After reading you concerns and now knowing that the octopus is not just out in the open all the time, I remembered a study done observing Vulgaris and Briareius and it mentions that Vulgaris will often build a den where the Briarius will more often take what is available:
file:///C:/Documents%20and%20Settings/Owner/Local%20Settings/Temporary%20Internet%20Files/Content.IE5/0H8W8VYA/An%2520Analysis%2520of%2520the%2520Cohabitation%2520of%2520Octopus%2520Vulgaris%5B1%5D.ppt#256,1,An Analysis of the Cohabitation of Octopus vulgaris Cuvier and Octopus briareus Robson at Bailey’s Cay, Roatan, Honduras