They really are remarkable animals and unbelievably inquisitive. Trying to keep them from escaping, even from a relatively small 250 gallon tank was always a challenge and I can only imagine how difficult it is to retrofit the larger tanks they inhabit at public aquariums to be escape proof.
They two I kept were incredibly tactile in their curiosity and loved exploring my arms/hands whenever I put them into their tank. They would readily eat small crabs and shrimp right from my fingers and seemed to like playing tug of war, clinging to my hand with a few tentacles and them some of the heavier rocks with the others. Lego blocks, clear acrylic piping, small plastic toys would keep them busy for hours on end.
Many people don’t realize this but they are unfortunately (or fortunately for humans) remarkably short lived. Most species live only for 1-1.5 years and even the Giant Pacific Octopus usually only lives for 3-5 years. I would always jokingly remark to friends that if they had a longer life span on the order of a decade or more, they probably would have invented some kind of breathing apparatus and conquered land long ago. They would probably have a much more advanced space program by now as well.
I suppose that also means they reproduce very quickly, which is a good thing in light of the other discussion here about sustainability of consuming them as food.
In captivity octopuses are considered relatively difficult to keep due to their sensitivity to water quality. They have a fairly narrow band of acceptable levels of pH, ammonia, nitrite, and salinity. Copper is pretty much Kryptonite to an octopus. I’m unsure what the potential is for large scale changes in oceanic chemistry but I imagine it would be devastating to octopuses, along with overfishing.
I’m actually curious how serious a threat overfishing is. Seeing as how difficult they are to contain I wonder if most commercially available octopus are specifically fished for or are just a byproduct of some other fishing and whether this has a large affect on their populations.
We are down from pre-industrial 8.25 pH to 8.08, and no sign of slowing. Major changes are certain. I don't know if this means the water will dissolve more metals like Cu, but it seems possible.
Also, the cuttlefish: http://www.pbs.org/wgbh/nova/camo/anat-nf.html