Knowing when a hungry fish is about to slurp you up is a good skill if you're a 5-day-old zebrafish larva. Karla Feitl from the University of California, Irvine, explains that she and Matt McHenry knew that the tiny larvae use flow sensors (mechanosensors known as superficial neuromasts) along the fish's lateral line to detect threatening fluid flows, and if they inactivated the flow sensors with antibiotics the youngsters became ‘deaf’ to the water's motion and vulnerable to predation. Feitl and McHenry also noticed that stationary fish might be more sensitive than moving fish to fluid flows generated by suction, potentially leaving active fish at risk from hungry predators. Realising that moving fish are always surrounded by apparently ‘flowing’ water, the duo decided to take a closer look to find out if the fluid flows generated by a fish's own movements reduced it sensitivity to threatening gulps ().
First, the duo built a tank attached to a computer-driven piston so that they could ‘suck’ water through the tank to produce fluid flows similar to the suction produced by a slurping fish predator. Having encouraged the larvae to swim in the dark by switching the lights off and on, Feitl filmed their responses before and immediately after a computer-generated slurp with high-speed video in infrared light. ‘We had no say over what the larvae did or how they oriented within the tank,’ says Feitl, ‘so we ran huge numbers of larvae until we got the necessary sample sizes in each category to allow us to statistically evaluate whether or not there were differences in responsiveness between the stationary and swimming larvae,’ she explains.
Next, Victoria Ngo analysed the high-speed videos, determining each larva's orientation toward the fluid flow as the piston sucked water toward it, and then painstakingly checked each frame of video to see if the larva was unperturbed or startled into a C-shaped escape response. The team saw that 76% of the stationary fish twisted into a ‘C’ ready to beat a hasty retreat; however, only 40% of the moving fish picked up the warning. The moving larvae were less sensitive to suction.
Wondering if the moving larvae were simply less sensitive to the fluid movement than stationary larvae, the team checked how quickly the two groups of larvae responded to the simulated slurp. If the sensitivity of the moving fish's flow detectors was reduced by the relative flow of fluid over their bodies, then the team expected them to respond more slowly than the stationary fish: but they didn't. ‘We think it is more complicated than a straight reduction in sensitivity,’ says Feitl.
Having found that stationary fish are more sensitive to slurped water than moving fish, Feitl and McHenry suggest that intermittently swimming larvae could benefit from their stop-and-start swimming style by having a higher chance of escaping hungry predators than continuous swimmers. The team is also keen to find out if the larvae escape in a particular direction relative to the fluid flow, to improve their chances of swimming for another day.
