Blindsight: Animals that see without eyes [slideshow] (2023)

Light bathes our planet, spilling over mountaintops, flooding deserts, tundras, savannahs and forests, and penetratingas deep as 1,000 meters in the ocean. Bacteria, plants, animals and all kinds of living things have evolved different ways of detecting and responding 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 very sophisticated eyes, rely on other body parts to see. Exactly how organisms perceive light without eyes, in many cases, remains a mystery. In recent years, with the help of new tools such as genome sequencing, scientists have discovered light-sensitive cells and proteins in unexpected places and established that creatures once thought to be blind can indeed see. Light-reactive proteins coat the ends of a sea urchin's legs, for example, possiblytransforming the entire body of the spiky animal into a compound eye. Likewise, small jellyfish-like hydras canfeel the light with its stinging tentacles. And although nematodes live in the dark underground, some of themyour neurons respond to light, helping them escape danger.

View a slideshow of animals that see without eyes.

These new insights arechanging the way biologists understand the evolution and function of vision. Two important questions are when light-sensitive proteins first evolved and how animals originally used these proteins. Another question is whether complex eyes evolved many times in different groups of animals, or whether later generations inherited and played with a single primitive eye archetype. While biologists still don't have definitive answers to these questions, it's already clear that vision and light detection are older, more diverse, and more widespread than researchers previously believed. Here are six amazing examples of animals that have stunned researchers with eyeless vision.

SEA URCHINS
Sea urchins respond to light in a variety of ways: they can change color, contract their spines, or move closer or further away from light. Scientists have known this for a long time, but they've never been sure how hedgehogs detect light, because no known species has eyes of any kind. His best guess was that the network of nerves surrounding a hedgehog's body included some diffuse light-sensitive tissue. The remarkable truth is that sea urchins have a much more organized visual system than expected.

When researchers sequenced the genome of the purple sea urchin (Strongylocentrotus purpuratus), were surprised to discover a number of genes important for the development of the vertebrate retina, the thin layer of light-sensitive tissue that lines the back of our eyes.Maria ArnoneAnton Dohrn of Stazione Zoologica in Italy and colleagues revealed that the ends of a porcupine's tubular feet are tagged with opsins, the same light-sensitive proteins that our own eyes depend on. When certain wavelengths of light hit an opsin protein, it changes shape and triggers a chemical cascade that opens tiny gates in cell walls called ion channels. Depending on the animal, this sequence of molecular events results in reflex behavior, such as moving toward or away from light, or it informs the nervous system about some aspect of vision. The hundreds of feet of a sea urchincan act as a giant compound eye, allowing them to see as well as a horseshoe crab or nautilus, both of which have genuine, if primitive, eyes.

HYDRA
Hydra, tiny relatives of jellyfish, look like dandelion seeds: they have thin tubular bodies covered in thin tentacles. They typically latch onto weeds, bite and eat even smaller aquatic invertebrates such as water fleas (daphnia). Like sea urchins, hydras also respond to light even without eyes. When scientists sequenced the genome ofhydra magnipapillata, found many opsin genes.

Scientists have recently confirmed that hydras have opsins in their tentacles, specifically in their stinging cells known as cnidocytes.David Plachetzkiat the University of California, Davis, and colleaguesshowed that hydras respond not only to touch and chemicals, but also to changes in lightin your immediate environment. Hydras sting harder in dim light than they do in bright light, perhaps because they evolved to recognize shadows as signs of prey or predators: the more they shoot in the presence of a shadow, the more likely they are to hit their targets.

Hydras belong to one of the oldest groups of animals on the planet, the cnidarians. Although hydras don't have eyes, other members of their family have simple eyes called eyespots. Box jellies have remarkably sophisticated eyes with lenses and retinas. The fact that hydras, which evolved long before most cnidarians, could detect light with their tentacles suggests that the origins of vision go back further than previously thought. Later, jellyfish and other animals may have modified these existing primitive visual systems to form more complex eyes.

SQUID, SQUAD AND POWDER
Octopuses have big eyes and huge occipital lobes, the parts of the brain that process vision. These crafty, cute masters of marine disguise can match the texture, color, and pattern of just about anything in their environment. But they cannot see colors, at least not with their eyes. The eye of the octopus is technically colorblind. So is the eye of the cuttlefish, a related mollusk.

roger hanlonof the Laboratory of Marine Biology in Woods Hole, Massachusetts, and colleagues recently discovered that cuttlefishactively express opsin genes throughout the skin, particularly on their fins and bellies. ANDDesmond Ramirezfrom the University of California, Santa Barbara, hasOpsin genes detected in octopus skin. The skin of octopus, squid, and cuttlefish is also studded with chromatophores, elastic sacs of pigment that expand and retract, allowing the mollusk to change 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 unknown ways to detect and mimic the color of nearby objects.

C. ELEGANT
Caenorhabditis elegans—small worm-like nematodes—live in complete darkness in soil, so scientists assumed they couldn't perceive or respond to light. WhenXZ Shawn Xuat the University of Michigan and his colleagues shined beams of light on the heads of the nematodes, however, they slowly stopped moving and changed direction. When the researchers illuminated a nematode's tail or body moving in reverse, the creature began to squirm forward. By killing several neurons in the nematode heads with a laser, Xu and his colleagues identified four cells without which C. elegans cannot perceive light. The researchers propose that light avoidance is an adaptation that helps nematodes stay in the soil, outside of which they won't survive for long (unless scientists keep them alive in the lab).

In subsequent work, Xu and his team showed that light-sensitive neurons in nematodes do not depend on opsins. Instead, they use LITE-1, a protein that functions as a taste receptor in invertebrates. A separate team of scientists.found that neurons in fruit fly larvae detect light with a protein closely related to LITE-1. As with nematodes, it is advantageous for fly larvae to remain in the shade, away from bright light and predators.

TAIL OF GOLUNDA BUTTERFLIES
Japanese yellow swallowtail butterflies can see from their hind ends. More specifically, they have two light-sensitive neurons called photoreceptors in their abdomens, right next to their genitals.Kentaro Arikawa, now at the Graduate University for Advanced Study in Japan, found that these light detectors are essential for swallowtail butterfly sex and reproduction. When yellow swallowtails mate, they precisely line up their genitals as they move away from each other. Typically, butterflies successfully complete their mating dance about 66% of the time. When Arikawa and his colleagues heat-destroyed the photoreceptors on the males' abdomens, or covered the buttonholes with black mascara, the insectssuccessfully matched only 23 to 28 percent of the time.

Ema related study, Arikawa and his team removed or smeared the photoreceptors on the abdomen of pregnant females and released the butterflies into a cage with a potted lemon tree. Insects successfully laid eggs on leaves 14% of the time, far below the usual 81% success rate. Taken together, the evidence suggests that male swallowtails rely on light detection to approach females during mating and that female swallowtails rely on hindsight to confirm that they have correctly extended their ovipositor, the organ with which which the eggs adhere to the leaves.

SCORPIONS
Scorpions instinctively avoid light. During the day, eight-legged arachnids seek shelter under rocks, in underground crevices or in people's boots. At night, they go out to hunt small insects. Scorpions detect light and can even perceive images, with two main eyes on top of the head and up to five pairs of smaller eyes close together. Scientists recently investigated whether scorpions can also detect light with their skin. The answer is a preliminary yes.

Most scorpion species have a dark, waxy exoskeleton that looks like black or amber armor in daylight. However, if certain wavelengths of ultraviolet light hit a scorpion, it will glow an eerie neon turquoise due to fluorescent molecules in its cuticle. Biologists speculated that this fluorescence could help scorpions attract prey or warn predators to stay away; alternatively, sheen may be an unavoidable physical property of scorpion skin that offers no adaptive benefit.

douglas gaffinfrom the University of Oklahoma exposed 40 Texas scorpions to green light and ultraviolet light. Half the time, the animals wore tiny aluminum eye patches; the other half, her eyes were unobstructed. Scorpions were much less active under green light when their eyes were covered compared to when they were unmasked, butwere equally active under ultraviolet light, regardless of whether their eyes were exposed. One interpretation of this pattern is that scorpions change their behavior in response to ultraviolet light, even when they cannot see with their eyes, because their skin detects ultraviolet rays on its own. Another possibility is that the scorpions somehow perceive the green light from their armor's turquoise fluorescence. Using the whole body to sense the light, rather than just the eyes, can increase a scorpion's chances of finding shelter during the day.