Re: Why is blood red on land and green under water?

Rick--

This question's answer largely involves perception and light filtering. Blood is essentially the same whether its in the ocean, in a lake, or in a cup of water in your kitchen.

The question is the depth of the object (blood) from the surface. The perceived color of objects changes based on the "ROY G. BIV" order of the spectrum. The deeper you go, the more blue in appearance heading toward indigo/violet. The water is acting as a giant filter, the sun/daylight is your light source. This is gradual and overlapping, so when the "red is gone" that doesn't mean its now orange, and so on. For example, at more than 10 feet, most red is gone, some of the orange and yellow are gone as well. The website here: http://www.underwaterphotography.com/Underwater-Video/Lighting/Color-Underwater.asp
gives a good indication of some of these changes from a photography perspective. Many objects below about 30 feet will appear dark green, dark dark blue/indigo, to black.

So, as you suggest in your question, its not necessarily that its specifically a question of fish, if a diver were to be bleeding beyond a certain depth, say 50 feet, the blood would be a green-black color.

If you can, next time you're diving, bring a bright underwater flashlight and see what colors become more apparent with that as the main light source vs. the light from the surface. Remember though, this filtering in water happens in all directions, so if the object you're shining the flashlight on is greater than 5-10 feet away, much of the red will be gone. Perhaps bring something bright red with you that you can hold in one hand, look at it without the flashlight and then with, see if the colors are different!

Now, there are a few points that will alter this observation some. If where you're diving has a lot of green organic material, whether plants or green algae or whatever, this will skew things to the green range a bit more. Another issue is the oxygenation of the blood and the effects that the water's pH/oxygen content might have once the blood is outside the fish... that is, blood that has a lot of oxygen is bright red whereas blood that is depleted of oxygen is a darker deeper red, almost purple.

Hope this helps answer your question.

-Keith Anderson
Brigham & Women's Hospital, Boston