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 can see 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.
Which animal's eye allows them to see without the blindspot? ›Octopuses have no blind spot
Having rectangular pupils and eyes positioned laterally on their head helps octopuses see in all directions in addition to giving them control over how much light they let into their eyes.
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 are the two types of blindsight? ›Over the years, however, the definition of blindsight has changed and two forms of it have been described (Weiskrantz, 1998): type 1 blindsight is the classical type in which the patient reports no awareness of any kind, and type 2 blindsight where the patient reports the feeling that something has occurred in the ...
What can blindsight people see? ›People with blindsight can see normally, but they are lying in order to get attention. Blindsight is caused by none of these explanations, but by something else altogether. The memory system has been damaged, so people with blindsight can report what they see, but then they forget that they saw it.
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.
“Naked mole rats are fascinating creatures,” says Sonja Pyott at the University of Groningen in the Netherlands. Not only do they live exclusively underground, they are blind, have poor hearing and make loud, piercing cries.
Is it possible to see without eyes? ›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 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.
Is there an animal with only one eye? ›But do one-eyed animals exist outside Greek mythology and Hollywood? The answer is yes. And they are everything but big monsters. There are 44 species of the genus Cyclops, also known as water fleas, all with a single eye that is either red or black.
What animal does not blink? ›
Some animals like fish, snakes and some lizards don't blink because they don't have eyelids, and some animals don't have eyes at all.
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.
How do these people read around the world who don t have eyes? ›Braille (/breɪl/ BRAYL) is a tactile writing system used by people who are visually impaired, including people who are blind, deafblind or who have low vision. It can be read either on embossed paper or by using refreshable braille displays that connect to computers and smartphone devices.
Who was the man who could see without his eyes? ›This story was inspired by the real life Pakistani mystic Kuda Bux, who claimed to be able to see without his eyes.
Can blindsight see invisible objects? ›Blindsight lets you spot an invisible creature in range, but that creature can still try to hide behind something with Stealth.
How would you describe blindsight? ›Blindsight is defined by the Oxford Concise Dictionary as "Medicine: a condition in which the sufferer responds to visual stimuli without consciously perceiving them," implicitly referring, of course, to human patients. The historical origins, however, stem from animal research and neuroanatomy.
What is blindsight in simple terms? ›Summary: Blindsight is a phenomenon in which patients with damage in the primary visual cortex of the brain can tell where an object is although they claim they cannot see it. Scientists now provide compelling evidence that blindsight occurs because visual information is conveyed bypassing the primary visual cortex.
Can blindsight see color? ›It can therefore be concluded that creatures with Blindsight does mechanically have normal vision (not including details such as color, writing, reading creatures expressions etc.).
Does blindsight see through darkness? ›Blindsight functions as normal sight without using vision. It doesn't specifically see invisibility or ignore illusions and darkness, but that is a natural consequence of those things being visual in nature. It can't see through creatures or objects, and isn't even super hearing able to hear heartbeats.
What does blindsight feel like? ›Nevertheless, some individuals with a damaged visual cortex experience a phenomenon called 'blindsight': they report an inability to see, yet are able to navigate and react to sudden movements or facial emotions correctly and above chance.
What animal has see through skin? ›
Hyalinobatrachium pellucidum, also known as the “glass frog,” is native to the cloud forests and rivers of Ecuador. Hyalinobatrachium pellucidum's pale green skin is translucent to the point that the majority of its vital organs are clearly visible.
What animal has the weirdest 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.
The two-tailed spider is the master of camouflage, perfectly able to hide in plain sight on trees and trunks, easily trapping insects. Their body colouring is usually brown, matching with dry barks, they change their colour according to the surface they are resting on.
Are Dolphins blind or deaf? ›They are nearly deaf, in a world where hearing can be as valuable as sight. That understanding — gained from a study of dolphins' brain activity — could help explain why such intelligent animals do something so seemingly dumb.
Is bat a blind animal? ›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.
What mammal is born blind? ›Many species of mammals, such as rabbits, are actually born blind (born with their eyes closed and eyelids fused together), with their eyes opening a little while later. Of course, treatment exists for animals with blindness, however, not to the extent that it does with humans.
How do dolphins see without using their 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).
Can an eye see itself? ›The eye sees not itself But by reflection, by some other things. . . . And since you know you cannot see yourself So well as by reflection, I, your glass, Will modestly discover to yourself That of yourself which you yet know not of. This issue of Gestalt Review is dedicated to the memory of Edwin C. Nevis, Ph.
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.
What animal can see itself? ›In Gallup's view, only three species have consistently and convincingly demonstrated mirror self-recognition: chimpanzees, orangutans, and humans. Others, though, think the list is longer.
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 a third eye? ›Purpose : The third eye, commonly known as the parietal eye, is a non-visual, photosensitive parapineal organ found in most lizards, frogs, the tuatara, and some species of fish.
Is there an animal with 8 eyes? ›Spiders usually have eight eyes (some have six or fewer), but few have good eyesight. They rely instead on touch, vibration and taste stimuli to navigate and find their prey.
Is there an animal with 6 eyes? ›Six-eyed spiders are spiders that, unlike most spider species miss the principal pair of eyes, leaving them with only six eyes instead of the usual eight.
Do snakes have feelings? ›Generally, reptiles do demonstrate basic emotions. According to Dr. Sharman Hoppes, clinical assistant professor at the Texas A&M College of Veterinary Medicine & Biomedical Sciences, the main two are fear and aggression, but they may also demonstrate pleasure when stroked or when offered food.
Can a fish blink? ›No. Like snakes, they don't have moveable eyelids. Instead, they have a transparent eye protector permanently in place. Fish have excellent eyesight and can see parts of the spectrum we can't.
Can sharks blink? ›Many people do not realise that sharks actually have eyelids, however, they do not need to blink like us humans as the surrounding water cleans their eyes. Just like us, the eyelids are also used to protect their eyes from damage.
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.
What if you were blind in one eye? ›Your ability to judge distances accurately may be affected and you may be less aware of objects on the side that has lost vision. Making better use of your wing mirrors will help. You will also need to bear in mind that blind spots caused by your car's design will be larger for you if you only have vision in one eye.
How does a blind man sees the world? ›
A person with total blindness won't be able to see anything. But a person with low vision may be able to see not only light, but colors and shapes too. However, they may have trouble reading street signs, recognizing faces, or matching colors to each other. If you have low vision, your vision may be unclear or hazy.
How does a blind person perceive the world? ›Even when blind from birth, sightless people understand how others see the world in the same way that sighted people do -- though they have never personally experienced a single visual image, according to a new study conducted by researchers from the Massachusetts Institute of Technology and Johns Hopkins University.
Why dont we notice that we have a blind spot in each of our eyes? ›Why You Don't Notice It. You probably don't notice your blind spot because your other eye makes up for it. Each eye sends data to your brain on its own, so your brain fills in what's missing. What one eye doesn't see, your other eye does.
What would you see if you had no eyes? ›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.
Who Has 1000 eyes but Cannot see? ›Riddle answer: What has an eye but cannot see? The answer to the “what has an eye but cannot see” riddle is a needle.
Why the blind man was trying to hide his blindness in the story the eyes are not here? ›The narrator had an impression that the girl was normal-sighted but when the fellow traveler revealed to the narrator that the girl was blind, it was then the narrator is outwitted by her. He tried his best to hide his blindness considering that the girl was normal-sighted.
What animal has a good sense of sight? ›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.
Mantis shrimps probably have the most sophisticated vision in the animal kingdom. Their compound eyes move independently and they have 12 to 16 visual pigments compared to our three. They are the only animals known to be able to see circular polarised light.
What two animals can see behind them? ›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.
What other animals have binocular vision? ›Examples include rabbits, buffalo, and antelopes. In such animals, the eyes often move independently to increase the field of view. Even without moving their eyes, some birds have a 360-degree field of view.
What animal has 8 eyes? ›
Spiders usually have eight eyes (some have six or fewer), but few have good eyesight. They rely instead on touch, vibration and taste stimuli to navigate and find their prey.
What animal has a sixth sense? ›Bats. Bats have a trifecta of sixth senses, or perhaps a sixth, seventh, and eighth sense: echolocation, geomagnetic, and polarization. Bats use echolocation to find and capture prey. They have a larynx capable of generating an ultrasonic buzz, which they emit through their mouths or nose.
What animals see in color? ›Good Color Vision
Some types of animals can see as many colors as we do. Monkeys, ground squirrels, birds, insects, and many fish can see a fairly good range of color. In some cases it's not as good as what we humans see - but it's much better than cats and dogs.
“There is one species that has only one eye naturally and they are from a genus called copepods.” Unlike the mythical one-eyed giant Cyclops, these real-world creatures are pretty small. In fact, some copepods are even smaller than a grain of rice.
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.
Which animal has super sense of tongue? ›Catfish (Sense of Taste)
Ever wondered what animal has the highest sense of taste? Well, the award goes to the catfish! Apparently, this fish with cat-like whiskers has up to more than 175,000 taste sensitive cells (as compared to an average person with only 10,000 taste buds) located in its entire body.
Emus. Like kangaroos, emus are from Australia. They are flightless birds similar in looks and characteristics to ostriches, though they average about 10 inches shorter in height. Unlike ostriches, emus cannot walk backwards; however, it's not known why.
Do rabbits have a blind spot? ›Rabbit's eyes are placed laterally so they have a good field of vision but struggle to see objects close up and have a blind spot right in front of their nose. For this reason, they have an acute sense of smell and remarkable hearing, with each ear being able to move independently.
What are six animals with monocular vision? ›Animals with Monocular Vision
Another animal group with monocular vision is most lizards. Herbivores or prey animals, such as rabbits, goats, cows, deer, commonly have monocular vision. Humans and predators, such as eagles, lions, and tigers, are commonly known for having binocular vision.
Deer see in 2D and have monocular vision (apart from humans where we have binocular and see with both our eyes), which means that they have larger field of view 310 degrees.
What animal has binocular vision and nails instead of claws? ›
The anatomical and behavioral features that distinguish primates from members of other mammalian orders include a lack of strong specialization in structure; prehensile hands and feet, usually with opposable thumbs and great toes; flattened nails instead of claws on the digits; acute vision with some degree of ...