Bioluminescence, the ability of certain living things to produce and emit their own light, has fascinated scientists for centuries. The best-known example is the firefly, which flashes on warm summer evenings to attract a mate. Yet the phenomenon is far more common in the ocean, where many jellyfish, squid and deep-sea fish glow in the dark. These creatures use light for very different purposes: some lure prey towards them, others startle or confuse predators, and a few use it to communicate or to blend in with faint sunlight filtering down from above. The light itself is produced by a chemical reaction that gives off almost no heat, making it extraordinarily efficient. Researchers now study these organisms not only to understand how the reaction works, but also to borrow it — the light-producing molecules are already being used in medicine and genetics to track cells and diagnose disease.
