Sharks: The Animal Answer Guide

3 Shark Colors

53 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 “countershading .” 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 countershaded 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 sidewelling 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 viewed Chapter 3 Shark Colors 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, creating 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 conspicuous 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 background . Countershading is the best of all possible coloration worlds. With one minor exception. When viewed from directly below, any shark, regardless 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 (wobbegong 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 background 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 colorbackground combinations eliminate contrast between the shark and its background. Helfman et al. (2009); used with permission of Wiley-Blackwell 55 Shark Colors may or may not be countershaded. Regardless, their backs, instead of being a solid dark color, are a mottled gray or brown, often arranged as lighter regions between dark saddles. In some sharks, such as triakid Leopard Sharks and scyliorhinid shysharks, the saddles and blotches wrap around almost the entire body before grading to a lighter belly. Such blotchy coloration is similar to that of benthic (bottom-living) bony fishes such as scorpionfishes , flatfishes, and sculpins. This color pattern most likely makes the animals...