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From flickering plankton near the shoreline to the steady glow of deep-sea anglerfish, bioluminescence is a pervasive feature of the oceans. Scientists estimate that more than three-quarters of marine species can generate light, yet the phenomenon is unevenly distributed: it reaches its peak in the aphotic, or lightless, zones where sunlight never penetrates. There, flashes and glimmers replace vision based on reflected light.

Although the precise chemistry varies among groups, most bioluminescent reactions follow a common pattern. The enzyme luciferase catalyzes the oxidation of a small molecule called luciferin, releasing energy as photons. Some animals package the reactants in dedicated organs, while others rely on symbiotic bacteria that perform the reaction externally. Because the process generates little heat, it is far more efficient than the incandescent bulbs invented by humans.

The light serves multiple ecological purposes. Deep-sea shrimp eject luminous clouds to distract predators, squid use it for counter-illumination to erase their shadows, and a few fish emit rare red wavelengths to communicate in secrecy because most marine eyes are blind to red. Beyond ecology, the same chemistry is guiding technological innovation: researchers insert the luciferase gene into cells to track tumors in live mice and to develop low-energy medical imaging devices. Thus, studying glowing creatures illuminates both ocean life and human health.