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Sea urchin -

Sea urchins are a group of sea animals that live on the ocean floor and are known for their spiny exteriors and wide variety of colors. Although they don't have eyes, they react to light in several different ways. They sometimes twitch their spines, move towards or away from the light, and even change color.

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Sea urchin -

Researchers discovered that purple sea urchins have numerous genes related to the development of the retina. The ends of their tubular feet are also covered in opsins. Opsins are the same light-sensitive proteins found in the human eye. When light hits an opsin protein it changes shape, causing some sort of reflexive reaction. In the case of the sea urchin, it causes movement or color change.

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Swallowtail butterfly -

Japanese yellow swallowtail butterflies don't see with eyes. In fact, they see with their butts! To be specific, they have two light-sensitive photoreceptors at the end of their abdomens near the reproductive organs.

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Swallowtail butterflies -

The photoreceptors are essential for reproduction. The male and female butterflies copulate while facing away from each other and use the photoreceptors to ensure they align. The female swallowtail also uses them to locate a leaf where she can lay her eggs.

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Mexican tetra -

There are two forms of the Mexican tetra fish: the surface-dwelling kind that have eyes, and the cave-dwellers that lost their eyes over millions of years of evolution.

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Mexican tetra -

The blind Mexican tetra fish lives in waters inside dark caves, so their need for eyes waned over time and they simply developed without them. They also produce less skin pigment and have lost most of their color, unlike their surface-dwelling cousins.

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Octopus -

Octopuses have large eyes and good vision, but their eyes aren't able to see color. Regardless, they are able to change the color of their skin to match almost any background and blend in. They are masters of disguise, but how do they match what they can't see?

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Octopus -

Octopuses have a great deal of light-sensitive opsin proteins in their skin to help them detect light. Their skin is also filled with chromatophores. Chromatophores are elastic sacks of pigment that allow them to change their color.

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Cuttlefish -

Cuttlefish are also colorblind but can change their appearance and camouflage themselves. They seem to use a combination of opsin proteins and chromatophores in their skin to perceive the colors around them and blend in.

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Squid -

Another similar creature is the squid. When it comes to the color-changing talents of the octopus, cuttlefish, and squid, scientists still don't know exactly how it works. One thing, however, is for sure: they can perfectly mimic the colors around them, but they certainly aren't seeing them with their eyes.

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Star-nosed mole -

The star-nosed mole is one of the strangest-looking creatures you're likely to come across in the woods and marshes of North America. The hamster-sized animal has a star-shaped appendage on its face, which it uses to feel its way around in the dark. The fleshy tendrils can target 13 different areas at once and effectively allow them to chose their path.

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Scorpion -

Scorpions are nocturnal arachnids that always try to avoid light. They are usually found sheltering under rocks, in crevices, or in an unlucky person's boot! They have two eyes on their heads and can have up to five more pairs of smaller eyes. You'd think this would take care of all their visual needs, but it's believed that they can also detect light with their skin.

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Scorpion -

Most scorpions have a dark black or amber exoskeleton. However, they can turn bright turquoise when hit with certain kinds of UV light. They have fluorescent molecules in their skin that react to UV light, which may be an evolutionary feature to scare off predators. Preliminary studies also suggest that scorpions can detect light even when all of their eyes are covered.

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Olm -

The olm is a European cave-dwelling salamander. They don't need eyes due to their dark surroundings and have developed other ways of surviving. They can smell their prey when they hunt, and can even detect the quality of the prey due to their sensitive noses.

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Olm -

It's believed that they have also developed the ability to sense electric and magnetic fields. Olms have a lifespan of up to 100 years.

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Hydra -

Hydras are tiny little relatives of the jellyfish. They have small tubular bodies and thin tentacles radiating from the head. They usually attach themselves to weeds in the sea and eat even tinier sea creatures like water fleas. They sting their prey with their tentacles, which are also covered in opsin proteins.

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Hydra -

Research shows that hydras sting more forcefully in dim light than in bright light. Scientists believe this is because they have evolved to feel safer when exposed to more light, while dim light suggests shadows and is more likely to harbor a real predator.

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Blind crayfish -

The blind crayfish is found underground in the caves of Missouri. They are translucent and blind due to their lack of exposure to light. They move around and sense their prey with the help of their long antennae.

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Blind albino cave crab -

The blind albino cave crab is more or less self-explanatory! This cave-dwelling sightless crab has lived underground for millennia, losing its sight and its color. One interesting fact is that it's actually a very small lobster and not a crab!

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Blind albino cave crab -

The blind albino cave crab can be found in the caves of Jameos del Agua in Lanzarote in Las Palmas, Spain.

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Worm lizard -

The worm lizard is an eyeless, earless, and limbless reptile. It burrows into mud and sand with its head and spends most of its life underground like a worm. They evolved differently from other lizards, losing their limbs and eye to better aid their underground lifestyle. 

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Worm lizard -

Worm lizards are still in the evolutionary process, however, and rudimentary eyes and ears can be found below the skin.

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Texas blind salamander -

The Texas blind salamander is another animal that evolved without eyes in response to its surroundings. They live in water underground at the Edwards Aquifer near San Marcos, Texas.

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Texas blind salamander -

The blind salamander no longer has working eyes, just two black dots under the skin where the eyes used to be. They hunt their prey by sensing their movement in the water through ripples and waves.

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Kentucky shrimp -

The Kentucky shrimp can only be found in the Mammoth Cave area of Kentucky. They have evolved without working eyes thanks to their dark home in the caves. They survive by feeding on minute organisms in the water. Unfortunately, they're now endangered due to contaminated groundwater entering the caves.

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Sea anemone -

Sea anemones are sea animals in the same family as hydra and jellyfish. They mainly consist of a mouth surrounded by tentacles! They tend to stay in one place and sting any prey that happens to pass by. If they sense danger, they can completely change their shape and retract all of their tentacles.

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C. elegans -

The C. elegans is a tiny little worm-like creature that lives in total darkness in the soil. Scientists always assumed they couldn't detect light because they are eyeless and live in an environment that's completely deprived of light.

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C. elegans -

But when scientists shone a torch at the head of the C. elegans, it started to inch its way backwards away from the light. When they shone the light at its tail, it began inching forwards. Cells containing a photosensitive protein called LITE-1 were found in the C. elegans. It's believed that they evolved to detect and avoid light, as they cannot survive out of the dark soil for long.

Sources: (Scientific American) (Mom.com)

See also: Amazing animals under threat of extinction

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