flat rock crab (#67, 99K)
This crab is commonly found in shallow water near or above the surface, hiding among narrow rock crevices. Flat rock crabs are extremely quick, moving so fast with their sideways motion they are often difficult to identify. Their speed and elusiveness make them unlikely photo subjects.

This particular specimen seems to be short a few limbs (nobody is that quick with a shutter).

Percnon planissimum