Interesting - I remember swimming with a blue crab once and he did not like me there - he raised his claws in a defensive nature but was also in retreat.
I have little or no experience with other species crabs so have no way of comparing them.
I have heard via a grape vine there have been some crab fights held and they do seem to be a territorial creature when held in captivity and entering another into the arena from what I have been told.
This example would not give any real retreat ability unless it was done in a very large tank. Would the same thing happen in open water? How much provocation would it take?
Don't know, but my eyes are open......