How nature designs eyes
Nature has a reason for everything. A creature's environment plays the main part in deciding how its eyes develop and function. For instance, starfish have five eyes at the end of each arm, but they can only see shade and light and huge masses of land. They don’t need to see any more than looming coral reefs, so that is how their eyes have developed. The eyes of the nocturnal elephant hawk moth excel at collecting the tiniest traces of light. Even in faint starlight, it can distinguish the colours of blossoms bearing nectar. Lemurs, who feel safer foraging at night, have such excellent night vision that they can make out colours in complete darkness.
Here's the explanation behind some of the pupils you find in nature.
Those animals with vertical pupils, like cats, foxes, crocodiles, snakes, geckos, are ambush predators, active in hunting both by day and night. A vertically oriented pupil is a precision instrument, capable of razor-sharp focus in many light conditions. A crocodile’s eyes can adapt to twilight or night-time. One reason is their ability to open very wide or close to tiny slits, depending on how much light they need to let in. A cat's pupils can expand by 135 times, for example. Compare that to the 15-fold expansion of our own pupils!
Horizontal pupils belong to grazing prey animals like goats, sheep, horses, moose, deer. An animal with horizontal pupils tends to have its eyes on each side of its head, so these pupils can give it a panoramic view that lets in more light from its front, back, and sides. That helps them both detect an oncoming predator and keep tabs on it while they flee.
Crescent pupils belong to water-dwelling animals that spend a lot of time motionless. For example, skates, rays, flatfishes, catfish and some whales. Light bends differently in water than it does in air. Crescent shapes can take in more information than a circular pupil with the same surface area. This pupil turns points of light that are in front of its owner into U shapes (or crescents facing up) and points of light that are behind it into n shapes (or crescents facing down). The closer the object, the larger the U shape, and vice versa. For a flounder on the seafloor watching schools of fish swim by, the ability to know exactly how far away each one is comes in handy.
Camels have three eyelids, but because they're as thin as paper, they can still see even with all three eyelids closed - which is handy in a sandstorm. Snakes have two sets of eyes. One set is used to see and the other to detect heat and movement. Penguins have eyes that allow them to see better underwater.
The hexagonal lenses of a bee are so strong that they can see ultraviolet rays that humans can’t. Macaws and other parrots see everything in ultraviolet vision, allowing them to see the maturation of fruits. The upward-looking left eye of the squid is twice the size of its right eye, the better to spot prey in light from above. The squid’s smaller eye points down into the dark waters below.
If there are no benefits of seeing, some animals lose their eyes altogether. Mexican Tetras are small freshwater fish which, in the Pleistocene epoch, swam into several deep caves. Their eyes were of little use in the pitch blackness, so their descendants evolved into different populations of blind cavefish—pinkish-white creatures with skin covering where their eyes used to be.
The Ganges river dolphin can only live in freshwater and is essentially blind. It lost its eyes in the course of evolution to adapt to the muddy water of rivers. Gangetic river dolphins navigate using sound waves made by clicking sounds through their throat that bounce off targets and return to their large, flat heads equipped with extremely sensitive auditory systems. They have rods and cones, found in mammal eyes, but these are feeble and just help tell light from dark.
Cephalopods, like Octopuses, squids, cuttlefish, are the smartest creatures on Earth - they can communicate by changing the patterns on their skin. They can even change their own DNA. You can find cephalopod pupils in the shape of Ws, crescents, dumbbells, and whatever. Their eyes don't have cells for colour vision, but they can see colour - they just do it in a completely different way. They might be able to spread out these colours, and bring certain wavelengths into focus, by changing not only the shape and position of their pupils but also the depth of their eyeball and the distance between their lens and their retina.
Some creatures have more than two eyes.
The weirdest lenses in nature, and the largest number of eyes, belong to chitons—a group of marine molluscs that look like ovals adorned with armoured plates. These plates are dotted with upto a thousand eyes of amazing rock crystal on the back of their shell. Each eye has its own lens and these are made of aragonite, which the chitons assemble from calcium and carbonate molecules in seawater. Simply put, this creature looks through rocks. And when their rock lenses erode, the chitons just fabricate some new ones. Chitons live mostly under rocks, in the shallow water near the coast. While a chiton does not have crystal clear vision like humans, it is able to see a blurry image of a fish from 6.5 feet away.
The Horseshoe Crab has 2 compound eyes on each side of its shell and 3 other eyes on the shell, plus 2 more eyes in the front near their mouth. They also have eyes on their tails, so totally, a horseshoe crab has 10 eyes.
The Box jelly is just a gelatinous, pulsating blob with trailing bundles of stinging tentacles. It doesn’t even have a proper brain—merely a ring of neurons running around its bell. The Box Jellyfish has 24 dark brown eyes grouped into four clusters, called rhopalia, anchored on a flexible stalk. Four of the six eyes in each rhopalium are simple light-detecting slits and pits. But the other two are sophisticated; they see images. Its lower lensed eyes spot approaching obstacles, like the mangrove roots that it swims among.
The upper lensed eyes are always looking upward, even if the jellyfish swims upside down. If this eye detects dark patches, the jellyfish senses that it’s swimming beneath the mangrove canopy, where it can find the small crustaceans that it eats. If it sees only bright light, it has strayed into open water, and risks starving. With the help of its eyes, it can find food, avoid obstacles, and survive.
A Praying Mantis has 5 eyes: 2 big compound eyes and 3 other tiny eyes in the middle of their head. The big eyes are for detecting movement, while the other 3 are for detecting light.
Scorpions have 2 eyes on top of their heads. There are also 5 more eyes along each side of their body as well.
Scallops have rows of eyes along the edge of their mantles, one scallop can have between 50 and 100 eyes. The mantle of the bay scallop is festooned with up to 100 brilliant blue eyes. Each contains a mirrored layer that acts as a focusing lens while doubling the chance of capturing incoming light. These eyes let them know if there are threats approaching.
The Purple Sea Urchin's entire body acts like one big eye. On the other hand, Hydra, a small relative of jellyfish, has no eyes, but it does have photoreceptors in its body. These receptors control the hydra’s stinging cells, so that they fire more easily in darkness. This allows the creature to react to the shadows of passing victims, or to fire at night when its prey is more common.
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