Look beneath the ocean’s surface, and you will instantly see how water transforms light. Colors appear differently because water scatters the cooler colors like blue (known as short wavelength light) and absorbs warm bright colors like yellow and red (known as long wavelength light).

If you take a red object under water with you, the deeper you go the less red it will appear. This happens because there is a decreasing amount of red light available to reflect back to your eye. Remember that a red object appears red because it absorbs all other colors and reflects red light. If there is no red light available, the object will look black or gray.

You can demonstrate this for yourself. Take a flashlight, cover it with a blue filter, and use it to look at a red object in a dark room. Against a black background the red object will seem to disappear. It is important to remember this fact when you see pictures of a bright red animal in the deep ocean. We can see the red pigmentation, because we use bright lights. But in the dim blue light, where this animal lives, it will appear gray or black.

Water not only changes the color of sunlight, it significantly changes its intensity. In clear ocean water, visible light decreases approximately 10-fold for every 75 m that you descend.

This means that at 75 m below the surface the light is just 10% as bright as it was; and at just twice that depth, 150 m, it is another 10-fold dimmer, or 1% of surface light. Below this depth there is insufficient light for photosynthesis, but there is still plenty of light for seeing. This is because eyes are useful over an astonishing range of intensities.

Light conditions affect the functionality of both fish eyes and human eyes. Human eyes, for example, are functional in bright sunlight at high noon and under dim starlight on a moonless night. This is a range of about 12 "orders of magnitude," with each order of magnitude representing a 10-fold difference. Under water (where light decreases 10 fold with every 75 m of descent), the human eye theoretically can detect light down to almost 900 m. The eyes of deep-sea fish, however, may be functional down to 1000 m. Their eyes show remarkable adaptations and may be 10 to 100 times more sensitive than ours. Even below 1000 m there are many animals with functional eyes. These eyes have evolved to detect bioluminescence (the emission of light by a living organism).

Ocean animals that can make light are everywhere, often in very high abundance. Bioluminescence is common because it provides a significant survival advantage in the dim dark depths. It helps animals find food, either with the aid of a built-in "flashlight"or by means of a "lure," and it is used to attract mates and to defend against predators.

The light is the product of a very efficient chemical reaction. Light produced in this way is called chemiluminescence. Light sticks are a well-known example of chemiluminescence. Bioluminescence is just one form of chemiluminescence -- one where the chemicals are synthesized by living organisms. These chemicals are referred to as luciferin and luciferase, which often leads people to assume that all animals produce the same light-producing chemicals.

However, there are number of different luciferins and luciferases, which indicates that bioluminescence evolved multiple times during evolutionary history -- a clear indication of what an important survival advantage it provides.

Edith Widder
Senior Scientist
Harbor Branch Oceanographic Institution
Ocean Explorer