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I'm getting concerned with Owen. He has stopped taking frozen foods from the stick and if I had to guess his last two crab kills were just kills and not to eat them. His habits have also changed. He seems to spend more time in hiding and less time out and about. The water tests fine and I changed 50% over the weekend to be sure.
I know with female octopus, it is easy to tell when the end of life is drawing near but with males is there a key indicator I should be looking for? (Asides from lack of appetite and behavioral changes)
The most common observation for senescence is loss of appetite followed by fully not eating. However, you do need to (as you have done) try offering a change of diet when this is observed as we do see a reversal of the appetite problem with a change of menu sometimes.
Sometimes they will start doing very strange things near the end (wandering aimlessly around the tank, trying to leave the water, moving objects with no obvious intent). One of mine moved mounds of sand from one side of the aquarium to the other. I had to laugh (and video) in spite of knowing why I was seeing the behavior.
You might remove the pumpkin if the behavior started after it was added just to see if anything changes. I never quite trust the "aquarium safe" stuff that is painted but I cannot give you examples or written evidence to defend the concern.
We even went and bought him a shrimp for a tank mate and it hasn't gotten eaten like we hoped.
The pumpkin has been in there as long as he has. I can take it out but it has been 6 + weeks with no effect.
He hasn't eaten in over a week now. A few examples of behavioral changes, he jets around the tank more than I have ever seen him, he used to wander around the glass and rarely jetted anywhere. He seems to have relocated from his cave to sleeping under a rock ledge. At times he will go around like he used to but that is getting rarer and rarer.
Behavior changes, particularly restless/aimless wandering are a sign of senescence. However, the same symptoms occur when the water is uncomfortable. You might try another large water changes since you have removed the pumpkin and double check your salinity and PH.