Boy, that raises a lot of tricky issues... one of the reasons I like learning about cephalopod nervous systems is that they're so different from us, so I'm thinking that it's hard to know how much our own sense of what "pain" and "consciousness" and "awareness" and "suffering" mean carries over to cephalopods. I'm pretty sure that most of us agree that those words have some meaning for cephs, but the way their nervous systems work is different enough that it's hard to know if their experience of anything is analogous to ours.
One thing that comes immediately to mind is that, AFAIK, there is no blood-brain barrier in cephalopods like there is in vertebrates. So any chemical that's in their bloodstream or tissue, can directly effect the neurons of the brain. But also, the nervous system has a lot of peripheral ganglia that respond to stimuli, as well, but are sort of autonomous... the brain is known to be responsible for learning, and it's probably safe to say it handles awareness and consciousness as well, so maybe the place to draw the line of "feels pain" is "brain receives a signal of pain" but severed arms will show some responses to stimuli, including and "ouch" like withdrawal from uncomfortable stimuli, without any connection to the brain at all. Is making a severed arm "feel pain" something that should be avoided?
Although I find that argument about pain related to internal infections and organ failure to be plausible, I can also imagine other plausible views... there are a lot of human organ ailments that have little or no pain at all, because we don't really have detectors for some things that can go wrong, like holding your breath when you're coming up from a deep SCUBA dive(!), but other ones, like kidney stones, where internal organ problems cause immense pain.
I don't know if there have been any studies that determined criteria for measuring the pain or discomfort level for cephs... we know anecdotally that octos will try to climb out of their tanks more if the water quality is bad, and they'll certainly try to escape if you poke them or annoy them. So they seem to have ways of expressing displeasure at outside sources of pain, but it's not really clear to me how to tell from their behavior whether they're "frail but happy" and wanting to be pampered in their last days, or "miserable and despondent." Maybe some octo-keepers have a sense of this... I guess there have been a number of studies which I've only seen referred to in books like Hanlon & Messenger that studied senescence (like the one that showed a delay in it when the optic gland was removed)-- do those provide any behavioral criteria for assessing senescence or quality of life? The table at the end of Greg's article has some criteria for "stages" and "planes," and it sounds like that's the accepted standard, but it seems to be more about level of consciousness than any sort of "quality of life" assessment or chronic pain and suffering, or even the cephalopod's experience of what's happening in the short or long term.
Even in humans and mammals this stuff is pretty poorly understood, and has a lot of weird rationalizations of morality, correctness, and so forth that I find are more emotional than rational... a lot of anesthesia doesn't so much prevent pain as much as it inhibits memory formation, so the patient may be in agony during the operation, but doesn't remember that afterwards. Are these sort of things immoral because the pain is momentarily experienced, even if there's no memory of it later? They seem to be established as acceptable. And I've heard (although possibly from unreliable sources) that it was an accepted belief at some point frighteningly recently that babies didn't need anesthesia because they don't experience pain, or at least that they don't remember it or it doesn't count or something... which, of course, the babies didn't argue with, but when someone came up with (obvious) things to look for, they noticed that babies given anesthesia before surgery seemed to be showing less signs of extreme discomfort.
I'm intending all these questions as inquiry and investigation rather than criticism of any position... I'm trying to raise questions that have some bearing on the discussion about antibiotics for life extension as either an extension of the animal's experience of life or experience of suffering, but I don't really know how they fit in to reaching some sort of conclusion... heck, we as a society can't even decide on who gets to make what decisions and by what criteria for people, who we can probably assume experience pain and suffering and awareness and consciousness just like the rest of us, on life support or with fatal illnesses or who have been condemned to death.
Getting a bit far afield of cephalopods, I read recently that when Richard Feynman was going through cancer treatments, he was concerned that he couldn't make an objective decision on when to stop fighting the cancer and give up, so he arranged that when he thought he should stop his own treatment, he'd ask his wife and sister if they thought that was appropriate, and if either of them thought that he was suggesting it out of temporary depression or something, they would be a check on the decision. When they all agreed that the time had come, he asked the doctors to stop the extreme measures preventing the cancer from taking its course. I have mixed feelings about this (it seems to put one's loved ones in an uncomfortable position) but it does address some of the sticky issues.
