With a hungry predator on the loose looking for its next meal, you've got to keep your wits about you if you want to survive. But what sensory tools do you employ to detect and escape in time from this peckish hunter? For zebrafish larvae trying to evade their cannibalistic elders, the answer seems to lie in a special sensory system – the lateral line – that detects water motions near the body, explains William Stewart (p. 388).

Whilst watching videos of adult zebrafish snacking on their young, Stewart, a PhD student in Matthew McHenry's lab at the University California, Irvine, USA, noticed that larvae were actually quite effective at dodging these attacks. He realised that the ability to escape was unlikely to rely on the visual system because this system processes information relatively slowly and – given the speed with which some predators strike – processing

speed can be a matter of life or death for the victim. Stewart therefore turned his attention to another sensory system, the lateral line. This sensory system detects water movements or flows over the body and Stewart points out that while ‘the system is sensitive to artificially generated water flows, its role in predator evasion is unclear’.

To investigate the role of this sensory system in the survival of onslaughts from adult zebrafish, Stewart enlisted the help of undergraduate student Gilberto Cardenas. Together, they recorded the outcome of predator–prey interactions using larvae in which the sensory system had been removed. To their surprise, larvae without the lateral line system survived a mere 5% of attacks. On the whole, larvae with a functional detection system were 14 times more likely to survive and they survived four times longer than their unfortunate relations without a lateral line system.

To understand the details and precise timings of both the attacks and escape manoeuvres, Stewart turned to videoing the zebrafish larvae–adult encounters with a high-speed, high-magnification video camera. After tackling the problems associated with filming fish that seemed to love swimming rapidly out of the camera's field of view, Stewart was able to determine the exact timing of a larva's escape relative to an incoming attack. The results showed that the larvae responded as soon as the hungry adult started moving and not, as expected, when the adult opened its mouth and started suction feeding (the way in which adults capture larvae). ‘We were intrigued that the larvae initiated their response prior to predator suction’, says Stewart. He adds, ‘The water flows generated by suction are of [a] higher magnitude than the flows produced by the predator's approach’.

It is likely that the movement of the hunter zebrafish, a bit like a ship, creates a bow wave that the larvae sense. Sensing this bow wave, as opposed to water movement caused by suction, allows the targeted larvae more space and time to escape. While other sensory systems, such as vision, may still be important in determining when to perform an emergency escape manoeuvre, Stewart highlights that ‘Unlike vision, flow sensing can be effective in both light and dark conditions or when the water is turbid’, in addition to being the faster of the two senses. Sensing water movement thus seems to be the best tool in a fish's arsenal to avoid being someone's lunch or dinner, whether it's night or day.

W. J.
G. S.
M. J.
Zebrafish larvae evade predators by sensing water flow
J. Exp. Biol.