from the point of view of the head, the gun shrimp (Alpheidae family) is not impressive, but they make a lot of noise under water. Gun shrimp have a special structure of large pliers, as long as the open pliers are quickly closed, they can make a "gunshot" and locally emit shock waves for attack.

Let's first listen to the sound of gun shrimp:

(recorder: Mike Biddle)

although it can only deal with small animals, this gun is so powerful that it can even kill small prey.

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however, how on earth does the shrimp "shoot" underwater with a clamp? The answer is that they take advantage of cavitation.

cavitation means that, in some cases, the local pressure in the liquid decreases, resulting in a low-pressure cavitation close to vacuum. The internal pressure of such bubbles is very low, and the surrounding environment also has a strong effect of liquid pressure and atmospheric pressure, so they are quickly flattened under the surrounding high pressure. The process of the bubble being flattened is very shocking. under the action of the surrounding high pressure, the liquid will impact quickly in the local part of the bubble, which can produce a strong shock wave.

previously introduced an example of smashing a glass bottle with an empty hand, from which you can experience the power of cavitation:

take a closer look at the bottom of the bottle, when the bottle moves down quickly, the water in the bottle does not keep up with the rhythm, so there is a low-pressure bubble at the bottom of the bottle, and because of the pressure imbalance, the water above quickly flattens the bubble and hits the bottom of the bottle, causing the bottle to break.

the "gunshot" and shock wave emitted by the gun shrimp also come from the moment when the low-pressure bubble is flattened. The pliers of the gun shrimp have a special structure on both sides: a groove on one side and a bulge on the other.

according to a recent study, when the pliers close quickly, the bulge squeezes the water out of the groove quickly, creating a high-speed current (arrow above). And around the high-speed current, there will be a vortex. When the flow rate is fast to a certain extent, local low pressure will be produced in the whirlpool, which will promote the formation of cavitation. Then, as the bubbles are flattened and local liquids collide heavily together, the shock wave is generated.

in high-speed photography, you can also clearly see the process of bubble generation and being "flattened":

(videographer: Michel Versluis)

in many cases, human beings are worried about the side effects of cavitation (such as causing damage to the propeller), and gun shrimp are really good at turning it into their own weapon.

PS: you may have seen the cartoon in which mantis shrimp (Oratosquilla) struck to produce shock waves. It should be noted that gun shrimp and mantis shrimp are two different things. Some crickets do produce cavitation, but they are not the ones we usually eat.

Source: https://www.nature.com/articles/s41598-017-14312-0

https://www.newscientist.com/article/2151700-how-a-tiny-shrimp-fires-a-savage-shock-wave-using-just-its-claw/