Fish that swim in the open ocean have nowhere to hide from hungry predators. But some may have evolved a kind of natural invisibility cloak that helps them hide in plain sight. Understanding how it works may let scientists and engineers develop new forms of ocean camouflage.

Some species of fish have microscopic structures called platelets in their skin and scales. These platelets look like thin, flat crystals. And they are so small that 100 could fit in a grain of sand! “It’s why fish seem so shiny and reflective,” says Molly Cummings. She is a biologist at the University of Texas at Austin. She also is a co-author of the new study.

For many years, scientists thought fish camouflaged themselves by reflecting light in the same way a mirror does. But much of the light beneath the ocean’s surface is polarized. That means the light is scattered in a specific pattern, with all the light waves moving along the same plane. Biologists have long known that many ocean animals could detect changes in polarized light. They wondered if some fish might also have evolved a way to blend in to these light patterns.

To find out, Cummings and her colleagues built a video polarimeter (PO-lar-IM-eh-ter). This is a camera that lets scientists see polarized light as fish do. The team used netting to hold a fish in place on an underwater platform against a mirror. Then they took pictures of the fish from more than 1,500 different angles.