Male octopus often eat their own arms during senescence. As D said, bacteria can cause it. I would guess that too much "stress" could also cause the bacteria problem. Regarding the senescence, I believe that could also be a problem in their central nervous system resulting from the aging process...
I'm still leaning to some of the instances being a prion, it's simply too infectious to be much else, I do agree stress is a contributor, but not the whole story. We've had it a couple of times and neither stress nor senescence seemed to play a part, nor could any bugs be cultured. THe cleaning of the system is extremely prolonged and shortcuts led to further cases, once sterilisation of EVERYTHING took place (over more than a month!) we didn't see it again. A complex issue methinks!
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