Lookdown Fish Manipulate Polarized Light To Camouflage Themselves

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The lookdown has the capability to hide in polarized light due to the structure of platelets in the fish's skin cells. Photo by John Virata
John Virata

The lookdown (Selene vomer) has the capability to change how light is reflected on it, further enabling it to camouflage itself well in the open ocean, according to researchers at the University of Texas at Austin. In their study, just published in the Proceedings of the National Academy of Sciences and funded by the U.S. Navy, the researchers determined that the lookdown uses a complex manipulation of polarized light after it hits the fishes’ skin, a type of camouflage that they say outperforms, by 80 percent, the mirror strategy that was thought to be the best type of camouflage in fish.

Researchers previously thought that the best way for fish to camouflage themselves in the open ocean was to reflect sunlight like a mirror, which works well for certain elements of light, such as the light’s intensity and color. However, this type of mirror reflection does not work when light is polarized, which happens when individual waves of light are aligned in parallel to one another. The lookdown, which is known as a good camouflager is able to manipulate how light bounces off its body by tweaking their skin’s reflective properties in ways that were better than that of mirroring light.

“The nifty thing is when we mimicked the light field when the sun is overhead, as it would be at noon, the fish just bounced back that light field,” said Molly Cummings, associate professor of integrative biology in the College of Natural Sciences at the university. “It acted like a mirror. Then we mimicked the light field when it’s more complex, and the lookdown altered the properties of the polarized light it was reflecting so that it would be a better blend into its specific background at different times of day.”

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According to Cummings, the lookdown’s skin selectively reduces the degree of polarization and transforms the angle of polarized light depending on the conditions of the light and the water. Cummings’ next task is to determine how the fish is able to manipulate the light as it bounces off its skin. She speculates that the fish hasten their camouflage by subtly altering the way in which its body is oriented relative to the sun or by neurologically ramping up certain processes within its body to achieve the change, or, she said, it may be an entirely passive process in which different elements of the lookdown’s skin automatically responds to the angle in which the sunlight strikes and filters down through the water column.

“From an evolutionary biologist viewpoint, I am always excited when evolution is one step ahead of humans,” said Cummings. “There is this problem out there — how to blend in to this environment, and though we haven’t quite solved it yet, an animal has. We can identify these basic biological strategies, and perhaps materials scientists can then translate them into useful products for society.”

Researchers who worked on the study include Parrish C. Brady, Kort A. Travis, and Molly E. Cummings of the University of Texas, and Tara Maginnis of the University of Portland.

An abstract of their paper Polaro–cryptic mirror of the lookdown as a biological model for open ocean camouflage can be found here.

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Fish · Lifestyle

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