If you can't get out of the way fast when you're tiny, there's little hope for your future and even less for your offspring. However, nimble larvae appear to be able to sense approaching doom and take evasive action even before the attacker is within range. ‘It is unclear how the sensory systems of prey fish operate quickly enough to coordinate an evasive manoeuvre,’ says Matt McHenry and colleagues from the University of California, Irvine, USA, and the Woods Hole Oceanographic Institution, USA. However, there was a clue that the fish might be using a sensory system – the lateral line – that detects the motion of fluid flowing over the surface of their body to trigger their lightning-fast reaction. According to the team it takes a fish 200 ms to respond to a visual threat, whereas it takes them only 4 ms to respond to alterations in fluid flows. Intrigued by the possibility that the tiny victims might be able to sense water that is propelled in front of a predator, McHenry and his colleagues, William Stewart, Arjun Nair and Houshuo Jiang began investigating the reactions of minute zebrafish larvae to an approaching adult (p. 4328).
Stewart designed a motor-driven sled that could be immersed in water to carry the body of a dead adult fish at speeds ranging from 2–20 cm s–1 and then filmed the responses of larval fish in the dark, forcing them to rely on their flow sensors alone as they fled from the approaching predator. Analysing the fish's escape manoeuvres, the team realise that the larvae reacted when the predator was within 2 cm. They also noticed that the larvae that were off to the side of the predator's line of attack performed the most effective escapes, consistently turning away from the approaching adult. And, when the team inactivated the sensors in the larvae's lateral lines the larvae failed to respond to the adult's approach; they would have been snapped up by a hungry predator.
Having shown that the lateral line is the sensory system that allows the larvae to take evasive action, the team turned their attention to the way that fluid is propelled by an approaching predator to find out what aspects of the fluid motion triggers the fish's reaction. Building a computer simulation of the fluid motions generated by approaching predators, Jiang could see a pulse of fast-moving water – which they describe as a bow wave – preceding the model fish. And when the team visualised how the water around an approaching fish moved in real life, the bow wave was clearly visible.
Combining the detailed information about the bow wave structure from the simulation with their measurements of the larvae's escape manoeuvres, the team realised that the fish's swift reactions were triggered by the lateral line's direct connection to the nerves that trigger the escape response on the opposite side of the larva's body. ‘This circuit accounts for the ability of flow on one side of the body to stimulate motion on the opposite side’, says McHenry, adding that understanding the neuroscience and mechanics of the larvae's evasive manoeuvres will help us build a better understanding of the relationship between predators and their prey.