eli5 How does the gun of a pistol shrimp work?



How can a little thing like this, create such an incredible amount of energy?

In: Biology

The amount of energy is actually not that high. It’s estimated that the energy output is only around 1 joule. 1 joule is about the amount of energy it takes to lift up a 100 gram (0.22 lbs) object 1 meter

What is impressive about pistol shrimps is power. Power is simply energy divided by time. Pistol shrimps can close their claws in less than 1 ms which means this small 1 joule is applied extremely fast. That’s why the power output it about 200W (for reference most of your typical household appliances consume around 100-1000W).

The third thing is that the energy gets concentrated into a very small space. That’s precisely why this 1 joule of energy can create a pressure imbalance and cause devastating damage to small fish.

The way pistol shrimps do this is with a special muscle configuration called a torque-reversal joint. It’s a bit complex but there’s a somewhat appropriate analogy. Instead of having to contract its muscles when it wants the claw to move, the joint allows for the muscle to already be contracted before the movement starts. It’s a bit like an arbalest where you draw the string back long before you actually shoot the arrow.

The shell of their pistol claw is flexible and acts like a spring, along with springy tendons. The pistol shrimp opens its claw against the spring action of the claw shell and tendons, which stores the energy as tension. When the shrimp releases its claw, it pulls it shut with muscles *and* the stored tension in the spring, slamming it closed with great force.

One half of the claw has a groove in it, and the other has a slot that fits into that groove. The groove is full of water until the claw closes and the slot shoves the water out of the way with all of that force. The groove has a narrow outlet that limits how much water can leave. A quirk of fluid dynamics is that when you narrow the flow without decreasing the force behind it the fluid has to accelerate. This means that as the water is forced out of the groove it rapidly accelerates until it’s going faster than the speed of sound in water.

This creates a shockwave not unlike a [mach cone](https://upload.wikimedia.org/wikipedia/commons/e/e1/FA-18_going_transonic.JPG), except water can’t flow fast enough to fill the space caused by the shockwave. This creates the cavitation bubble – a bubble of vacuum that rapidly collapses. As the sides of the bubble slam into each other, it creates *another* shockwave that is the loud pop of the pistol shrimp.