Finding food
Lures, like the anglerfish's glowing esca, draw prey in. Searchlights, like the dragonfish's red beam, reveal prey without being noticed.
How living light works
Bioluminescence is light made by a chemical reaction inside a living thing. No sunlight needs to be stored, and almost no heat is produced -- which is why it is sometimes called "cold light".
The basic reaction
Almost all bioluminescence follows the same recipe:
- Luciferin -- a light-emitting molecule (the fuel).
- Luciferase -- an enzyme that speeds up the reaction (the spark).
- Oxygen -- which reacts with the luciferin.
When luciferase helps oxygen react with luciferin, the luciferin is left in an energized, "excited" state. As it relaxes, it releases that extra energy as a particle of light. Fireflies also need ATP, the energy molecule found in every living cell.
The names come from the Latin lucifer, meaning "light-bringer". They were coined in the 1880s by French scientist Raphael Dubois, who studied a glowing clam and glowing click beetles.
"Luciferin" is not one chemical but a family name. Fireflies, dinoflagellates, bacteria and fungi each use a different luciferin. A molecule called coelenterazine is the most widespread in the sea, used by jellyfish, shrimp, squid, fish and many others.
Some animals, like certain jellyfish, store a ready-to-go version of the chemistry called a photoprotein, which flashes the moment it meets calcium.
Why most living light is blue
In seawater, blue-green light travels farthest. Red light is absorbed within a short distance. So most ocean creatures both make and see blue or blue-green light -- and many deep-sea animals have lost the ability to see red at all. That is exactly what makes the red-light dragonfish so sneaky.
On land the rules are different. Fireflies glow yellow-green to yellow-orange, glowing fungi shine green, and the railroad worm adds a red headlamp.
Why glow? Four big reasons
-
-
Hiding
In counter-illumination, an animal's belly glows to match the dim light from above, so its shadow disappears to anything looking up from below.
-
Defense
A sudden flash can startle a predator. Glowing clouds act like a smoke screen. "Burglar alarm" lights may call in a bigger predator to attack the attacker.
-
Finding a mate
Fireflies and ostracods use species-specific light codes to find partners, and some deep-sea fish have light organs that differ between males and females.
Glowing is not the same as fluorescing
These three words are easy to mix up:
- Bioluminescence
- A living thing makes light from a chemical reaction. It works in total darkness.
- Fluorescence
- A material absorbs light of one color and instantly gives it back as another color. It stops when the light source is removed. Scorpions under ultraviolet light are fluorescent, not bioluminescent.
- Phosphorescence
- Like fluorescence, but the material keeps glowing for a while after the light is gone -- think glow-in-the-dark stickers.
Sailors once called glowing waves "phosphorescence", but the glow in the water is almost always bioluminescent plankton.
Invented again and again
Bioluminescence did not evolve once and spread. It appears to have evolved independently many separate times across the tree of life. Older estimates put the number at about 40 or more, and more recent studies suggest it may be closer to 100.
A large survey of animals filmed in the waters of Monterey Bay, California, found that roughly three-quarters of the animals observed from the surface down to about 4,000 meters were capable of making light.
How people use living light
- Watching life inside cells. Green fluorescent protein (GFP), first found in the crystal jelly, lets scientists make cells, proteins and even whole organisms glow so they can be tracked under a microscope.
- Hygiene testing. Because firefly luciferase needs ATP, it can reveal traces of living material. Many food factories and hospitals use luciferase-based swab tests to check how clean a surface is.
- Medical and drug research. Researchers add luciferase genes to cells so that a glow shows when a particular gene switches on.
- Glowing plants. Scientists have moved the light-making genes of glowing fungi into plants, and a glowing petunia has been sold to home gardeners in the United States since 2024.