Light bathes our planet, splashing off the mountaintops, flooding the deserts, tundra, savanna and forests, and seeping as deep as 1,000 meters into the ocean. Bacteria, plants, animals and all kinds of living things have evolved different ways to detect and respond to light. Despite their familiarity and prevalence, eyes are not essential.
Biologists have known for several decades that some eyeless animals perceive light. Likewise, some animals with eyes—even rather sophisticated eyes—rely on other body parts to see. Exactly how organisms sense light without eyes has, in many cases, remained mysterious. In recent years, with the help of new tools like genome sequencing, scientists have discovered light-sensitive cells and proteins in unexpected places, and have established that creatures once thought to be blind can in fact see. Light-reactive proteins cover the ends of a sea urchin's feet, for example, possibly turning the spiky animal's whole body into a compound eye. Similarly, tiny jellyfish-like hydras can sense light with their stinging tentacles. And although nematodes live in darkness underground, some of their neurons respond to light, helping them wriggle away from danger.
View a slide show of animals that see without eyes.
These new insights are changing how biologists understand the evolution and function of vision. Two important questions are when light-sensitive proteins first evolved and in what ways animals originally used these proteins. Another question is whether complex eyes evolved many times in different groups of animals or if later generations inherited and tinkered with a single primitive eye archetype. Although biologists do not yet have definitive answers to these inquiries, it's already clear that vision and light-detection are older, more diverse and more widespread than researchers previously realized. Here are six striking examples of animals that have surprised researchers with eyeless sight.
SEA URCHINS
Sea urchins respond to light in various ways: they might change color, twitch their spines or move toward or away from light. Scientists have known as much for a long time, but they were never certain how urchins detect light, because no known species has eyes of any kind. Their best guess was that the net of nerves enveloping an urchin's body included some diffuse light-sensitive tissue. The remarkable truth is that sea urchins have a much more organized visual system than anyone expected.
When researchers sequenced the genome of the purple sea urchin (Strongylocentrotus purpuratus), they were surprised to discover a number of genes important for the development of the vertebrate retina—the thin sheet of light-sensitive tissue that lines the back of our eyes. Maria Arnone of the Stazione Zoologica Anton Dohrn in Italy and her colleagues revealed that the ends of an urchin's tubular feet are pockmarked with opsins, the same light-sensitive proteins our own eyes depend on. When certain wavelengths of light hit an opsin protein, it changes shape, triggering a chemical cascade that opens tiny gates in cell walls called ion channels. Depending on the animal, this sequence of molecular events results in a reflexive behavior—like moving toward or away from light—or informs the nervous system about some aspect of vision. A sea urchin's hundreds of feet may act as one giant compound eye, allowing them to see just as well as a horseshoe crab or nautilus, both of which have genuine, if primitive, eyes.
HYDRAS
Hydras, tiny relatives of jellyfish, look like dandelion seeds: they have thin tubular bodies crowned with slender tentacles. They usually cling to weeds, stinging and eating even tinier aquatic invertebrates that swim by, such as water fleas (daphnia). Like sea urchins, hydras also respond to light even though they lack eyes. When scientists sequenced the genome of Hydra magnipapillata, they found plenty of opsin genes.
Recently, scientists confirmed that hydras have opsins in their tentacles, specifically in their stinging cells, known as cnidocytes. David Plachetzki of the University of California, Davis, and his colleagues showed that hydras respond not only to touch and chemicals, but also to changes in the light in their immediate environment. Hydras sting with greater force in dim light than in bright light, perhaps because they have evolved to recognize shadows as signs of prey or predators—the more they fire in the presence of a shadow, the likelier they are to hit their targets.
Hydras belong to one of the oldest groups of animals on the planet, the Cnidarians. Although hydras do not have eyes, other members of their family have simple eyes called ocelli. Box jellyfish have remarkably sophisticated eyes with lenses and retinas. The fact that hydras, which evolved much earlier than most Cnidarians, can detect light with their tentacles suggests that the origins of vision stretch further back in time than anyone realized. Later, jellyfish and other animals may have modified these existing, primitive visual systems to form more complex eyes.
SQUIDS, CUTTLEFISHES AND OCTOPUSES
Octopuses have large eyes and humongous occipital lobes—the parts of the brain that process vision. These wily, squishy marine masters of disguise can match the texture, color and patterns of almost anything in their environment. But they cannot see color—at least not with their eyes. The octopus eye is technically color-blind. So is the eye of the cuttlefish, a related mollusk.
Roger Hanlon of the Marine Biological Laboratory in Woods Hole, Mass., and his colleagues recently discovered that cuttlefishes actively express opsin genes throughout their skin, particularly in their fins and underbellies. And Desmond Ramirez of the University of California, Santa Barbara, has detected opsin genes in octopus skin. Octopus, squid and cuttlefish skin is also peppered with chromatophores—elastic sacks of pigment that expand and retract, allowing the mollusk to change its color. Other cells called iridophores and leucophores make the skin more or less reflective. Hanlon and his colleagues propose that opsins work with chromatophores, iridophores and leucophores in an unknown way to detect and mimic the color of nearby objects.
C. ELEGANS
Caenorhabditis elegans—tiny worm-like nematodes—live in complete darkness in the soil, so scientists assumed they could not perceive or respond to light. When X. Z. Shawn Xu of the University of Michigan and his colleagues shined beams of bright light at the nematodes' heads, however, they stopped inching forward and reversed direction. When the researchers flashed light at the tail or body of a nematode moving in reverse, the creature began wriggling forward instead. By annihilating various neurons in the nematodes' heads with a laser, Xu and his colleagues identified four cells without which C. elegans cannot perceive light. The researchers propose that avoiding light is an adaptation that helps nematodes stay in the soil, out of which they will not survive long (unless scientists keep them alive in the lab).
In later work, Xu and his team showed that the light-sensitive neurons in nematodes do not depend on opsins. Rather, they use LITE-1, a protein that functions as a taste receptor in invertebrates. A separate team of scientists discovered that neurons in fruit fly larvae detect light with a protein highly related to LITE-1. Like nematodes, it's advantageous for fly larvae to remain in the shadows, unexposed to harsh light and predators.
SWALLOWTAIL BUTTERFLIES
Japanese yellow swallowtail butterflies can see with their rear ends. More specifically, they have two light-sensitive neurons called photoreceptors on their abdomens, right next to their genitals. Kentaro Arikawa, now at The Graduate University for Advanced Studies in Japan, discovered that these light detectors are essential for swallowtail butterfly sex and reproduction. When yellow swallowtails mate, they precisely align their genitals while facing away from one another. Usually, the butterflies successfully complete their mating dance about 66 percent of the time. When Arikawa and his colleagues destroyed the photoreceptors on males' abdomens with heat, or covered the eyelets with black mascara, the insects mated successful only 23 to 28 percent of the time.
In a related study, Arikawa and his team ablated or painted the photoreceptors on pregnant females' abdomens and released the butterflies into a cage with a potted lemon tree. The insects successfully laid eggs on leaves 14 percent of the time, much lower than their usual 81 percent success rate. Together the evidence suggests that male swallowtail butterflies rely on light-detection to cozy up to females during mating and that female swallowtails depend on their hindsight to confirm that they have properly extended their ovipositor—the organ with which they attach eggs to leaves.
SCORPIONS
Scorpions instinctively avoid light. During the day, the eight-legged arachnids seek shelter beneath rocks, in underground crevices or in people's boots. At night they emerge to hunt small insects. Scorpions detect light, and may even perceive images, with two main eyes atop their heads as well as up to five pairs of nearby smaller eyes. Recently, scientists investigated whether scorpions can detect light with their skin as well. The answer is a preliminary yes.
Most scorpion species have a dark, waxy exoskeleton that looks like black or amber armor in daylight. If certain wavelengths of ultraviolet light strike a scorpion, however, it glows an eerie neon turquoise because of fluorescent molecules in its cuticle. Biologists have speculated that this fluorescence might help scorpions lure prey or warn predators to stay away; alternatively, the sheen might be an inevitable physical property of scorpion skin that offers no adaptive benefit.
Douglas Gaffin of the University of Oklahoma exposed 40 scorpions from Texas to both green light and UV light. Half the time the animals wore tiny aluminum foil eye patches; the other half their eyes were unobstructed. The scorpions were much less active under green light when their eyes were covered compared with when they were unmasked, but they were similarly active under UV light regardless of whether their eyes were exposed. One interpretation of this pattern is that scorpions change their behavior in response to UV light even when they cannot see with their eyes because their skin detects UV on its own. Another possibility is that scorpions somehow perceive the green light from their armor's turquoise fluorescence. Using its entire body to sense light, rather than its eyes alone, might improve a scorpion's chances of finding shelter during the day.
ABOUT THE AUTHOR(S)
![Blindsight: Animals That See without Eyes [Slide Show] (1)](https://i0.wp.com/static.scientificamerican.com/sciam/cache/file/1560B231-E024-4EE4-9D26200BD165427A_small.jpg?h=65&w=65 "Blindsight: Animals That See without Eyes [Slide Show] (1)")
Ferris Jabr is a contributing writer for Scientific American. He has also written for the New York Times Magazine, the New Yorker and Outside.Follow Ferris Jabr on TwitterCredit: Nick Higgins
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FAQs
What animal sees without eyes? ›
Researchers said on Thursday that the red brittle star, called Ophiocoma wendtii, is only the second creature known to be able to see without having eyes - known as extraocular vision - joining a single species of sea urchin.
What animals are eyeless? ›- Star-nosed Mole. This mole is virtually blind and is nocturnal in nature. ...
- Atretochoana. This animal has been recently discovered in the Amazon River. ...
- Sea Urchins. ...
- Hydras. ...
- 5. Japanese Swallowtail Butterflies. ...
- Mexican Tetra. ...
- Texas Salamander.
The answer, of course, is nothing. Just as blind people do not sense the color black, we do not sense anything at all in place of our lack of sensations for magnetic fields or ultraviolet light. We don't know what we're missing.
What is the blindest animal? ›#1 Animal that's Blind: Golden Mole
The horrific tentacles that consume the face of the star-nosed mole allow it to create a map of its environment, but how does the comparatively normal but blind golden mole make sense of its world?
- Transparent Amazonian Fish. Cyanogaster, a recently discovered translucent fish. ...
- Golden Tortoise Beetle. Golden tortoise beetles can shine metallic gold or change color to red. ...
- Glass Frogs. The translucent glass frog Hyalinobatrachium pellucidum. ...
- Sea Angels.
The meerkat was unmasked in a scientific study as the most homicidal of over 1,000 mammals. One-in-five will be violently dispatched by another meerkat, likely their own mother, sister or auntie, demonstrating how the female of the species really is more deadly than the male.
What is the rarest see animal? ›Vaquita, the world's rarest marine mammal, is on the edge of extinction. The plight of cetaceans—whales, dolphins, and porpoises—as a whole is exemplified by the rapid decline of the vaquita in Mexico, with about 10 individuals remaining.
What animals are silent killers? ›Snakes and spiders seem to compete for fear in humans. Both have species that bite and inject a deadly venom. And both are silent killers.
What are 2 animals that can see in the dark? ›Animals like cats, wolves, foxes, mice, bats and owls are active almost exclusively in the dark. They hunt at night and stay safe from predators because of their ability to see in the dark. In fact, for these animals which are also called nocturnal animals, their vision is better at night as compared to the day.
What animals can see behind them without turning around? ›ONLY two animals can see behind themselves without turning their head and they are rabbits and parrots. Their eyes are on the sides of their head, instead on the front and this allows them to swivel 360 degrees.
Can any animal see in pitch black? ›
Cats and dogs can see in the pitch dark
Their eyes have a high density of rods, which are the cells that are sensitive to dim light. This means they can see normally with as little as 15% of light that humans would need.
Some blind humans are able to use echolocation; they can tell the size and proximity of nearby objects from the echoes of sounds they produce. Perhaps the most incredible method of seeing without eyes is electrolocation, used by sharks, rays, bony fish, dolphins, platypuses, and even bees.
What age your eyes can't see? ›The first signs begin to appear when people are in their 60s, although vision may not be seriously compromised until age 70 or older. There is no approved treatment. However, reducing sun exposure and eating a diet rich in fruits and vegetables — particularly dark leafy greens — may help prevent or control it.
How much vision do you lose if you go blind in one eye? ›What happens when I lose the vison in one eye? Because the sight in each eye covers the same field of vision to some extent, when you lose the ability to see in one eye you do not lose half of your sight. Rather, your overall visual field is reduced by about 30%, which might not be as bad as you first thought.
What is the smallest animal we can see with your bare eyes? ›Paramecium (0.5 mm)
A paramecium is a small unicellular (containing only one cell) living organism that can move, digest food and reproduce - and can be seen with the naked eye.
Few animals are as slow and dumb as sloths. No wonder they have a spot on the top 10 dumbest animals in the world list. In the nearby trees, they spend much of their time sleeping, although they never defecate there.
Are bats blind or deaf? ›While some bats may not have as good color vision as humans, their overall vision may be better than humans during dawn and dusk. Bats have both excellent hearing and good eyesight. Public domain image, source: CDC.
What can animals see that we can t? ›Some animals can detect forms of energy invisible to us, like magnetic and electrical fields. Others see light and hear sounds well outside the range of human perception. Scientists believe a light-detecting protein in the eye called cryptochrome functions as a magnetic field sensor.
What animal has see through wings? ›-- Many animals have evolved camouflage tactics for self-defense, but some butterflies and moths have taken it even further: They've developed transparent wings, making them almost invisible to predators.
What is the see through fish called? ›These domes are huge spherical lenses that sit on a pair of long, silvery eye tubes – hence its common name, the barreleye fish (Macropinna microstoma). The green tint (which actually comes from a yellow pigment) acts as sunglasses, of a sort, to help them track down their prey.
What is the most silent killer animal? ›
It's Natural to assume that animals with large teeth and aggressive reputation animals like lions or poisonous snakes are the world's deadliest killers. But appearance not sufficient to judge the creature's deadliness. Mosquito has recorded the maximum killing of people every year.
What is the most bloodthirsty animal? ›A study on violence in more than 1,000 mammals has revealed that pretty much all of them are murderous, but meerkats are the most bloodthirsty of all.
What mammal kills the most humans? ›Ungainly as it is, the hippopotamus is the world's deadliest large land mammal, killing an estimated 500 people per year in Africa. Hippos are aggressive creatures, and they have very sharp teeth. And you would not want to get stuck under one; at up to 2,750kg they can crush a human to death.
What is the 2 rarest animal in the world? ›| Rank | Animal | Location |
|---|---|---|
| 1 | Kakapo (Strigops habroptila) | New Zealand |
| 2 | Philippine Crocodile (Crocodylus mindorensis) | The Philippines |
| 3 | Amur Leopard (Panthera pardus orientalis) | The Amur Region, Russia & China |
| 4 | Addax (Addax nasomaculatus) | Termit Tin Toumma, Niger |
3. Boto/Pink River Dolphin. Place 3 for the Rarest animals in the world. The Amazon river dolphin, also known as the pink river dolphin or boto, lives only in freshwater.
What is the rarest animal that no one knows about? ›The rarest animal in the world is the vaquita (Phocoena sinus). It is a kind of critically endangered porpoise that only lives in the furthest north-western corner of the Gulf of California in Mexico. There are only 18 left in the world.
What animal kills the least humans? ›- Llama. Llamas (Lama Glama) are related to camels. ...
- Manatee. The manatee (Trichechus) is an herbivore that is not aggressive and doesn't prey on other species. ...
- Red Panda. ...
- Manta Ray. ...
- Giant African Millipede. ...
- Opossum. ...
- 3. Aye-Aye. ...
- Rabbit.
Animals with no natural predators are called apex predators, because they sit at the top (or apex) of the food chain. The list is indefinite, but it includes lions, grizzly bears, crocodiles, giant constrictor snakes, wolves, sharks, electric eels, giant jellyfish, killer whales, polar bears, and arguably, humans.
What animal can scream loudly? ›Howler Monkey- 140 Decibels. Howler Monkeys are the loudest animal in the New World with screams that can reach 140 decibels.
Are bats fully blind? ›No, bats are not blind. Bats have small eyes with very sensitive vision, which helps them see in conditions we might consider pitch black. They don't have the sharp and colorful vision humans have, but they don't need that.
Are dogs color blind? ›
Human eyes have three types of cones that can identify combinations of red, blue, and green. Dogs possess only two types of cones and can only discern blue and yellow - this limited color perception is called dichromatic vision.
Can horses see in the dark? ›Horses have excellent night vision. Horse eyes are large, they have a large pupil, and this allows ample amount of light to enter even in dark situations. On a night lit by a partial moon or stars, horses can see just as well as people can in full daylight.
What animal has 360 vision? ›Chameleon (Chamaeleonidae)
Chameleons have some of the strangest eyes on the planet, which are able to move independently of each other. This results in almost 360-degree vision.
In Gallup's view, only three species have consistently and convincingly demonstrated mirror self-recognition: chimpanzees, orangutans, and humans.
What animals lack a blind spot? ›Cuttlefish
A "W" shaped pupil and ability to see the polarization of light instead of color as humans see light, is a fascinating divergent optical system. They have no blind spot and their eyes are developed while still in the egg.
Depth Perception and Peripheral Vision
Dogs use it to sense the world in three dimensions (3D) and gauge the distance between objects. Peripheral vision pertains to the ability to see out of the corner of the eye.
So What's the Truth About Dog Color Blindness? Having yellow-blue dichromatic vision means that dogs are most similar to a red-green color blind person. They are very good at distinguishing between variations of blues and yellows (and whites and grays), but cannot really see red and green all that well.
What animals are color blind? ›Only one animal cannot see in colour
The only animal that has been confirmed to see only in black and white is a fish called a Skate. This is because it has no cones in its eyes.
Echolocation allows dolphins to 'see' much further than the eyes could manage. It is believed they can see an object the size of an orange from over 80 meters away using their echolocation. They produce a series of clicks which produce sound waves just below the blowhole and are emitted through the melon (forehead).
Is it possible to read blindfolded? ›This is how on closed eyes, when they touch any word they visualize it in their third eye and can read out without seeing," Anil said. The blindfolded children were able to read messages on phone, words written on paper, making drawings and recognizing colours.
Who can t see 3D? ›
Individuals who have vision conditions such as amblyopia (an imbalance in visual strength between the two eyes), strabismus (misaligned eyes), or other conditions that inhibit focusing and depth perception will have difficulty seeing 3D.
At what age do eyes stop changing? ›Although you can't predict the exact age your baby's eye color will be permanent, the American Academy of Ophthalmology (AAO) says most babies have the eye color that will last their lifetime by the time they're about 9 months old. However, some can take up to 3 years to settle into a permanent eye color.
Does your eyesight change after 40? ›After age 40, it's common for your vision to change so that you require reading glasses. “As we age, the lens inside the eye becomes less flexible, so we have a harder time viewing things clearly when they are up close,” says Erley.
Why can't babies look you in the eye? ›Some babies become overstimulated by eye contact and the neural messages their brain is receiving! They might refuse to have eye contact for some time afterwards, even for weeks. Over time you will learn your baby's particular personality and know their cues for when they're keen to have eye contact.
Can I drive if I am legally blind in one eye? ›If you only have vision in one eye, you can still drive a noncommercial vehicle in all 50 states and the District of Columbia. However, to drive a noncommercial vehicle, you must still pass an eye exam, and prove that you have adequate peripheral vision for driving.
What happens if you lose 1 eye? ›In fact, many eye doctors have underestimated the time required to adjust to losing one eye,” Dr. Whitaker said. But studies have shown that adults who lose the sight in one eye have declines in their abilities to accurately track moving objects, to judge distances, and to perceive depth.
Can blind eyes see again? ›The study out of the University of Pisa, Italy, found that the adult brain can actually learn to “see again” many years after a person went totally blind. The findings were published Tuesday in the journal PLOS Biology.
Which animal has human like vision? ›Shark Vision Vs.
Their structure is remarkably similar to ours. So much so, that their corneas are sometimes used in cornea replacement surgery in humans. You can in fact have shark vision! Sharks do have some differences though.
Some animals can detect forms of energy invisible to us, like magnetic and electrical fields. Others see light and hear sounds well outside the range of human perception. Scientists believe a light-detecting protein in the eye called cryptochrome functions as a magnetic field sensor.
What animal has best vision? ›Eagles – Best Eyes in the Animal Kingdom
To put that into perspective, an eagle has the visual acuity of 20/5 – meaning that it can see at 20 feet what a human with 20/20 vision would need to be 5 feet away from to see. By this standard, an eagle's visual acuity is 4 times stronger than ours.
Do animals have 3D vision? ›
While primates, cats, and owls have all demonstrated stereopsis, praying mantises are the first invertabrates known to posess this unique 3D vision. This previously unknown type of vision is based on object movement over time and operates under relatively unsophisticated brain-power.
Which animal has 360 degree vision? ›Chameleons
Chameleons are able to move each eye individually, and because their eyes are located on the sides of their heads, this affords them a nearly complete 360-degree field of vision.
Chameleon (Chamaeleonidae)
Chameleons have some of the strangest eyes on the planet, which are able to move independently of each other. This results in almost 360-degree vision.
You can see for miles and miles. On a clear day, you can see for up to 3 miles before the horizon due to the curvature of the earth. Yet you can see skyscrapers in a further distance than 3 miles due to no horizon obstruction. If you look into the sky you can see stars during the night that are millions of miles away.
What colors we Cannot see? ›Red-green and yellow-blue are the so-called "forbidden colors." Composed of pairs of hues whose light frequencies automatically cancel each other out in the human eye, they're supposed to be impossible to see simultaneously.
What are the 5 sense animals? ›The more common animal senses for external environment information are sight, hearing, smell, taste and touch. In vertebrates, the eye is the sensory organ for detecting visible light.
Do animals have a sixth sense? ›Some people say that animals have a special kind of power for sensing the future. They call it a “sixth sense,” above and beyond the five senses we normally talk about—taste, smell, touch, sight, and hearing. The scientific evidence for an animal sixth sense, however, is slim, says John Caprio.