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From the appearance of white spots on the arms, and arms apparently longer from arms 1 (front arms) to arms 4 (back), I'd say it is one of the "Octopus macropus" group of species (genus Pinnoctopus according to Steve O'Shea, 1999: NIWA Biodiversity Memoirs112) but I can't say which species it might be. See if you can identify it yourself from the FAO Guide (Vol. 3: http://www.fao.org/3/a-i3489e.pdf under "Callistoctopus"). The size is difficult to gauge but it looks big enough to be mature, which would make it an adult.