Is vision a vestigial sense for deep ocean species?

Question

Sunlight doesn't penetrate beyond a couple of hundred feet from the surface of the ocean. Species that exist at greater depth probably live in a state of perpetual night; yet from a quick google image search (i agree it isn't a very authoritative source) for deep ocean species I see that plenty of these have eyes.

So ...

• Are the sight organs in deep oceans species merely a remnant from an earlier era?
• If they are a remnant, has evolution/mutation modified these organs over time?
• Regardless of whether the organs of sight are vestigial organs for deep ocean species, how do the eyeballs withstand the extreme pressure of water in the deeps?

Answer 1

I can't answer your third, but I can answer your first two. With one word, in fact:

Bioluminescence

http://brightnepenthe.blogspot.com/2010/08/palate-cleanser-90.html

That's the deep ocean at night for ya. Unlike underground environments and caves, it's not pitch black pretty much anywhere in the ocean. There are things to see everywhere, and they play an important role in predator/prey relationships and/or mating. The absence of natural light does make things harder to see, but that's why you see the enormous eyes on species which live far down. So, nope! The eyes on deep ocean critters are not vestigial. They're very functional (and, if anything, probably increased in sensitivity).

The basics are summed up here: http://www.lifesci.ucsb.edu/~biolum/functions.html