How fish minimize their visibility to predators in open waters

November 19, 2015

Though the open ocean leaves few places for fish to hide from predators, some species have evolved a way to manipulate the light that fills it to camouflage themselves, a new study finds. 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 (see related video), 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.
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American Association for the Advancement of Science

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