A firefly blinking in a summer backyard and an anglerfish dangling a lure in the pitch-black deep sea are doing the exact same trick: making their own light. So why do some animals glow in the dark? It comes down to a chemical reaction called bioluminescence, and animals have evolved to use it for everything from finding mates to luring dinner to disappearing from predators entirely.

The Chemistry Behind the Glow

Bioluminescence happens when a molecule called luciferin reacts with oxygen inside an animal’s body, releasing energy as light instead of heat, according to the Smithsonian Ocean Portal. A second molecule, luciferase, acts as a catalyst that speeds the reaction along, and different species carry different versions of luciferin, which is part of why bioluminescent light can appear blue, green, or occasionally red. Some organisms skip the enzyme entirely and instead use photoproteins — light-producing molecules that sit pre-loaded and ready to fire the instant a trigger, usually calcium, is present.

Luring Prey Into a Trap

For the anglerfish, bioluminescence is a hunting tool. A modified fin spine tipped with a light-producing lure, called an esca, dangles in front of its mouth like bait, drawing curious smaller fish close enough to snap up, according to National Geographic Education. Some deep-sea dragonfish take the opposite approach, producing red light that most of their prey cannot even see, letting them hunt like they have night-vision goggles in a part of the ocean with no natural light at all.

Turning Light Into a Shield

Glowing can also mean survival rather than hunting. Certain squid flash a sudden burst of light to startle attacking predators, buying just enough time to escape, while the vampire squid famously ejects a cloud of sticky, glowing mucus to confuse anything chasing it. Hatchetfish use a subtler defense called counterillumination, tuning the light on their undersides to match the faint sunlight filtering down from above, which erases their silhouette from predators hunting upward from below.

A Universal Language for Attracting Mates

On land, the most familiar bioluminescent display is the firefly, whose flash patterns work like a species-specific code. Each species blinks its own distinct rhythm and timing, and females typically use those flash patterns to identify — and choose between — potential mates from the same species flying nearby. The same basic idea, using light as a signal rather than a weapon or a lure, shows up throughout the animal kingdom in different forms.

The Deep Sea’s Most Common Trait

While fireflies get most of the attention on land, bioluminescence is overwhelmingly a deep-sea phenomenon. The Smithsonian Ocean Portal notes that most deep-sea animals produce some form of bioluminescent light, making it arguably the most common form of communication on the planet — it just happens almost entirely out of human sight, hundreds or thousands of meters below the surface where sunlight never reaches.