I looked up the average monthly temperature for Haiti (where most of the hummelincki we can get come from (?) and found that it is as low as 78 in winter and 83 in summer. 73 may be a little cooler than their natural minimum temp, however the 78-83 figure is surface water temp, and even 30 feed deeper than that is likely to be a few degrees cooler, so maybe 73 is not unnaturally cool for them. I've kept several octopus, all of them bimacs, which I read only live twelve to eighteen months. I find that they live well over two years if I keep the water at the cold side of their natural temp range (or a little below), and feed them sparingly (two or three times per week). At 56 degrees they were less active (natural annual temp range is 58-68), so it's good to see that yours is still active. I think the low temp, and/or the reduced feeding, significantly extends their life. I would even suggest restricting food down to 75% of what they eat when food is always available, and only increase it if they start getting smaller, or try to eat tank mates. Did you drop temp to try to extend life span, or for some other reason?