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These shallow-water fish can use their eyes like flashlights

If you’ve ever visited an aquarium, you might have noticed tiny “sparks” of light dancing around the eyes of some fishes. Now, scientists have shown that certain species of fish can use that illumination—a reflection of the light streaming down from above—just like a flashlight, and redirect it at prey. Seven years ago, a German zoologist realized that the sparks, produced when fish rotated their eyes, came in two colors: blue and red. Blue was the “regular” color, and red appeared when fish with special fluorescing cells in their irises turned them on. To find out whether the fish were indeed controlling the flashes, the zoologist and his colleagues experimented with triplefins, finger-size fish that live in the shallow coastal waters of the eastern Atlantic Ocean and the Mediterranean Sea. In one experiment, they showed the fish either prey or another object. The sparks appeared only in the presence of prey. In another experiment, the fish changed the color of the redirected light to blue or red, based on the background. On blue-hued backgrounds, they used a red light, and on red-tinged backgrounds, they used a blue light, to better illuminate prey, the scientists say. That suggests that the fish have complete control over when and how they spark, and may even be using their flashlight eyes to detect prey, the researchers report today in Royal Society Open Science. Still to be seen: whether other fish do it, too.