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article imageOp-Ed: Scientists discover how camouflage 'trick' helps fish disappear

By Megan Hamilton     Nov 21, 2015 in Science
Scientists have discovered that some fish use a clever mechanism to disguise themselves and avoid hungry predators in the open ocean.
These findings may also help the military create more effective ways to use this type of camouflage.
The researchers found that these fish have microscopic structures within their skin cells that reflect polarized light, and this camouflages them, HNGN reports. Polarized light consists of light waves that travel along the same plane, and light that travels under water is usually polarized. Fish can easily detect variations within this type of light.
Two species of ocean fish — the big-eyed scad and the lookdown — have honed this method of predator avoidance by hiding in light, The Washington Post reports. Their skin is silvery and gleams in the light, making them nearly impossible to see. It's such an effective strategy, in fact, that the U.S. Navy funded the study, published Thursday in the journal Science, to better understand how fish do this, and how it could be used in the navy's favor.
I have seen this myself while snorkeling with lookdown. These fish are living mirrors, diaphanous in the brilliant waters of the Caribbean where I live. They are flattened from side to side like a knife and I can easily see why a predator moving along the bottom might have a difficult time seeing them. As I swim among them, their numbers envelope me so that they are the only thing I can see. What's most amazing is that they appear to be almost the same color as the water around them, so I can easily see why this would interest the navy.
So what does the navy want to do?
The study, conducted by researchers from the University of Texas, noted that the fish have the ability to blend in with the light waves, having evolved a microscopic element on the surface of their skin called guanine platelets. These elements manipulate the way the fish reflect in polarized light, said Parrish Brady, one of the authors of the study and a research associate at the university, The Washington Post reports.
This adaptive feature is particularly impressive because these platelets are designed to reflect polarized light more effectively when the fish is viewed from "chase angles," the positions that a predator would likely approach prey from, IFLScience reports. An example? When the fish were viewed from a perpendicular perspective, the researchers found that they exhibited "exceptional camouflage."
Hoping to measure the reflection of polarized light produced in these two species of fish, the scientists used a polarimeter and were able to compare the results against other fish from reef and coastal habitats. Specimens were held in place against a mirror, and recordings were taken from a variety of angles.
The water needed to be so deep that the bottom couldn't be seen, so the experiment took place in the open ocean off the Florida Keys and Curaçao, per The Washington Post.
The scientists found that the open-ocean species displayed greater polarization than the other fish, and they were also more reflective of polarized light than the mirror was. This discredited earlier theories that held that silvery, mirror-like scales was the most effective camouflage for these species, IFLScience reports. Reef fish didn't exhibit this adaptation because the ocean floor depolarizes aquatic light.
The results have led the study authors to conclude that this method provides a superior type of camouflage than many modern devices like invisibility cloaks. They suggest future attempts to improve the technology "should turn to natural systems for new materials and the means to use them effectively."
In fact, the navy has been trying over the last several years to find ways to hide vessels in deep open water. The methods to do this are a little bit closer, but still well into the future, Brady noted, per The Washington Post.
"I think it's a great example of how human applications can take advantage of evolutionary solutions and the value of evolutionary biology," said study co-author Molly Cummings, a professor of integrative biology in the College of Natural Sciences. "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."
Lookdown and scad are part of the Carangidae family, which includes numerous species such as the bluefin trevally, mackerel scad, rain bow runners and several jacks, including the threadfin, almaco, green and thick-lipped jack.
Lookdown (Selene vomer) are definitely distinctive-looking, with their steeply pitched forehead, low-slung eyes and pouting mouths. They do look like they are looking downward, and that is compounded by the fact that as they swim, they tilt downward slightly, making them look a bit comical.
Preferring marine and brackish waters, lookdown can be found at depths of one to fifty-three meters (more than 150 feet down) where it lives in large schools, FactZoo reports. They are also very popular in home aquariums. Lookdown eat small worms, and small fish and crustaceans. Interestingly, they lay eggs into the water column.
Fortunately, despite their popularity as pets and being openly fished, the lookdown isn't currently in danger of extinction and aren't the subject of specific conservation efforts.
It's heartening to know this. When I snorkeled into that wall of fish, they absolutely gleamed and sparked brilliant colors of blue, red, gold, and green, and I felt like I was at home in the ocean, where life was born.
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