While studying deepsea creatures, I came across a paper that discussed
their colors. Blue (short-wave) light penetrates more deeply into the ocean
than red (long-wave) light by some hundreds of feet/meters.
 
Animals in very shallow water can have any color at all, and there is good
evidence that some of these colors are perceived by other animals (mates,
competitors, predators, prey, etc.).
 
Animals in clear water a few hundred feet (or meters) deep are commonly red
or reddish in artificial light, but in their native habitat they appear
gray or black, and can therefore be considered to be camouflaged. Light red
= gray and dark red = black under these conditions.
 
Animals in very deep, clear water, but too deep for sunlight to penetrate,
are commonly dark or black. As Gary Rosenberg stated, this is an effective
form of camouflage in a realm where some predators have light-producing
organs. (Another form of camouflage is for an animal to be transparent!) As
to how deep sunlight penetrates, well, it differs widely from place to
place. The clearest waters are in mid-ocean, such as the Sargasso Sea. Off
southern California, where I did my work, the depth was more than about 700
feet, or about 200 meters. At this depth, the people who have been there
report the water to appear "bright blue," but this is an optical illusion.
The water is really very dark; evidently, it is a very *pure* and (forgive
me) *deep* blue.
 
Please don't ask me for the citation. I read the article for my doctoral
work about 15 years ago, and didn't cite it in my thesis. But it was
interesting enough to stick in my memory.
 
I like this thread; keep it up! Gary, have a pleasant weekend.
 
Andrew K. Rindsberg