Who Turns on the Light Underwater?

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

A passing wave can trigger flashes in plankton, a glowing lure can draw prey closer, and light from a fish’s belly can soften its outline. The effect of the light depends on how it is produced and what the animal seeing it can detect.

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

In many bioluminescent systems, an enzyme called luciferase catalyses a reaction involving luciferin. Some of the energy released leaves as photons. The molecules involved vary among organisms, and in some species bacteria living in a light organ produce the glow.

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.

A disturbance in the water can trigger a brief flash in light-producing Noctiluca cells. When enough cells flash together, their light makes the movement of the water visible. Other populations of the same species do not produce light.

The jellyfish that shines at night

The mauve stinger’s body colour depends on the light that falls on it. A flash triggered by contact is light the jellyfish produces itself. To test whether the flash helps defend it, compare the predator’s response with and without the flash.

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

The lure of the female humpback blackdevil evolved from a dorsal fin ray. Symbiotic bacteria in the organ at its tip make light. Prey that approach the signal can come within reach of the fish.

Lighting up to become harder to see

Ventral photophores send light downwards. A close match with the faint light from above reduces the contrast of the animal’s silhouette for a viewer below. The effect depends on the ambient light and the position of the observer.