[JoeAltmaier]

Is that really true? There are many photo luminescent creatures there, so light must be important to them. I'm thinking they appear to be blind to us in normal daylight, perhaps because their sensitivity is to extremely low levels of light. In their environment, they can see perfectly well?

[panglott]

Deep ocean may be an unusual case because species and individuals can relatively easily move between different layers. Unlike caves.

But marine creatures have many different adaptations. Cetaceans IIRC have relatively good visual acuity, but are monochromants. Lots of animals lose color vision. Some animals develop huge eyes to pick up tiny bits of light; some have a bioluminescent lure for those fish with sight. Others go blind. But no marine creatures adapted to deep ocean are going to have the level of visual acuity and sensory dependence on sight that terrestrial creatures often have.

Blindness is a continuum.