Blindsight: Animals That See without Eyes [Slide Show]
Recent insights into how animals see without eyes reveal that vision and light-detection are older and more widespread than biologists previously realized
Credits: (Photo by Gerd Alberti and Uwe Kil, via Wikimedia Commons
GO WITH THE GLOW: Some preliminary evidence suggests that scorpions can detect UV light with their skin, even when their eyes are covered. Alternatively, scorpions may be attuned to the green light in their armor's turquoise fluorescence. Using its entire body to detect light, rather than its eyes alone, might improve a scorpion's chances of finding shelter during the day.
Image courtesy of Douglas Gaffin, University of Oklahoma
NIGHT LIGHT: Most scorpion species have a dark, waxy exoskeleton that looks like black or amber armor in daylight. If certain wavelengths of ultraviolet (UV) light strike a scorpion, however, it glows an eerie neon turquoise because of fluorescent molecules in its cuticle.
Jonbeebe, via Wikimedia Commons
OUT OF SIGHT: Scorpions instinctively avoid light. During the day, the eight-legged arachnids seek shelter beneath rocks, in underground crevices or in people's boots.
Hernán Colombo, via Wikimedia Commons
BUTTERFLY KISSES: Studies suggest that male swallowtail butterflies rely on their hindsight to cozy up to females during mating and that female swallowtails depend on light-detection to confirm that they have properly extended their ovipositor—the organ with which they attach eggs to leaves.
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HINDSIGHT: Japanese yellow swallowtail butterflies can see with their rear ends. More accurately, they have two light-sensitive neurons called photoreceptors on their abdomens, right next to their genitals. These light detectors are essential for swallowtail butterfly sex and reproduction.
KENPEI, via Wikimedia Commons
DOWN TO EARTH: Although
tiny worm-like nematodes named Caenorhabditis elegans live in complete darkness in the soil, scientists recently discovered that they have light-sensitive neurons and consistently wriggle away from light, an adaptation that likely helps them avoid the dangerous surface world. National Human Genome Research Institute, via Wikimedia Commons
PIXELATED PIGMENTS: 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. Some scientists propose that opsins work with chromatophores in an unknown way to detect and mimic the color of nearby objects.
Minette, via Wikimedia Commons
COLORLESS CUNNING: Although octopuses, cuttlefishes and some squid can match the texture and color of almost anything in their environment, their relatively large eyes cannot see color. Scientists recently discovered, however, that these mollusks express opsin genes throughout their skin.
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SHADOWBOXER: In this image of a hydra tentacle, cnidocytes are stained red. Hydras sting with greater force in dim light than in bright light, perhaps because they recognize shadows as signs of prey or predators. Since hydras belong to one of the oldest groups of animals, the Cnidarians, the origins of vision likely stretch further back in time than anyone realized.
Image courtesy of David C Plachetzki of the University of California, Davis
STING OPERATION: Like sea urchins, tiny relatives of jellyfish called hydras also respond to light even though they lack eyes. Recently, scientists confirmed that hydras have opsins in their tentacles, specifically in their stinging cells, known as cnidocytes.
Oinari-san, via Wikimedia Commons
TUBULAR VISION: It turns out that the ends a sea urchin's tubular feet are pockmarked with opsins, the same kind of light-sensitive proteins our own eyes depend on. A sea urchin's hundreds of feet may act as one giant compound eye. Opsins are stained red in this juvenile urchin.
Photo courtesy of Sam Dupont, University of Gothenburg
NO EYE IN URCHIN: Sea urchins respond to light in various ways: they might change color, twitch their spines or move toward or away from light. Until recently, scientists were not certain how urchins detect light; no known species has eyes of any kind.
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MORE TO VISION THAN MEETS THE EYE: Some eyeless animals perceive light. Likewise, some animals with eyes—even rather sophisticated eyes—rely on other body parts to see. Here are six striking examples of animals that have surprised researchers with eyeless sight.
Compound eye of the Antarctic krill Euphausia superba. Photo by Gerd Alberti and Uwe Kils, via Wikimedia Commons Advertisement Expertise. Insights. Illumination.
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