Invisible Fish Hide in Plain Sight

August 23, 2016 | Erica Tennenhouse

Snub-nosed dart fish
Photo credit: Justin Marshall, University of Queensland, Australia

Shiny-scaled silvery fish go undetected, even by animals with the ability to detect polarized light.

Their mirror-like skin keeps herring, sardines, and tuna camouflaged against their watery background. According to a new study published in Current Biology, this disguise keeps silvery fish undetected even by animals that have super underwater vision.

Octopuses and squid are among the animals that have the ability to detect the polarization of light, which is “kind of like wearing polarised sunglasses," said first author Sonke Johnsen, from Duke University in North Carolina, in a press release.

“A long-held hypothesis is that polarisation vision in open water is used to break the mirror camouflage of silvery fish, as biological mirrors can change the polarisation of reflected light,” the researchers write.

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When they analyzed hundreds of photos of silvery fish in the waters around Australia's Great Barrier Reef, the researchers found that the light bouncing off of the fish’s shiny scales did, in fact, reflect a polarization pattern that contrasted with the background water, which could make it easier for animals with polarization vision to detect these fish.

But a mathematical model of visual perception that the researchers developed told a different story: through this model, they discovered that even animals with well-developed polarization vision would be unable to make out the silvery fish from a farther distance than they could without their specialized sight.

"Sighting distance is important, because hunting and avoiding being eaten in the open ocean is about seeing other animals before they see you," Johnsen said. "Once you're seen, you're dead. It's over."

Although polarized vision does not appear to offer an advantage from a distance, the researchers will explore the possibility that polarization cues, along with coloration and brightness, could be useful at close range when fish are assessing the appearance of potential mates.

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