These guys look as if they should really not be messed about with - but to be fair they are pretty harmless on the whole.  However, when we think of crabs most of us think about things that are much, much smaller that these particular crustaceans.  Mind you, as harmless as they are they can still be a nuisance to their human cohabitors.  Perhaps, though, we should leave well alone and go live somewhere you can't catch crabs this size - literally and metaphorically!