A study released Thursday says that two ocean fish the big-eyed scad and the lookdown have fine-tuned a method of avoiding predators by hiding in light. Elements in their silvery skin render them nearly impossible to see. The Navy funded the study as part of an effort understand how fish do this, and how it could be used to the Navy’s advantage. 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 bring 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.
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 lookdown 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.
The U.S. Navy has sought for many years to learn how to hide in open water and supported the work of Cummings and her colleagues exploring how fish avoid detection in the open ocean. In a previous study, the researchers demonstrated in the lab that a fish called the lookdown was able to manipulate polarized light to its advantage. The new study conducted in the actual ocean, not just in test environments shows lookdowns and other fish that live in the open ocean camouflage themselves this way. Parrish Brady, a collaborator with Cummings and lead author on the new study, built a video polarimeter that can record polarized light in real time, allowing the researchers to essentially see polarized light as fish do. Brady and Cummings worked with scientists from City College of New York, Texas A&M University and other institutions to build an automated rotating platform that would hold the fish in place in the water while Brady’s polarimeter took constant measurements.
Notably, the open ocean fish were most camouflaged in what are called “chase angles,” which extend out 45 degrees in all directions from the tail or head. These are the directions from which a predator would chase the fish, or from which the fish would pursue its own prey. As to how the fish accomplish this concealment, the Cummings lab found that the fish’s ability to camouflage in polarized light is due to the structure of platelets within the skin cells, which scatter polarized light differently depending on the angle. The researchers will next look into whether the fish can actively manipulate this ability, maybe by changing the angle at which they swim or somehow adjusting the platelets within their skin.
“I think it’s a great example of how human applications can take advantage of evolutionary solutions and the value of evolutionary biology,” said Cummings. “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. ”Many fish that live in the open ocean are silvery, which allows them to reflect light as a mirror does. For many years, experts assumed this was the main means of camouflage among such fish, but this camouflage approach works well only if the surrounding water appears uniform, as it does to human eyes. Polarized light turns out to be an important component of the underwater light field, and it is not uniform but instead highly variable. Using mirrors for camouflage in such an environment can actually backfire and make it easier to stand out in the open ocean
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