Scientists discover new camouflage mechanism fish use in open ocean

November 19, 2015
Researchers have found that fish that live in the open ocean, such as these lookdowns, reflect polarized light from their skin in a way that makes them seemingly disappear from predators' view. Credit: The University of Texas at Austin

Scientists have solved a longstanding mystery about how some fish seem to disappear from predators in the open waters of the ocean, a discovery that could help materials scientists and military technologists create more effective methods of ocean camouflage.

In a paper published this week in Science, a team led by researchers at The University of Texas at Austin reports that certain use microscopic structures called platelets in their skin cells to reflect polarized , which allows the fish to seemingly disappear from their predators.

Polarized light is made up of all traveling in the same plane, such as the bright glare you sometimes see when sunlight reflects off the surface of water.

Under the surface of the water, light tends to be polarized. Many fish—and sophisticated modern satellites—have the ability to detect variations in such polarized light.

"Fish have evolved the means to detect polarized light," said Molly Cummings, professor of integrative biology in the College of Natural Sciences. "Given that, we suggested they've probably evolved the means to hide in polarized light. If we can identify that process, then we can improve upon our own camouflage technology for that environment."

Whether it's a predator pursuing a fish or a satellite seeking an adversary, light patterns help with detection of targets in the the in three ways: through brightness contrast, color contrast and polarization contrast. Of the three, polarization contrast is considered most effective for detection in the open ocean.

This video shows researchers in Curacao assessing the biological response fish have to polarized underwater light fields. The scientists learned some species living in the open ocean have a way of reflecting polarized light from their skin to better camouflage themselves in open waters. Credit: The University of Texas at Austin

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.

Researchers from FAU's Harbor Branch and collaborators collected more than 1,500 video-polarimetry measurements from live fish from distinct habitats under a variety of viewing conditions, and have revealed for the first time that fish have an "omnidirectional" solution they use to camouflage themselves, demonstrating a new form of camouflage in nature -- light polarization matching. The top panel in black/white shows the intensity and the two other panels resolve different aspects of polarized light scattered off the fish. Credit: FAU Harbor Branch Oceanographic Institute

On the platform, a fish is held in place on a mirror, while an adjustable arm holds the video polarimeter a meter away. Once started, the platform takes three minutes to spin 360 degrees, with the polarimeter recording the whole time.

After every revolution researchers would make an adjustment, such as moving the polarimeter to a slightly different angle or adjusting how much the fish was tilted, and then start the apparatus on another rotation.

More than 1,500 different angular configurations were recorded in this way for each fish, taking into account camera location, angle of the sun overhead and position of the fish. Recordings were conducted in the Florida Keys and Curaçao for five species of fish.

The results showed that the two fish from the open ocean, the lookdown and the bigeye scad, had much better camouflage in polarized light than a mirror had. They also blended in better than two reef-dwelling fish and one surface-skimming fish, all of which live in environments where polarized light is not as important.

A diver with a scientific research team examines fish in the open ocean to better understand how they use polarized light for camouflage. Credit: The University of Texas at Austin

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 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.

Explore further: Researchers discover a new way fish camouflage themselves in the ocean

More information: "Open-ocean fish reveal an omnidirectional solution to camouflage in polarized environments," Science, www.sciencemag.org/lookup/doi/ … 1126/science.aad5284

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FredJose
1 / 5 (3) Nov 20, 2015
Fish have evolved the means to detect polarized light

This is a non-scientific statement if ever there was one! How does she know this as there isn't any documented observation that fish actually didn't have this ability beforehand and that after a certain period of time it was seen to appear? HOW did they "evolve" this ability? The only reasonable statement that one can make at the present time is that fish do have this ability, end of story. Any statements relating to how it happened to get it in the past is just plain speculation that is as vacuous as space itself.
...Given that, we suggested they've probably evolved the means to hide in polarized light....
So now she goes deeper into the mud and extrapolates from an unproven guess to an even further non-scientific statement. There is no documented history to support any such evolutionary statements. Only current observations in the present are applicable.

FredJose
1 / 5 (3) Nov 20, 2015
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."

The solution exists. That's a fact. That it evolved is not. There's no documented observation and verification of any such "evolution" ever having occurred. So to pander to the establishment with words like " take advantage of evolutionary processes and solutions..." is simply making a bow at the altar. If there was a problem, then the "solution" would have had to have been devised, i.e. DESIGNED with both theoretical knowledge as well as with the ability to manipulate materials at the nano-scale level to achieve the desired outcome. Darwinian evolution does NOT design anything, hence one can conclude that it was not involved here.

Zorcon
3.7 / 5 (3) Nov 20, 2015
Fish have evolved the means to detect polarized light

How does she know this as there isn't any documented observation that fish actually didn't have this ability beforehand...?


So you are suggesting the means to detect polarized light had already evolved in primitive chordates by the Ordovician, and the first fishes inherited it? That is possible.

It would be interesting to look at the larvae of tunicates (which have rudimentary eyes) to see if they have this ability. If so, that would support your hypothesis because they are in the same clade and quite similar to the progenitors of vertebrates (including fishes, of course).

But even if they do... while it's clear the eyes of all chordates share a common origin, the ability to detect polarization (if demonstrated in tunicates) might be a case of parallel evolution. It has also appeared in arthropods, for example, and the eyes of arthropods appear to have evolved independently from those of chordates.

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