Why do Pistol Shrimp Snap?

[seacups]

I have a pistol shrimp that is paired up with a gobie and am curious why they snap. I know that they do it as a warning but what else?

[olemiss reb]

It stuns their prey, allowing an easy to catch meal.

how it works: http://www.youtube.com/watch?v=eKPrGxB1Kzc

[seacups]

thats a cool video i saw it before i had a pistol shrimp but did not pay much attention.
are some pistol shrimp more aggressive than others? i was under the impression that my tiger pistol would not harm any thing bigger than them