Fish with Flashlights

Sloan viper fish

Ocean twilight zone animals, like this Sloane’s viperfish, rely on their photophores for a variety of important survival functions. Photo by Paul Caiger, Woods Hole Oceanographic Institution

Down in the dark and shadowy mesopelagic layer of the ocean, countless species—bristlemouths, lanternfishes, jellies, and others—have a natural ability to generate their own light through chemical reactions. Most twilight zone animals produce blue light—the color that penetrates the farthest through seawater—but some also produce flashes of red, yellow, and green. Collectively, the lights form stunning constellations as the animals swim through the perpetual darkness. You might think they’re just trying to make up for the lack of sunlight coming down from the surface. But in fact, they’re using their natural flashlights for a variety of specific purposes.

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