A snapping or pistol shrimp. This is either Alpheus glaber or Alpheus macrocheles. Hard to say as I'm no expert. I only recognised it as a similar shrimp spotted a few years back made it into the national newspapers.
These shrimps are famed for their ability to produce a loud snapping sound with their claw. By loud I mean incredibly loud as in one of the loudest animals in the sea. The name pistol shrimp comes from the fact it wounds like a gun shot (only louder).
This little fellow was found whilst rock-pooling next to St Micheal's Mount in Marazion.