by Andre on 20 August 2007
The archer fish gets its name from its amazing ability to shoot flies out of the sky by spitting jets of water at them. Just check out this slow motion video of an archer fish in action:
If you’ve ever tried to grab a fish underwater you know how hard it can be. Most small fish can swim pretty fast and it’s hard to move your open hand through water fast enough to catch them, but that’s only part of the story. Since your intuition about catching is developed from things like balls flying through the air, you will be at an extra disadvantage when fish grabbing because the fish’s apparent position is changed by light refraction at the air-water interface. You can see how this works most clearly by looking at a diagram.
If you follow the refracted ray to its apparent source, you’ll miss the fish. Of course, when shooting water at its prey, the archer fish also needs to correct for this refraction to make accurate shots, but unless it’s an even more remarkable fish than I think it is, it’s not solving any equations to do it. All it needs to do is learn from experience how to shape its mouth and angle its body to hit prey when it appears at a certain position. I’m assuming it solves the problem by brute force, developing a kind of catalog of prey positions and firing postures and just puts the two together to make a hit.
Going deeper, both literally and intellectually, the bottom dwelling pistol shrimp uses physics at a whole new level. Using a specially adapted claw that it rapidly (~30 000 rpm) snaps shut, this amazing shrimp creates a stream of water that moves fast enough to vaporize a small area of water. This low pressure bubble collapses very rapidly. So rapidly in fact that a shock wave is created in the water that is strong enough to stun any prey that gets too close (actually, I’m not sure whether it’s the speed of the jet hitting the prey or collapse of the bubble that stuns it).
A perhaps even more incredible side effect of this process is so called shrimpoluminescence, named after its more famous cousin, sonoluminescence. It is the light that is produced by the very high temperatures that are achieved when the energy of a collapsing bubble is highly concentrated due its small collapsed size. Temperatures inside a collapsing bubble can rival those on the sun and this has inspired the controversial search for bubble fusion.
Even if the archer fish and pistol shrimp aren’t aware of the rich physics that governs their behaviour and that they exploit to survive, it is still a wonder to me that they do. With every snap, and every spit, they demonstrate the beauty and ingenuity that one can find in nature just by looking. With that said, we wouldn’t appreciate the true beauty of these animals without an appreciation of physics. These critters also serve to highlight the advantage we get in describing phenomena quantitatively. They may have mastered an intuitive kind of physics, but formal physics allows us to transcend our intuitions and learn reliable things about situations remote from our experience.
1. The Wikipedia page about archerfish. If you have the energy you could correct the part about swimming directly under their prey to minimize refraction effects. If you check the literature you’ll find that they can shoot from multiple angles successfully.
2. One of the first of many papers discussing the archerfish and refraction [only the abstract is free].
3. A website from the research group that discovered the cavitation and luminescence in snapping shrimp. (includes more cool movies!)
4. The original paper describing bubble collapse as the source of a pistol shrimp’s sound [only abstract is free].
5. The first paper observing shrimpoluminescence [only abstract is free].
6. Jennifer Ouellette discusses shrimpoluminescence amongst other things at Cocktail Party Physics.