@Domido - I find that information about some of the underwater animals glowing to be quite interesting. There is another possibility, though.
When I was a child, my parents took my older sister and me to the Blue Ridge Parkway for vacation. While we were there, we went to an immense underground cavern.
It was beautiful, and really very scary. One of the scariest things was the fish. You could look into all of these underground rivers from bridges and ridges. The fish were absolutely gigantic, and they all had white eyes.
They were able to grow that large because for hundreds, or possibly even thousands of years, they were completely unfished and more or less trapped in this underground haven.
Their eyes were white because they were all blind. Since they were always without light, their bodies adapted to this and they no longer had the sense of sight.
I wonder if this is how the creatures of the lesser known areas of the ocean trench are. Blind, and huge!
If so, I wonder how their other senses have adapted to take over for their loss of vision and what quantities of food must be available to them for their survival.