We do not normally see Rock Crabs in the tidepools. This one scared me by popping a large claw out of the water near my feet! It was so well camouflaged I did not even see it till then. They are much larger than the Shore Crabs we see all the time. This guy was about 4 to 5 inches across his carapace. (That is the shell.) I do see them  when snorkeling. But this was the first one I have seen in the tidepools. They really look prehistoric!