Sharks The Animal Answer Guide

54 Sharks: The Animal Answer Guide

by their predators or prey from the side, and countershading works equally

well then. When seen from slightly above the horizontal, the dark upper

side of the shark absorbs relatively bright, mostly downwelling light, cre-

ating a dark target that is seen against the darker background of slightly

upwelling light. Similarly, if viewed from slightly below the horizontal, a

lighter-colored upper belly surface reflects weak upwelling light, creating

a relatively bright target seen against the lighter background of slightly

downwelling light.

Countershading makes even more sense when one realizes how con-

spicuous a uniformly colored shark would be. A uniformly light-colored

shark viewed from above would reflect rather than absorb downwelling

light and become a bright target against a dark background. Similarly, a

uniformly dark shark viewed from below the horizontal would absorb weak

upwelling light and appear as a dark object against a relatively bright back-

ground. Countershading is the best of all possible coloration worlds. With

one minor exception. When viewed from directly below, any shark, regard-

less of color, is clearly visible as a dark silhouette against the background of

bright, downwelling light.

Which sharks aren’t countershaded?

Sharks that spend most of their time on or very near the bottom (wob-

begong carpetsharks, scyliorhinid catsharks and shysharks, swell sharks)

Most sharks are countershaded: darker on top, gradually lighter on their sides, and brightest on their bellies. This
color pattern makes them disappear in the water column because their colors reflect light that matches natural back-
ground light. (A) A uniformly gray shark illuminated from above, as in natural lighting, would have a relatively bright
back and a relatively dark belly. (B) A countershaded shark illuminated from the side, as in flash photography, stands
out against the natural light behind it. (C) In a countershaded shark under natural lighting, the gradual transition
from dark back to light belly has an averaging or cancelling effect. The top of the shark is seen as dark against dark,
the middle as intermediate brightness against intermediate, and the light belly as light against light. All shark color-
background combinations eliminate contrast between the shark and its background. Helfman et al. ( 2009 ); used with permis-
sion of Wiley-Blackwell

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