I lived in Ventura CA where there's a market at the harbor on Saturday's where you can buy fish, sea urchin, live shrimp, etc and octopus.

They sell them in jars where the octopus has holed itself up cozily and normally you put the jar in the freezer until the octopus falls asleep, then take it out before it's frozen and clean it.

I didn't know about this easier way and instead read on the internet a different method... you make a big bowl of ice water and throw the octopus in it, stunning it, then quickly disassemble it.

I made the ice water bath and dumped the octopus from the jar into it. As soon as it hit the water it jumped out of the bowl into the air and onto the counter. It ran across the counter to the edge and jumped to the floor. It ran across the hardwood floor. I ran after it with a kitchen towel and used the towel to grip it and pull it up. The suckers stuck to the floor and pop pop pop as I pulled. It detached from the floor and I threw it back in the ice bath and covered the bowl with the cutting board as it passed out from the cold.

When you clean it you see its central brain but there are also other noticeable nerve cluster at each leg. There is also a very hard and sharp beak.

Had to look up (by which I mean ask an LLM) what predator might bite off an octopus tentacle. Moray Eel was the first mentioned. Shark or grouper suggested next. That would be an unlucky octopus to lose several arms to a Moray or several Moray ... guessing shark then.