Animal eyes are as weird and wonderful as rest of the animal kingdom's collection of unique adaptations—from the elephant's trunk and the giraffe's neck to the electric eel's charge and the echolocation of bats.
Read on for eight fun facts about creature vision that will open your eyes wide with amazement.
1. Eyes have evolved as many as 50 or 60 times independently.
Eyes have been around for a long time, as early as the Cambrian period half a billion years ago. This is most of the time that complex animals have populated the earth.
Eyes are such a valuable tool that even simple light-sensing cells could help ancient creatures avoid capture, find suitable homes, or seek out a meal. The most basic light-sensing ability evolved just once, as we can tell from the fact that there is a “master gene,” Pax6, that controls eye development.
However, there is such a huge variety of vision strategies in the animal kingdom—think our human sight versus fly’s multi-imaged compact lenses—that scientists think that there were a few dozen separate instances of eye evolution.
There are at least that many different ways that animals process visual input.
The animal kingdom is truly diverse and amazing.
2. The mantis shrimp might have the most complicated visual system in the world.
That's according to National Geographic. While humans have a mere three types of cone cells, the peacock mantis shrimp has 12 or perhaps even 16 types of photoreceptors that seem to work in a whole different way than other animals’.
And now. scientists writing in Current Biology have found that some of the pigments in their eyes serve as a natural sunscreen!
3. The box jellyfish has 24 eyes.
How can an animal that doesn’t even have a brain have good vision?
While it lacks many organs found across the animal kingdom, including a true brain, the box jellyfish doesn’t lack for eyes. It has 24.
Of those two dozen separate eyes, it has two different types. Stalks called rhopalias each house six eyes, four of which are “simple light-detecting slits and pits. But the other two are surprisingly sophisticated” and help the blob-like animal navigate among the mangrove roots in its oceanside habitat.
4. Dogs aren't color blind.
No, dogs aren’t color blind, but many whales and dolphins are.
Dogs see color, although not in the same range we do.
While humans have three types of cone cells, one each for green, blue, and red, virtually all other mammals—except for primates—have only two. So canines and other mammals can perceive a more limited range of blues, greens, purples, and yellows.
Whales and dolphins, however, have only one type of photoreceptor (green) and therefore are functionally color blind. When their land-dwelling ancestors moved to ocean habitats, they adapted to the different color palettes in deep water.
The New Scientist points out that biologists aren’t completely sure what the advantage this change gave to the sea mammals. But like many animals that are color blind, many whales and dolphins have superior abilities to distinguish objects in dim light.
5. Cat eyes are reflective.
If you’ve ever driven in heavy fog, you’ve relied on a safety technology based on cat’s eyes.
Road reflectors are used in pavement paint to keep traffic in the right lane during low-light conditions. Their inventor, Percy Shaw, acknowledged that he was inspired by seeing a pair of cat’s eyes shining along the roadside one fog-filled night when he was trying to pick his way down a dark road.
Cat’s eyes, and those of almost all carnivores, shine reflectively because of a special layer in the eye called the tapetum lucidum. The reflective power allows more light to be concentrated on receptor cells in very dim light.
Many animals such as raccoons can shine across a color range from blue to green to yellow.
6. The red-eyed tree frog's eye watches predators.
The red-eyed tree frog boasts an eye straight out of horror flick.
Against the frog’s deep green body, its vibrant red eye startles potential predators when the frog opens it abruptly from a resting state.
In addition, the open eye can be covered by a clear third eyelid that is criss-crossed with bold slash shapes. This third eyelid, or nictitating membrane, allows the frog to see underwater and also protects the eye from drying out while on land.
Many other animals have nictitating membranes, including camels, who use their clear third eyelid during sandstorms to see despite the flying sand.
7. The barrel eye fish has eyes on the top of its head.
Where its eyes normally would be, there are nares—another sense organ.
Long tubes lead from to the openings at the top of the head and channel the light down into the head much like the long shaft of a microscope or telescope—hence the barrel eye name.
Two species of this fish are the only vertebrates known to use both a mirror and a lens. They collect light in two ways—both reflection and refraction.
As a result of these adaptations, they can see both up and down at the same time, detecting both bioluminescent animals below as well as silhouettes of animals in the waters above.
8. The anableps fish's eyes are split in half.
The eye of another fish—the anableps—also does double duty, but in a different way.
The anableps is aptly nicknamed the four-eyed fish because each of its two eyes is split in two. One half is perfectly adapted to function above water, and the other half to work ideally below the water’s surface.
Since this fish spends much of its time hovering at the surface of the water, this dual-purpose eye is a big advantage.
“Light travels at different speeds through air and water and changes direction when it moves between the two,” as SciShow points out.
Scientists don't know of any other animal that has used this strategy to deal with its half-aquatic, half-terrestrial lifestyle.