Imagine a sea that glitters blue-green in the dark, lit not by the moon but by millions of living creatures, each no bigger than a grain of sand. This is not a scene from a science-fiction film — it is a real phenomenon called bioluminescence, and it is one of the most widespread and astonishing abilities in the natural world.
Bioluminescence is the production of light by a living organism. Unlike a light bulb, which generates heat as well as light, bioluminescent light is almost entirely cold. It is produced through a chemical reaction between a compound called luciferin and an enzyme called luciferase. When oxygen is added, the reaction releases energy as light — typically blue, green, or yellow-green — rather than heat. Scientists estimate that at least 76 per cent of deep-sea creatures are capable of producing their own light.
Perhaps the most iconic bioluminescent animal is the deep-sea anglerfish. The female anglerfish carries a modified spine above her mouth that functions as a living fishing rod. At its tip dangles a small, fleshy bulb teeming with bioluminescent bacteria. In the complete darkness of the ocean floor — sometimes more than 1,000 metres below the surface — the glowing lure pulses gently, drawing in curious prey. When a small fish or shrimp drifts close enough, the anglerfish snaps shut its enormous jaw in less than a tenth of a second.
Not all bioluminescence is so predatory. Dinoflagellates — microscopic, single-celled organisms found in every ocean — flash briefly when the water around them is disturbed. A wave, a swimming fish, or even the hull of a boat can trigger thousands of flashes simultaneously, producing the famous 'sea sparkle' that coastal walkers sometimes witness on calm summer nights. In 2019, sailors crossing the Indian Ocean reported a vast patch of sea glowing continuously for several hours — an event scientists believe was caused by an enormous bloom of dinoflagellates stretching for hundreds of kilometres.
The firefly squid of Japan offers yet another variation. Watasenia scintillans — to use its scientific name — is a small squid, roughly eight centimetres long, covered in light-producing organs called photophores. Each spring, millions of firefly squid migrate into Toyama Bay on Japan's western coastline to breed, turning the shallow water a vivid, pulsing blue. Local fishermen have observed this annual event for centuries, and today it attracts tourists from across the world.
Scientists are now investigating whether the chemical processes behind bioluminescence could be harnessed in medicine — for example, to make cancer cells visible during surgery by attaching luciferase to specific proteins that bind to tumour tissue. The glowing sea, it turns out, may yet illuminate the future of human health.