EYES MIGHT NOT be able to speak, but they have plenty to say. Many animals use complex visual systems to help them survive in particular habitats. Sight is often a first line of defense against predators—or a crucial sense for catching prey. To dwell in Earth’s darkest locations or travel high through the sky, creatures have evolved special and bizarre ways to perceive their worlds.

UNSPECIFIED - CIRCA 2002: Barreleye, Opisthoproctidae family. (Photo by DeAgostini/Getty Images)
Barreleye, Opisthoproctidae family. Credit: DeAgostini/Getty Images

Spookfish: Mirrored orbs

It’s gloomy year-round for the spookfish, a spiked carnivore that swims in the dark depths of the ocean. Sunlight rarely penetrates more than a few hundred feet below the surface, so this critter uses mirrors instead as well as lenses to see—the only known vertebrate with this feature. The disks, made of guanine crystals, can focus light emitted by bioluminescent plankton and other glowing marine life. Weirder still, the funky fish’s eyes are split into two connected parts: one section gazes up while the other looks down. That way, it can spy food above and below it, as well as lurking predators.

Golden Eagle, Aquila chrysaetos, Scotland, winter. (Photo by: Education Images/Universal Images Group via Getty Images)
Golden Eagle in Scotland. Credit: Education Images/Universal Images Group via Getty Images

Golden eagle: Cones galore

In North America, Europe, and Asia, golden eagles soar on 7-foot wingspans while searching for small prey like rabbits, which they can spot more than a mile away. The birds can see from great distances thanks to an extreme density of visual cells known as rods and cones in their retinas. Rods generally register the overall shape of an object, whereas cones detect color and detail. In general, the density of rods and cones in a raptor’s eye is five times greater than in a human’s. 

WELLINGTON, NEW ZEALAND - APRIL 30:  (STRICTLY EDITORIAL USE ONLY) Dr Eric Warant from the University of Lund in Sweden displays the eyes of a giant squid at the Te Papa Museum on April 30, 2008 in Wellington,  New Zealand. The giant squid is currently defrosting to enable scientist to examine it and it will in the future be put on display at the Museum on Wednesday April 30, 2008 in Wellington, New Zealand. (Photo by Marty Melville/Getty Images)
The eyes of a giant squid at the Te Papa Museum in New Zealand. Credit: Marty Melville/Getty Images

Giant squid: Titanic eyes

The largest eyes in the animal kingdom, at up to 10 inches in diameter, belong to adult giant squids. And it makes sense: The biggest individuals can exceed 40 feet long, so the peepers on these invertebrates have to be proportionally massive. The species uses its dinner-plate-size eyes to draw in as much light as possible from its dark watery world to hunt fish and shrimp. Its line of sight can also detect a moving sperm whale—one of the squid’s main foes—up to 400 feet away in deep ocean zones that have little or no sunlight.

(GERMANY OUT)   Mantis Shrimp, Odontodactylus scyllarus, Ambon, Moluccas, Indonesia   (Photo by Reinhard Dirscherl\ullstein bild via Getty Images)
Mantis Shrimp in Indonesia. Credit: Reinhard Dirscherl\ullstein bild via Getty Images

Mantis shrimp: Extra light

Mantis shrimp, known for their herculean strength and swift clubbed appendages that can break a snail’s shell in a single strike, also boast a unique vision system. Their eyes process 12 channels of color and can detect ultraviolet (UV) and polarized light, which has waves that all vibrate in the same direction. By comparison, humans can process only three channels of color—blue, green, and red—and can’t process polarized or UV light at all. The crustaceans’ 10,000 small photoreceptive units are organized in strips that log objects around them, similar to how a scanner reads a bar code. 

Lone Svalbard reindeer grazing in the Tundra in Svalbard.
Lone Svalbard reindeer grazing in the Tundra in Svalbard. Credit: Getty

Arctic reindeer: Color play

In the Arctic reindeer, the tapetum lucidum, a part of the eye located behind the retina in many vertebrates, changes color with the seasons. Their eyes go from gold in summer to blue in winter, compensating for the drastic variation in sunlight at the North Pole. Blue eyes enhance the ungulates’ ability to detect shorter wavelengths of light during the dark and dreary winter, when they have only a few short hours per day to detect sunlight. During the extremely bright summer, their eyes no longer need to take in as much light in a short period and switch back to yellow. 

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