BillyZ the Idea of the displacement of water causing the sound is not really feasible, although gas is easily displaced and compressed, water is very difficult to compress. The idea that a shrimp can move fast enough to create a vacuum in an extremely viscous fluid is hard to believe. If this is not what you meant please explain.
P.S If that were true, the displacement caused by a tsunami would sound like thunder, and could be heard miles away.
P.P.S Test it... Take a capped bottle completely filled with water and put it under pressure, or take it into an airplane, and then do the same to one filled with air, see if the one filled with water changes shape.
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