You and I had the same idea : using snake as a dynamic part of the level design, allowing player to manipulate this one to complete the game loop. Your game is basically the big brain version of this concept, and mine the action one, so I'm really not surprise if you like it :)
The eels has been going through a LOT of different designs, the most important for me was precisely what you said : the affordance ( the object's sensory characteristics intuitively imply its functionality and use ), and the readability.
That's precisely why I thought :
-Wall eels = psycho-rigid squared shape with 90° moves and non aggressive color = safe
-Electric eels = chaotic sharp triangle shape with 0 predictability and aggressive color = danger
I though it will be sufficient to teach the player, but that doesn't worked for you. I will also considering what you said and tried to enhance the affordance while trying to not ruin the simplistic art style and readability. Thanks a lot for playing and for your feedback, there is always space for improvement.