Who Turns on the Light Underwater?

Some ocean organisms make their own light. Why shine when staying hidden can mean surviving?

A wave can light up for a moment as it breaks. Its flashes may come from living things too small to see on their own. Deeper down, a glow can bring a meal closer or make a fish harder to spot.

By DIVE.EXPLO360.FR ·

The blue line follows the wave. When the moving water disturbs many light-producing plankton cells, their flashes can become visible together. © dive.explo360.fr — All rights reserved. Reproduction requires permission.

Making light with very little heat

Bioluminescence happens when a chemical reaction gives off light. In many organisms, an enzyme called luciferase helps a substance called luciferin react. Blue and green light generally travel well through seawater.

The first clue is the source of the energy. The flash begins inside a living organism.

The mystery of glowing waves

Watch the path of the light. It follows the breaking wave, fades and starts again with the next one.

Moving water can set off flashes in tiny plankton called dinoflagellates. When enough cells flash together, they can trace a breaking wave or a boat’s wake. Some populations of one of these organisms, Noctiluca, have lost the ability to glow.

The jellyfish that shines at night

The mauve stinger looks pink when it reflects light that falls on it. In the dark, it can make a flash of its own. A flash after contact tells you what set it off, but you would need to watch the predator’s response to see whether it helps the jellyfish.

A mauve stinger jellyfish in dark water. — The mauve stinger looks pink or mauve under external light. The glowing points represent an emission triggered by disturbance. This is an illustrative reconstruction. © dive.explo360.fr — All rights reserved. Reproduction requires permission.

Female humpback blackdevil with a glowing lure above its mouth. — In the female humpback blackdevil, the organ at the tip of the stalk houses luminous bacteria. Prey approaching the lure may come within reach of the mouth. © dive.explo360.fr — All rights reserved. Reproduction requires permission.

A light above the jaws

A female humpback blackdevil carries a lure above her mouth. Bacteria living inside the lure make it glow. They have a place to live, and the fish can use their light to draw in prey. This close association is called symbiosis.

Lighting up to become harder to see

If you look up at a fish from below, its dark outline can stand out against the light from the surface. Light organs on its belly can reduce that contrast. This is counterillumination.