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Why are so many sharks dark on top and light on
the bottom?
Most animals that live up in the water have dark tops and light bottoms.
This is true not just of sharks and rays but also of dolphins, killer whales,
penguins, and even crocodiles. The coloration is known as “countershad-
ing.” Countershading is adaptive because it makes the animal blend into
the open-water background against which it is seen by predators and prey
alike. Blue Sharks and makos, which are open-ocean species, are also coun-
tershaded but tend to a dark bluish back color that is probably a match to
the indigo blue background of the open seas.
A countershaded shark usually has a dark back that grades to a light
belly, although in many sharks the transition from dark to light is abrupt.
Regardless, the principle in operation is that the shark reflects or absorbs
light that makes it blend into the background because dark colors absorb
incoming light and light colors reflect it. A countershaded shark disappears
because its color is opposite to the distribution of light in water, creating a
target that is identical to background. The fish therefore reflects or absorbs
light that is roughly equivalent to the background against which it is seen
at all viewing angles, dark against dark, light against light, intermediate
against intermediate.
Light coming from above is called downwelling, from the side sidewel-
ling or spacelight, and from below upwelling. Countershading is easiest
to understand when viewing a shark from above. A shark with a dark back
absorbs very bright downwelling light, creating a dark target against the
dark background of dim upwelling light. However, most fishes are viewedChapter 3
Shark Colors