On a moonless night in a handful of tropical bays, you can dip a paddle into the black water and watch it burst into cold blue light, every drop trailing luminous sparks, fish darting away as streaks of fire, your own hand glowing if you trail it in the sea. The effect is uncanny and beautiful, and it is produced by some of the smallest organisms in the ocean. This is bioluminescence — light made by living things — and the sea is full of it.
Cold light from tiny engines
Bioluminescence is the production of light through a chemical reaction inside an organism. The essential ingredients are a light-emitting molecule, generically called a luciferin, and an enzyme, a luciferase, that triggers it to react with oxygen and release energy as light. Crucially, almost none of that energy is lost as heat, which is why it is often called "cold light" — far more efficient than a light bulb.
In the glowing bays, the light comes from dinoflagellates: single-celled plankton, present in enormous numbers, that flash when the water around them is disturbed. Agitate the water — with an oar, a fish, a wave, a swimmer — and each jostled cell emits a brief spark of blue. A few places on Earth have the right combination of sheltered geography, warm water and nutrients to concentrate these organisms into permanent glowing lagoons. The most famous is Mosquito Bay on the Puerto Rican island of Vieques, often cited as the brightest bioluminescent bay in the world, along with others in Puerto Rico and scattered sites elsewhere in the tropics.
The milky seas, and why creatures glow
Stranger and far rarer than the sparkling bays is a phenomenon sailors have reported for centuries and science long struggled to believe: the milky seas. Unlike the fleeting blue sparks of disturbed dinoflagellates, a milky sea is a vast, steady, whitish glow that can stretch across thousands of square kilometres of open ocean, most often in the Indian Ocean, turning the sea into something like a field of snow that ships sail across for nights on end. It is thought to be caused not by dinoflagellates but by immense concentrations of luminous bacteria, glowing continuously rather than flashing.
For a long time milky seas were the stuff of mariners' tales, too rare and remote to study. Then, in the twenty-first century, researchers turned to satellites, sifting orbital imagery for the faint continuous glow. They succeeded — identifying and even photographing milky-sea events from space, and in some cases directing observers towards one — confirming that these enormous glowing patches are real and beginning, at last, to bring a centuries-old sea legend into the reach of science.
Why do so many marine organisms make light at all? Because in the sea, especially the deep sea — the largest habitat on Earth and one of near-total darkness — light is worth making. The reasons vary. Some creatures use it to find or attract mates, flashing coded signals in the dark. The anglerfish dangles a glowing lure to draw prey towards its jaws. Many animals use light defensively: a startled organism may emit a blinding flash to dazzle an attacker, or release a glowing cloud as a decoy while it escapes, or — most cunningly — use light to erase its own shadow. In the twilight depths, an animal seen from below shows as a dark silhouette against the faint light from the surface; by glowing gently on its underside to match that light, a creature can vanish, a trick called counter-illumination. And the dinoflagellates of the glowing bays may flash precisely to betray whatever is disturbing them — their spark drawing the attention of a bigger predator to the small creature trying to eat them, a burglar alarm of the plankton.
That is perhaps the loveliest thing about the seas that glow. The light in the bay that seems, to a wondering visitor with a paddle, like pure enchantment is, to the organism making it, a matter of life and death — a warning, a lure, a disguise or a cry for help, written in cold blue fire across the surface of the dark. The wonder we feel at the glowing water and the survival the organism wrings from it are, in the end, the same phenomenon seen from two sides.
Quiz nuggets
- Bioluminescence is light made by a chemical reaction using a luciferin molecule and a luciferase enzyme, producing almost no heat ("cold light").
- The blue sparkle of bioluminescent bays comes from single-celled plankton called dinoflagellates that flash when disturbed.
- Mosquito Bay on Vieques, Puerto Rico, is often described as the brightest bioluminescent bay in the world.
- "Milky seas" are vast steady glows, mostly in the Indian Ocean, thought to be caused by luminous bacteria and now confirmed by satellites.
- Counter-illumination is a defensive trick in which an animal glows on its underside to erase its silhouette against light from above.