Planet Earth

Subtle Cuttle

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In less than a second, a cuttlefish can go from being dark brown with yellow stripes to orange-red with dark spots. It makes a chameleon look--well, chameleons are hopeless, really, says zoologist John Messenger of the University of Sheffield in England. Amazingly, though, cuttlefish are essentially color blind; they’re sensitive only to shades of green, yellow, and blue. Messenger wondered how, with such a seemingly enormous handicap, the fish camouflage themselves so successfully. He placed cuttlefish in tanks with patterns of colored gravel on the bottom. A cuttlefish on a floor of red gravel on white (top) produced a bold mottled pattern. Encountering red gravel on blue, the fish produced a much lighter pattern. And with yellow gravel on blue, the mottle was hardly noticeable. Messenger then compared close-ups of the cuttlefish’s skin (middle column) with the gravel photographed through a green lens, as a cuttlefish would see it (photos at right). He realized that cuttlefish rely on contrast rather than color to make their patterns. Yellow, green, and blue gravels look almost uniformly bright to the cuttlefish, while red gravel, which absorbs most of the light that hits it, looks dark. On white gravel, which reflects almost all light, red stands out. The cuttlefish are able to distinguish red from blue, since blue reflects about half the light falling on it.

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