Although the study focused mostly on just two types of fish, the scad and lookdown are members of a family called the Carangidae, a bountiful clan that includes the white ulua, bluefin trevally, mackeral scad, rainbow runner and a collection of jacks: threadfin, almaco, ambers, green and thick-lipped.
The University of Texas study, published in the journal Science, said the fish use their skin as camouflage to blend in with light waves. They've evolved a microscopic element on the surface of their skin called guanine platelets. It manipulates the way the fish reflect in polarized light, said the study's author, Parrish Brady, a research associate at the university.
For years, the navy has searched for ways to hide vessels in deep open water. The study's findings brings the military branch one step closer to understanding a new type of camouflage. How and when that could happen are questions with answers far in the future, Brady said.
Polarized light is comprised of light waves moving on a single plane. Beneath the water's surface, fish in the Carangidae clan have found a way to detect variations in polarized light waves and use it to conceal themselves in plain sight. In the open ocean where there's hardly anyplace to hide from sharks and other predators, it's a significant advantage.
Hiding in light doesn't always work, but it increases life expectancy, allowing fish with the capability to live longer and breed. The big-eyed scad and lookout were chosen for the study because of their silvery skin, flat and mirror-like, with color reflective cells, and sharp angles that make hiding easier.
"If we can identify that process, then we can improve upon our own camouflage technology for that environment," said Molly Cummings, professor of integrative biology in the College of Natural Sciences and a co-author for the study.
For the study, the open ocean, so deep the bottom can't be seen, was used as a laboratory off the Florida Keys and Curaçao. "We put the fish in a restraining device and measured them with video," Cummings said.
The fish were held against a mirror as video rolled. A platform supporting the fish spun 360 degrees in three-minute cycles while a polarimeter recorded. A polarimeter is a device that measures how polarized light behaves as it passes through angles. After every revolution researchers would make an adjustment, then restart the apparatus.
More than 1,500 angles were studied. They revealed that the fish blended in with its surroundings far better than the mirror, and were hard to detect. They were hardest to see in what the research called "chase angles," 45 degrees in all directions from the tail or head, from which a predator would pursue. It's also the angle from which fish in the study chase smaller fish.
"I think it's a great example of how human applications can take advantage of evolutionary solutions and the value of evolutionary biology," Cummings said. "It's important for people to recognize that we take advantage of evolutionary processes and solutions all the time and that even our military does."