Tag Archives: reflect light

Omnidirectional fish camouflage and polarizing light

I find this camouflage technique quite interesting due to some nice writing, from a Nov. 19, 2015 Florida Atlantic University (FAU) news release on EurekAlert,

The vast open ocean presents an especially challenging environment for its inhabitants since there is nowhere for them to hide. Yet, nature has found a remarkable way for fish to hide from their predators using camouflage techniques. In a study published in the current issue of Science, researchers from Harbor Branch Oceanographic Institute at Florida Atlantic University and collaborators show that fish scales have evolved to not only reflect light, but to also scramble polarization. They identified the tissue structure that fish evolved to do this, which could be an analog to develop new materials to help hide objects in the water.

HBOI researchers and colleagues 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.

“We’ve known that open water fish have silvery scales for skin that reflect light from above so the reflected intensity is comparable to the background intensity when looking up, obliquely at the fish, as a predator would,” said Michael Twardowski, Ph.D., research professor at FAU’s HBOI and co-author of the study who collaborated with co-author James M. Sullivan, Ph.D., also a research professor at FAU’s HBOI. “This is one form of camouflage in the ocean.”

Typical light coloring on the ventral side (belly) and dark coloring on the dorsal (top) side of the fish also can help match intensity by differential absorption of light, in addition to reflection matching.

Light-scattering processes in the open ocean create spatially heterogeneous backgrounds. Polarization (the directional vibration of light waves) generates changes in the light environment that vary with the Sun’s position in the sky.

Polarization is a fundamental property of light, like color, but human eyes do not have the ability to sense it. Light travels in waves, and for natural sunlight, the direction of these waves is random around a central viewing axis. But when light reflects off a surface, waves parallel to that surface are dominant in the reflected beam. Many visual systems for fish have the ability to discriminate polarization, like built-in polarized sunglasses.

“Polarized sunglasses help you see better by blocking horizontal waves to reduce bright reflections,” said Twardowski. “The same principle helps fish discriminate objects better in water.”

Twardowski believes that even though light reflecting off silvery scales does a good job matching intensity of the background, if the scales acted as simple mirrors they would impart a polarization signature to the reflected light very different from the more random polarization of the background light field.

“This signature would be easily apparent to a predator with ability to discriminate polarization, resulting in poor camouflage,” he said. “Fish have evolved a solution to this potential vulnerability.”

To empirically determine whether open-ocean fish have evolved a cryptic reflectance strategy for their heterogeneous polarized environments, the researchers measured the contrasts of live open-ocean and coastal fish against the pelagic background in the Florida Keys and CuraƧao. They used a single 360 degree camera around the horizontal plane of the targets and used both light microscopy and full-body video-polarimetry.

The American Association for the Advancement of Science (AAAS), publisher of Science magazine where the researchers’ study can be found issued a Nov. 19, 2015 news release on EurekAlert further describing the work,

… The study’s insights could pave the way to improvements in materials like polarization-sensitive satellites. Underwater, light vibrates in way that “polarizes” it. While humans cannot detect this vibrational state of light without technology, it is becoming increasingly evident that many species of fish can; lab-based studies hint that some fish have even adapted ways to use polarization to their advantage, including developing platelets within their skin that reflect and manipulate polarized light so the fish are camouflaged. To gain more insights into this form of camouflage, Parrish Brady and colleagues measured the polarization abilities of live fish as they swam in the open ocean. Using a specialized underwater camera (…), the researchers took numerous polarization measurements of several open water and coastal species of fish throughout the day as the sun changed position in the sky, causing subsequent changes in the polarization of light underwater. They found that open water fish from the Carangidae fish family, such as lookdowns and bigeye scad, exhibited significantly lower polarization contrast with their backgrounds (making them harder to spot) than carangid species that normally inhabit reefs. Furthermore, the researchers found that this reflective camouflage was optimal at angles from which predators most often spot fish, such as from directly below the fish and at angles perpendicular to their length. By looking at the platelets of open water fish under the microscope, the team found that the platelets align well on vertical axes, allowing fish to reflect the predictable downward direction of light in the open ocean. Yet the platelets are angled in way that diffuses light along the horizontal axis, the researchers say. They suggest that these different axes work together to reflect a wide range of depolarized light, offering better camouflage abilities to their hosts.

The AAAS has made available a video combining recordings from the researchers and animation to illustrate the research,

Be sure you can hear the audio as this won’t make much sense otherwise.

Here’s a link to and a citation for the paper,

Open-ocean fish reveal an omnidirectional solution to camouflage in polarized environments by Parrish C. Brady, Alexander A. Gilerson, George W. Kattawar, James M. Sullivan, Michael S. Twardowski, Heidi M. Dierssen, Meng Gao, Kort Travis, Robert Ian Etheredge, Alberto Tonizzo, Amir Ibrahim, Carlos Carrizo, Yalong Gu, Brandon J. Russell, Kathryn Mislinski, Shulei Zha1, Molly E. Cummings. Science 20 November 2015: Vol. 350 no. 6263 pp. 965-969 DOI: 10.1126/science.aad5284

This paper is behind a paywall.