The Fluid Dynamics of Archerfish Hunting by Spitting

Aatish Bhatia in his blog Empirical Zeal at Wired:

Archerfish are incredible creatures. They lurk under the surface of the water in rivers and seas, waiting for an insect to land on the plants above. Then, suddenly, and with unbelievable accuracy, they squirt out a stream of water that strikes down the insect. The insect falls, and by the time it hits the water, the archerfish is already waiting in place ready to swallow it up. You have to marvel at a creature that excels at what seems like such an improbable hunting strategy – death by water pistol squirt…

…Technically, the term archerfish doesn’t refer to a single species of fish but to a family of 7 different freshwater fish, that fall under the genus Toxotes. They strike with remarkable accuracy, and just a tenth of second after the prey is hit, they quickly move to the spot where it will hit the water. Unlike most baseball players who have to keep their eyes on a fly ball to track it, in less than the blink of an eye, the archerfish is in place, waiting for the insect to arrive.

If that isn’t impressive enough, consider this. When these archerfish squirt water, their eyes are underwater. If you’ve spent any time in a swimming pool, you’ll know that light bends when it enters water. A less astute fish might not correct for this bending of light, and would be tricked into thinking that the insect is somewhere it isn’t. But not the archerfish. This little aquatic physicist is able to seamlessly correct for the bending of light. And it isn’t a minor correction – when the perceived angle of the target is 45 degrees, its true angle is off by as much as 25 degrees.

More here.