Imagine you are a small crustacean. You might be minding your own business, until suddenly you feel it: a wave of water signaling the approach of a predatory fish. Luckily, your escape response is incredibly fast. All you need is that brief warning wave to scoot away in the nick of time. Furthermore, your fishy pursuer probably uses a suction-based strike, and suction feeding requires a fish to be very close to its prey for success. Given the rapid approach of the fish, and the comparatively huge wave it must send toward the copepod, it seems remarkable that fish can catch anything at all. Yet many fish survive by suction feeding, and they are remarkably successful predators. This raises an interesting question: how can fish ever get close enough to prey for suction feeding while avoiding detection?
The wave produced by the front of a swimming fish is called a bow wave, and it should provide ample stimulation to frighten away prey. And somehow, the prey doesn't escape. They hardly seem to notice the bow wave at all. Fish must have some way of sneaking up on small prey that does not elicit an escape response.
Brad Gemmel and his colleagues at the University of Texas suspected that fish might be able to actively avoid detection by manipulating their own bow wave. The group of researchers filmed predatory interactions between zebrafish and copepods using tomographic particle image velocimetry, a technique to visualize fluid movement in a 3-D volume. By measuring the bow wave and fluid strain between the fish and the prey, they hoped to find some clue as to the fish's apparent stealth.
Surprisingly, the researchers noticed that during predatory strikes, the fluid forces from the bow wave on the prey were much smaller than expected. However, a normally swimming fish that was not approaching prey had a much stronger bow wave. In fact, somehow, the feeding fishes were targeting the change in their bow wave. The fluid strain was weakest in the plane of the prey. But how were the fish doing it? Gemmel and colleagues decided to look for some change in behavior just before the suction strike that did not appear during normal swimming and found there was one chief difference in the fish between predatory and non-predatory swimming. When a fish approached a copepod, it opened its mouth just slightly before the actual feeding strike. During routine swimming, the fish's mouth did not open.
By opening its mouth ever so slightly, the zebrafish created suction: sucking its own bow wave in to hide it from the copepod. This hydrodynamic stealth has never been observed before. It's a clever co-option of a mechanism that was already being used for feeding – and given the prevalence of suction feeding among fishes, hydrodynamic stealth may be more common than anyone expected.
