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If you're trying to hide from something, the open ocean is a tough place to do it. I mean, there's not much to hide behind, for one. And if you want to blend in with the background, that's going to be tough, too, because the background is constantly changing as the sunlight does and as predators approach from different angles. So a lot of nature's tried and true camouflage tricks aren't that useful in the big, wide ocean. But in spite of all this, some marine animals have evolved ingenious ways of hiding out in the open - by using light.

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Near the ocean's surface, camouflage is especially complicated. As the wind stirs up the surface of the sea, the light that passes through gets bounced around, too. So, the glow right below the surface is uneven, and that makes it harder to blend in. Many fish have silvery skin that acts like a mirror, reflecting that funky light pattern around them. And at least one of them takes camouflage even further; it's a flat, shiny fish called the "lookdown," and it lives in shallow waters all along the Atlantic coast of North and South America. Like many other fish, its skin acts like a mirror, but that's not all it's doing, because it's not just a difference in color or brightness that can blow a fish's cover.

Under water, patterns of polarized light can also be a big giveaway. Polarized light is made up of light waves oscillating in the same direction. Our eyes aren't very sensitive to polarization, so when we look at the ocean, we just see a lot of plain old blue. But many sea creatures see patterns of this polarized light. And as sunlight bounces off something like a fish, the reflected light forms a polarization pattern, so creatures that see those patterns can spot the fish.

But lookdowns have evolved a way to mask these disruptions. Inside their skin cells, they have flat, microscopic crystals that reflect light; these are called "guanine platelets," and depending on how they are stacked and oriented, they can reflect light at different polarization angles. Thanks to this adaptation, lookdowns can reflect polarized light at different angles depending on the time of day. Their skin changes how much the reflected light is polarized to match the conditions around them. So when a predator is looking at them, whether it's detecting brightness or polarization or both, these fish just look like part of the water. And these aren't the only fish able to use tricks of light like that.

Like the lookdown, many other silvery fish use their mirror-like skin as a kind of invisibility cloak that works by reflecting the ambient light, but sometimes that same trait can betray them. When schools of fish swim near the surface, they often catch direct sunlight, which gives off a flash as it bounces off their skin, and that makes them pretty conspicuous. But when they're in danger, these schools of fish change their geometry to minimize this effect.

Researchers have noticed that when schools of fish sense a threat, they pack together more densely and line up with the direction of sunlight and each other. Crowding together could be helpful for a number of reasons, like you're less likely to be plucked off by a predator if you're in a crowded group. But it also seems to make the flashes of light less conspicuous; by packing together more densely, they leave less skin exposed to the light, which makes the flashes harder to see. And aligning the school with the direction of the sun also helps; when sunlight is shining from behind them rather than hitting their sides, they also reduce the amount of light that flashes off of them. So, by collectively manipulating the light, these fish can keep the whole school safer.

That works great near the surface, but what about deeper in the ocean where there's almost no light that filters down? At a few hundred feet down in a layer known as the "twilight zone," things get extremely dim but not quite pitch black. At these depths, a lot of predators hunt by looking upward and picking our the silhouettes of other creatures against the dim sunlight.

And in the twilight zone, there's another master of camouflage - the glass squid. Glass squids avoid casting a shadow by just being transparent. There's just one problem - their eyes. They have these big, bulbous eyeballs protruding from their heads that cast shadows below them, kinda ruining the whole "being invisible" thing. So to protect themselves, these squids have evolved to hide by glowing. They have cells called "photophores" right underneath their eyeballs that are bioluminescent; in other words, they produce light through chemical reactions. And this light is the perfect brightness to blend in with the surface above when seen from below.

But the structure of these photophores is even more ingenious than you might think at first. See, the photophores are made up of fibers that guide light, so a lot of the light travels down the fibers and gets released beneath the eye, but some it escapes out the sides. Scientists studying these squids weren't sure what to make of these leaky fibers at first, it kind of seemed like a design flaw. But then they realized it was yet another trick for camouflage. By letting a little light escape, this squid cancels out its shadow as seen from the side, too, not just from below. So, no matter what angle a predator is coming from, the squid appears to blend right into the background.

What's incredible about all these species is that they've taken light, which is usually used to reveal things, and turned it into a tool that conceals them. And they've used this tool to master camouflage in one of the hardest environments to hide in. These guys really went from hiding in plain sight to hiding in plain light. [SFX: rimshot]

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