I find it interesting that many cave fish lack eyes, but the super deep sea fish have giant eyes. Both with no environmental light.
The deep sea fish use their giant eyes to look at bio-luminescent organisms, which seems to be a lot of the animals in the deep sea. I suppose the small river/pond fish that cave fish come from don't have bio-luminescence and neither does their food.
So cave environment: really nothing to look at.
Deep sea environment: lots of glowey animals to look at.