For many years it was thought that they did not hear at all as scientists could not discover a biological mechanism and they appeared deaf with testing. A couple of years ago, scientists discovered a hair like structure in shrimp that was later found in octopuses (reexamined because it appeared they could "hear" certain ground rumblings). It turns out that they can hear certain frequencies that our out of our hearing range. Here is a thread on the topic with several article references.
Okay, thanks for the answer. So day update: I removed the fish yesterday and he has actually barely been out since then. I think he got a bit firghtened when I tried to catch them as I thiiiink he inked, but I'm not sure, could've been sand or something. Still, he ate a piece of scampi that little time i saw him. Found him again earlier today, but havent seen him for a couple hours now. Tried to lure him out with some food, but he doesn't respond. Should I be worried?
Edit: Found him, but he is still hiding behind a rock
Octopuses are naturally shy creatures. It will take about a month before he fully acclimates to tank life and to begin to learn his/her personality. As long as he is eating and active, I would not worry about current behaviors
So not much changed except that I realized the shrimps are super jelous of his food so they'll try to take it from him, which they succeed to do. Therefore I've been trying to scare them off while he eats just now in the beginning till he gets comfortable and confident to scare them off on his own. So I was wondering three things:
1. What are signs of stress or him being uncomfortable?
2. I saw this color you can see in the pictures attached and he seems so... struggled. Could it be anything bad or is it just one of his camouflages? I also think he is missing one arm while a couple others seem hurt. Difficult to get a good picture.
3. How do I determine sex and age?
Not sure if I would say corkscrew, but they sure are tucked in more than I would think is natural. I'm trying to remove the shrimp as we speak. Also I don't have any testers here at the moment, but I'll try to get some after I removed the shrimp. The salinity had dropped to 1.23 so I'm also going to fix that immediately.