Deepsea fish use bio-lumenescence and photo cells of some sort.
The fish that use photo cells use light to blend in to their surroundings, so from below looking up at the fish, they use a bluish light on their
stomachs to match the sun filtering down from the surface and from above looking down on the fish its dorsal section turns dark to match the depths
below them.
Has any navy researched this for there subs???
Maybe I should have started this as a thread!! lol