The explosive physics of a mantis shrimp punch

HostIf you have ever tried to run through the water at a pool or even just throw a punch while you're swimming, you know how much the water fights you. It feels like you're trying to move through heavy syrup. The resistance just kills your speed. But there's a tiny animal in the ocean that has figured out how to move through that thick water so fast it actually breaks the rules of physics.

GuestIt really is incredible. The mantis shrimp is basically a living weapon, and it has to deal with that same water resistance you feel in the pool. Most animals move their limbs by just pulling on their muscles, but muscles have a built-in speed limit. They can only shrink so fast. If this shrimp only used its muscles to strike, it would be way too slow to hunt the way it does. Instead, it uses a trick called power amplification.

HostSo it's not just about being strong? I mean, I always assumed it just had really beefy arm muscles for its size.

GuestWell, it does have big muscles, but it uses them in a different way. Think of it like a crossbow. You use your relatively slow arm muscles to pull the string back, and then you lock it in place with a latch. All that energy is just sitting there, stored up. The shrimp has a special part on its arm made of a springy material called chitin, and it's shaped like a saddle. It uses those muscles to slowly bend that saddle-shaped spring back, and then a little latch holds it in place. When it sees a crab, it lets the latch go. All that stored energy snaps forward at once. It hurls its claw, which people call a dactyl club, at fifty miles an hour. In terms of how fast it gets moving from a standstill, it's about the same as a bullet firing out of a small gun.

HostThat sounds like it would create a massive splash, but I have heard it actually boils the water. Is it moving so fast it creates heat?

GuestThis is the part that trips people up. It does boil the water, but not because of heat. It's a thing called cavitation. Because the claw is moving so fast, it pushes the water in front of it out of the way with a lot of force, but it moves too quickly for the water behind it to fill in the gap. This leaves a tiny empty space, a void, behind the claw. The pressure in that void drops so low that it hits what's called the vapor pressure of the water. When the pressure is that low, water turns into a gas instantly. It boils because there's no pressure holding the liquid together.

HostWait, water boiling without any heat? That sounds like a magic trick. How does that help the shrimp catch anything?

GuestIt creates a one-two punch. First, the shrimp hits the prey with its physical claw. But then, a few thousandths of a second later, the water rushes back into that empty void to fill it. When that bubble of vapor collapses, it creates a second shockwave. It's so powerful that it can stun or even kill a crab even if the shrimp misses with the actual claw. The water itself becomes the second hit.

HostAnd is that when the light happens? I have heard there's a flash of light when they strike.

GuestYeah, and that bit is even wilder. When those bubbles collapse, the water rushes back in at speeds faster than sound. It squeezes the gas inside the bubble so tightly and so quickly that it releases a burst of energy. Scientists call this sonoluminescence. Some people even call it a star in a jar. For a tiny fraction of a second, the temperature inside that collapsing bubble can reach several thousand degrees. That's about as hot as the surface of the sun.

HostOkay, if the water is getting as hot as the surface of the sun, why is the shrimp not coming home as a boiled dinner? I mean, how does its own arm not just melt or shatter?

GuestIt's all about how tiny the spot is and how fast it happens. The heat is gone in a billionth of a second, so it doesn't have time to actually cook the shrimp or the prey. But the physical force is a real problem for the shrimp. To keep from breaking its own arm, the dactyl club is built like a piece of high-tech armor. It has three different layers. The outside is very dense and hard, full of minerals. But the middle layer is the real secret. It has a spiral layout, which scientists call a Bouligand structure. It's made of layers of fibers that sit in a spiral pattern. If the impact starts a tiny crack in the arm, that spiral shape forces the crack to keep changing direction. It gets lost in the maze of the spiral and can't spread. That lets the shrimp throw thousands of these bullet-speed punches over its life without the hammer ever breaking.

GuestIt's just one of those cases where nature found a way to use pure physics to turn a simple arm into a supersonic weapon.

HostWhile we're still down here struggling to just walk through a waist-deep pool, that shrimp is busy turning its own arms into tiny, sun-hot bullets.