Different organisms have evolved to see the world differently, with eye structures optimized for diverse modes of existence.
Cuttlefish
No other animal has pupils like the cuttlefish. Its pupils are shaped like the letter W, a feature that biologists determined helps the animal maintain balance when the light field is unstable, which is quite common in the deep seas where they live. Cuttlefish have only one type of photoreceptor, meaning they can only see in monochromatic colors. However, the wide pupils of cuttlefish and many other cephalopods may promote a completely different way of seeing color, based on how light travels through the lens and splits into a rainbow.
Although this can produce a blurry image, the blurring depends on the color. Cuttlefish can see colors that humans don’t even know about. That’s how they adjust their body color according to the environment to disguise themselves. But unlike many cephalopods, cuttlefish’s eyes can rotate, allowing them to see the world in three dimensions.
Four-eyed fish
This fish does not actually have four eyes as its name suggests, but its two eyes have evolved a special adaptation. Four-eyed fish specialize in living at the water’s surface, where they spend most of their time hunting for flying insects. Their eyes are located on the top of their heads to observe insects flying in the air. But part of the eye is below the water’s surface. Each pupil is divided into two halves, half above the water surface and half towards the bottom of the turbid water.
Thanks to that, four-eyed fish can simultaneously see both above and below the water surface to watch for predators and prey. The thickness of the eyeball and precorneal epithelium varies to accommodate the different extractions of the aerial and aquatic environments. Proteins in photoreceptor cells are more sensitive to blue light in the upper retina and yellow light in the lower retina. Because four-eyed fish often live in muddy water environments such as mangrove forests, this structure helps enhance their vision.
Shrimp
The most complex eyes in the animal kingdom known to man belong to crustaceans that live on the ocean floor, spending their lives in burrows in rock crevices and the sea floor. Humans have 4 photoreceptors, but mantis shrimps of the order Shrimps have up to 16 photoreceptors in their compound eyes. They have both regular colored photoreceptors and photoreceptors sensitive to ultraviolet light. They can see 5 different ultraviolet frequency bands.
In addition, mantis shrimp can see polarized light. Many animals can see linearly polarized light, including cuttlefish. Mantis shrimp are the only animals that can see curved polarized light. Each eye sits on a long stalk and can move independently with depth perception. Humans rely on two eyes to perceive depth while mantis shrimp only need one. They can even detect cancer before symptoms appear.