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I believe this species are what I have 2 of. If his mantle is the size of a golf ball, then it is an adult and you will not have much time left with him. I just lost one of the 2 that I have. Octo#2 I only had for 2 weeks. Squiggles about 2 months but she's stopped eating and quarding eggs.
I currently have an Aculeatus in my 55 gallon [I wouldn't recommend going much smaller than that for tank size]. He is a confirmed Aculeatus and his body is bigger than a golf ball and he's been with me for months....so I don't know what the deal is with that, as far as life span goes.
I've found mine to be extremely active, as well as diurnal. Sometimes he plays with me, but to be honest, they seem to be very moody individuals [I've observed several other before this one].
If this is your first octo, I wouldn't recommend an Aculeatus...they don't seem to be as social or friendly as a bimac.