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.
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: 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.
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.
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.
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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