d)) for treated larvae with 95% confidence intervals (shaded areas). A linear least-squares curve fit (black line: R=0.12h+0.11, r ²=0.98, p=0.001) to the five sets of measurements made 25 hours after treatment.