Carlos, You have reaised an interesting topic, it would be interesting to 
see if the ink released by the octopus utilises the same 'mechanism' as the
purple dye in the group of shells which produce this purple pigment.  I am
told that the octopus selectively extracts Vanadium from the seawater, and
it is a Vanadium concentrate it releases, together with an organic compound
and the two interact to form this ink.  As a result of this finding, a new,
very accurative and sensitive colourometric method for the measurement of 
Vanadium has now been developed.  I am sure that some of our members can
give you more information on octopus ink production.  I am afraid that the
above information was just passed on to me by an analytical chemist who was
aware of my interest in shells. I have no reason to doubt the information ,
but I do not have any documentation.