Caribe's ar Pygocentrus Notatus, from the Orinoco drainage. They have a dark spot near the gill.
Red bellies are Pygocentrus Nattereri...they have red bellies and blacks spots ( when young ) on their body. They are the most common all over south america.
they are competely different fish, both are piranhas but they are diff species. caribe grow bigger and have a humeral spot. thats the spot they are talking about on the gills. caribe are also more aggressive than rbps. RBPs have been bread in captivity while caribe have only been bread once. or so the story goes. i've owned both and i prefer the caribe much more agressive and less skittish