How archerfish hit insects by spitting water

HostIt's a strange thing to think about, a fish that hunts out of the water. Usually, if you're a fish, you just wait for a bug to fall in or swim by, but these little guys take matters into their own mouths.

HostHow does a fish actually manage to knock a beetle off a leaf with just a spray of water?

GuestIt's all about how they use their mouth like a gun barrel. They have this long, thin groove that runs along the roof of their mouth. When they're ready to shoot, they press their tongue up against that groove to turn it into a tiny tube. Then they snap their gill covers shut really fast. That squeeze forces a jet of water out through the tube at high speed. It's not just a random squirt. They can actually change the shape of the tip of their mouth to aim the water and control how much comes out.

HostBut water is soft. If I splash a bug, it just gets wet. It doesn't usually fall off a branch. There must be some real force behind it.

GuestThat's where the real trick comes in. If the fish just sprayed a garden hose at the bug, the bug would probably just hang on. But the fish does something much smarter. It makes the water bunch up in the air. When the fish starts the shot, the first bit of water leaves the mouth at one speed. But then, the fish keeps pushing and makes the tail end of the water jet move even faster than the front part.

HostSo the back of the water stream is catching up to the front while it's flying through the air?

GuestYeah, and that means by the time the water hits the bug, it has turned into one big, heavy drop. It hits like a solid slug. Scientists have found that the force of that water hitting the bug can be ten times stronger than the bug’s grip on the leaf. It's like being hit by a tiny, wet fist. The fish can even change the timing of its mouth movements based on how far away the bug is, so the water always bunches up at just the right moment of impact.

HostThat sounds like a lot of work for a fish brain. But there's another problem. If you have ever looked at a straw in a glass of water, the straw looks bent or broken. That's because light changes direction when it moves from water to air. Does the fish see the bug in the wrong spot?

GuestIt does. From under the water, the bug looks like it's higher up in the air than it really is. If you or I tried to shoot that bug, we would miss every time because our eyes would lie to us. But the archerfish has a way to fix that. They're born with the ability to account for that bend in the light. They can look at a bug from almost any angle and their brain automatically tells them where the bug actually is.

HostI find that hard to believe. A fish can't be doing math in its head. It has a brain the size of a grain of rice. How can it know how much the light is bending?

GuestIt's more like an instinct, or the way your brain knows how to catch a ball without you thinking about the laws of gravity. But even for them, it's not perfect. The math is easiest if they're sitting directly under the bug, because if you look straight up, the light doesn't bend at all. Many times, they'll swim until they're right below the target before they take the shot. But if they have to shoot from an angle, they can still do it. They have been doing this for millions of years, so their eyes and brains are just built for this one job.

HostSo they shoot the bug, the bug hits the water, and then they just swim over and eat it at their own pace?

GuestNo, they can't afford to be slow. These fish live in groups, and they're all very hungry. If one fish knocks a bug down, every other fish in the area is going to race to get it. If the shooter waits for the splash, they'll lose their dinner to a faster neighbor.

HostSo they have to be quick, but they still have to wait for the bug to land, right?

GuestThey actually start moving before the bug even hits the surface. The moment the water jet leaves their mouth, the fish starts swimming toward the spot where the bug is going to fall. They can work out the path of the falling bug in a split second. By the time the bug touches the water, the fish is already there with its mouth open. They can reach the landing spot in less than a tenth of a second. It's one of the fastest responses in the animal world.

HostThey have to know where that bug is going to be in less than a tenth of a second, which is faster than you can blink.

HostThat little fish is basically a living slingshot that has learned how to beat the tricks that light plays on our eyes.