My apologies, folks from UK and Australia -- I don't mean it that way. In videos that I've seen, octopus blood is rarely visible, and when it is it's not guaranteed to be distinct from an inking response to fear/injury. Even in videos showing cutting of live animals (ugh!), I understand that essentially no bleeding is visible. Is this an artifact of our poor vision? Octopuses have apparently around 4% to 6% blood by body mass, and according to the paper below they have the ability to recover quickly from a loss of 40% or so of their blood using fluid filtered from their digestive tract. (Apparently, all of their water intake comes from this source; tie those ducts off and they quickly die of thirst.) So -- is octopus blood ever visible in the water? Do body injuries have the "instant seal" effect that a lost arm has? Is that effect different by species? This was the paper that talks about blood loss: "FLUID UPTAKE AND THE MAINTENANCE OF BLOOD VOLUME IN OCTOPUS" http://jeb.biologists.org/cgi/reprint/175/1/211.pdf A question that occurs to me: If they never lose blood, how did they evolve to be so good at replacing it? Also, would blood in the water be visible to octopus eyes, with their much greater sensitivity to low light and polarization? And how about the octopus's sense of smell? Can he smell the blood of another octopus?