An underwater glow. A fleeting gleam across a field. These lights seem mysterious, but organisms generate them for practical purposes. Bioluminescence fends off predators, lures prey, and attracts mates. Making light is such a useful trait that it has evolved independently at least 40 times. It occurs most commonly in the ocean, where bioluminescence is often the only source of light. Under the right conditions, a bioluminescent flash can be seen a hundred yards away.

(Read more about “Luminous Life” in this month’s issue of National Geographic.)

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