How Fish Camouflage in the Open Ocean with Light

When fish venture into the open ocean, they seem to disappear. Now, researchers have learned a bit more about how fish play with light in order to camouflage themselves from predators.

"We've known that open water fish have silvery scales for skin that reflect light from above so the reflected intensity is comparable to the background intensity when looking up, obliquely at the fish, as a predator would," said Michael Twardowski, one of the researchers, in a news release. "This is one form of camouflage in the ocean.

Typical light coloring on the bottom side of the fish and dark coloring on the top side of the fish can also match intensity by differential absorption of light, in addition to reflection matching.

Light-scattering processes in the open ocean create spatially heterogeneous backgrounds. Polarization, which is the directional vibration of light waves, generates changes in the light environment that vary with the Sun's position in the sky.

Polarization is a fundamental property of light, like color. However, human eyes do not have the ability to sense it. Many fish, though, may be able to do so.

Even though light reflecting off of silvery scales does a good job at matching the intensity of the background, if the scales acted as simple mirrors then they would impart a polarization signature to the reflected light that's different from the more random polarization of the background light field.

In order to take a closer look at this phenomenon, the researchers measured the contrasts of live open-ocean and coastal fish against the pelagic background. They found that the fish actually match polarized light.

The findings may help researchers design materials that are better able to camouflage open-ocean vehicles.

The findings are published in the journal Science.

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