Bioluminescence : when ocean makes light

Your movement through the water at night can ignite a glowing blue trail that is actually a biological alarm system. Tiny single-celled organisms called dinoflagellates produce light when they are disturbed, specifically to attract the attention of even larger predators. By lighting up the creature that bumped into them, they turn their personal attacker into a bright, visible target for something higher up the food chain.

This light is produced by a chemical reaction involving a molecule called luciferin and an enzyme called luciferase. When the two meet in the presence of oxygen, they release energy in the form of cold light. It is one of the most efficient processes on Earth, where nearly 100 percent of the energy is converted into illumination rather than heat. If a standard lightbulb were this efficient, it would never get warm to the touch.

Deep-sea dragonfish use this chemistry to create invisible searchlights. While most ocean life is blind to red light, the dragonfish produces a red glow from lanterns beneath its eyes. This allows it to see its prey in the dark without the prey ever realizing it is being watched. It is the biological equivalent of using night-vision goggles in a pitch-black room.

Some squid species use light to essentially disappear into the sky. By matching the intensity and color of the moonlight filtering down from the surface, they erase their own silhouette from the perspective of predators lurking below. This counter-illumination makes them invisible, even when they are swimming directly overhead.

Milky seas, massive glowing patches of ocean that can be seen from space, are caused by trillions of bacteria glowing in unison. These colonies are so large and intense that they can cover 15,000 square kilometers, roughly the size of Connecticut.

Even more strangely, some land-dwelling fungi share this identical chemical pathway, suggesting that the ability to glow evolved independently dozens of times across the tree of life.