No, I think you've got the wrong info somewere - what defence mechanism? The only few things you should note though:
1) Their bite - most octopi have VERY hard bites that can break skin
2) Suckers - if you have a large octopus, the suckers can be very strong and an attempt to peel off an octopus hanging on to you will result in sucker marks
3) Escapes - these guys are escape artists and you need a very secure tank
Another point to note - blue-ringed octopi are sometimes offered for sale. Unless you are trained in handling venomous animals, I'd strongly recommend you NOT to purchase these. Their venom is extremely potent and reports of fatalities are not uncommon.
Regards,
Hong Yee
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