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| A brittle star on an octocoral skeleton. They climb to get higher in the current. NOAA. |
Most creatures have some way of perceiving and responding to light. Many can do it without having any eyes.
Brittle stars have photoreceptors scattered across their bodies. Some can only tell whether they’re in light or darkness, but others can detect different contrasts of light, enabling them to seek shelter under a dark shape in the distance. Sea urchins can also see without having any eyes. They have clusters of photoreceptors in their tentacle-like tube feet and use their own shadows to discern the direction of light.[1]
Similarly, the hydra can see without eyes. This freshwater predator is basically a tube-shaped stalk with tentacles on one end. They can grow up to two inches in length (5 cm), but can stretch themselves to eight inches (20 cm). They use their minimal sight to detect and shoot their prey with harpoon-like stingers.
Scorpions have eyes, but they can also detect ultraviolet light with the waxy cuticle covering their bodies, making their exoskeleton a sort of eye. Creatures that can see with their skin include octopuses, a chameleon, a gecko, a wall lizard, a sea snake, a fish, a pond snail, a caterpillar, new-born pigeons and rats, and fruit flies. Even earthworms can detect light with photoreceptors in their skin. We do too, although we’re not aware of it. These receptors launch immediate repairs when our skin becomes sunburned.[2]
I See You
Many animals that we wouldn’t expect to be able to see, actually do have eyes. Chitons, the tide pool mollusk that looks like a flattened slug protected by a series of eight armored plates, have hundreds of tiny eyes built into their shells that have retinas and aragonite crystals as lenses. This relative of limpets and abalones can actually see the shadow of an eight-inch fish (20 cm) that’s six and a half feet away (2 m). And these are animals that mainly consist of a snail-like foot that can grip rock faces. Their brains are just a simple ganglion—a group of neuron cell bodies—but on seeing an approaching fish, the chiton clamps down on the rock. Even though these eyes seem primitive, they evolved in just the last 10 million years, so they’re pretty new, evolutionarily speaking. Other chitons that don’t have lenses or retinas are still able to detect small changes in brightness.[3]
Some species of starfish have between five and 50 eyes that are on the tips of their arms. They only see in black and white, but, judging from the position of their eyes, it’s likely they can see all around them for a distance, detecting things up to a dozen feet (nearly 4 m) away, including the surface of the water and whatever is right in front of them. They most likely use their sight to stay on or near the reef.
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| A scallop with blue eyes and a close-up from another scallop. Top: Rachael Norris and Marina Freudzon. Bottom: Matthew Krummins, CC BY 2.0. |
Scallops also have dozens of eyes—some have 200 of them—that protrude from their mantel between their shells. They move around a lot, so their eyes are quite useful. Oysters and mussels don’t have them, but then they’re mostly immobile. Scallop eyes are on the end of tentacles and protrude from under their mantle in a line along the edge of each of their shells. Some have red eyes, but many have blue eyes. Their eyes also have pupils that expand and contract simultaneously. Light passing into one of their eyes reflects off of a curved mirror and onto two retinas that detect different things, but it’s thought they’re mostly looking for movement. When they see large enough particles drifting by, they open their shells to investigate, probably by using their sense of smell.
Scallops can detect large objects, but their visual system is so slow that it’s probably not much use in detecting predators. Although the eyes of one species—the venomous crown-of-thorns starfish (Acanthaster planci)—are good enough to see predators. They also use their sight to hunt for prey, and they’re fast enough to chase it. It’s possible that all sea stars with eyes can detect the bioluminescence of other nearby starfish and they might even be able to communicate with each other using flashes of light.[4]
Even box jellies, which are nearly transparent, have 24 eyes distributed among four eye stalks. With eight of their eyes, which are similar to ours, they can probably see silhouettes at least 26 feet (8 m) above the water. This helps them hunt prey and the navigate mangrove swamps they sometimes live in.[5]
Scientists have found thousands of creatures that produce their own light. They include fireflies and mushrooms, but most of them live deep in the ocean, such as some sharks, fish, jellies, crustaceans, and octopuses. It’s thought that 80 to 90 percent of sea organisms luminesce. These creatures use light to communicate, attract mates, attract prey, camouflage themselves, ward off predators, and to attract bigger predators that eat their predators, among other things. The lights range from blue to green, but in barbeled dragonfish it can be red. Bioluminescence has evolved independently at least 50 times. Genetic engineers have transferred the ability to glow in the dark to other creatures, such as plants, marmosets, rabbits, cats, and dogs.
Oysters don’t have ears, but they hear sounds through a different organ called a statocyst. We don’t know how things sound to them, but it probably wouldn’t be like how our brains interpret sounds. Still, they can hear breaking waves, water currents, the approach of predators, thunderstorms, and they’re particularly sensitive to man-made noise pollution. They use the sounds to decide when to clam up, feed, and spawn. Also, oyster larva navigate towards the sound of snapping shrimp, which helps lead them to reefs. Scientists found that mussels and hermit crabs can also hear[6], and there are likely many other sea creatures that can. We don’t yet know how noise pollution affects them[7], but it can destroy the statocysts in octopuses, squid, and cuttlefish, making them permanently deaf and unable to move or hunt—effectively killing them.[8]
Also, since many people like to eat them alive, we can imagine what that experience might be like for them. Just as we can’t know what it’s like to be a bat. We may never truly know, but we can get a better idea the more we learn about them.
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[1] Ed Yong, “Sea urchins use their entire body as an eye”, National Geographic, May 2, 2011, https://www.nationalgeographic.com/science/article/sea-urchins-use-their-entire-body-as-an-eye, citing Esther M. Ullrich-Lüter, Sam Dupont, Enrique Arboleda, and Maria Ina Arnone, “Unique system of photoreceptors in sea urchin tube feet”, PNAS, 2011, https://www.pnas.org/doi/full/10.1073/pnas.1018495108, https://doi.org/10.1073/pnas.1018495108.
And University of Gothenburg press release, “Sea urchins see with their whole body”, ScienceDaily, September 12, 2011, http://www.sciencedaily.com/releases/2011/06/110630111538.htm, citing Esther M. Ullrich-Lüter, Sam Dupont, Enrique Arboleda, and Maria Ina Arnone, “Unique system of photoreceptors in sea urchin tube feet”, PNAS, 2011, https://doi.org/10.1073/pnas.1018495108.
[2] Wendy Zukerman, “Skin ‘sees’ light to prevent UV harm”, New Scientist, no. 2838, November 12, 2011, p. 20, and as “Skin ‘sees’ the light to protect against sunshine”, https://www.newscientist.com/article/dn21127-skin-sees-the-light-to-protect-against-sunshine/, citing Nadine L. Wicks, Jason W. Chan, Julia A. Najera, Jonathan M. Ciriello, and Elena Oancea, “UVA Phototransduction Drives Early Melanin Synthesis in Human Melanocytes”, Current Biology, vol. 21, no. 22, November 22, 2011, pp. 1906-1911, https://doi.org/10.1016/j.cub.2011.09.047.
[3] Ed Yong, “Chitons see with eyes made of rock”, National Geographic, April 14, 2011, https://www.nationalgeographic.com/science/article/chitons-see-with-eyes-made-of-rock, citing Daniel I. Speiser, Douglas J. Eernisse, and Sönke Johnsen, “A Chiton Uses Aragonite Lenses to Form Images”, Current Biology, vol. 21, no. 8, April 26, 2011, pp. 665-670, https://www.cell.com/current-biology/fulltext/S0960-9822(11)00305-8, https://doi.org/10.1016/j.cub.2011.03.033.
And Anna Nowogrodzki, “Mollusc sees the world through hundreds of eyes made out of rock”, New Scientist, November 19, 2015, https://www.newscientist.com/article/dn28520-mollusc-sees-the-world-through-hundreds-of-eyes-made-out-of-rock/, citing Ling Li, Matthew J. Connors, Mathias Kolle, Grant T. England, Daniel I. Speiser, Xianghui Xiao, Joanna Aizenberg, and Christine Ortiz, “Multifunctionality of chiton biomineralized armor with an integrated visual system”, Science, vol. 350, no. 6263, November 20, 2015, pp. 952-956, https://doi.org/10.1126/science.aad1246.
[4] Laura Geggel, “Starfish Can See You … with Their Arm-Eyes”, Live Science, February 7, 2018, https://www.livescience.com/61682-starfish-eyes.html.
And Christie Wilcox, “Sea Stars See!”, Discover Magazine, January 7, 2014, https://www.discovermagazine.com/planet-earth/sea-stars-see.
And Ed Yong, “Starfish Spot The Way Home With Eyes On Their Arms”, National Geographic, January 8, 2014, https://www.nationalgeographic.com/science/article/starfish-spot-the-way-home-with-eyes-on-their-arms.
All three citing A. Garm and D-E. Nilsson, “Visual navigation in starfish: first evidence for the use of vision and eyes in starfish”, Proc Roy Soc B, 281, 2013, http://dx.doi.org/10.1098/rspb.2013.3011.
[5] Cell Press press release, “Through unique eyes, box jellyfish look out to the world above the water”, ScienceDaily, April 30, 2011, http://www.sciencedaily.com/releases/2011/04/110428123938.htm, citing Anders Garm, Magnus Oskarsson, and Dan-Eric Nilsson, “Box Jellyfish Use Terrestrial Visual Cues for Navigation”, Current Biology, April 28, 2011, https://doi.org/10.1016/j.cub.2011.03.054.
And Ed Yong, “Single-Celled Creature Has Eye Made of Domesticated Microbes”, National Geographic, July 2, 2015, https://www.nationalgeographic.com/science/article/single-celled-creature-has-eye-made-of-domesticated-microbes, citing Gregory S. Gavelis, Shiho Hayakawa, Richard A. White III, Takashi Gojobori, Curtis A. Suttle, Patrick J. Keeling, and Brian S. Leander, "Eye-like ocelloids are built from different endosymbiotically acquired components", Nature, 2015, http://dx.doi.org/10.1038/nature14593.
[6] Louise Roberts, Harry R. Harding, Irene Voellmy, Rick Bruintjes, Steven D. Simpson, Andrew N. Radford, Thomas Breithaupt, and Michael Elliott, “Exposure of benthic invertebrates to sediment vibration”, Proceedings of Meetings on Acoustics, vol. 27, no. 1, 010029, January 5, 2017, https://doi.org/10.1121/2.0000324.
[7] Andy Coghlan, “Oysters can ‘hear’ without ears”, New Scientist, no. 3149, October 28, 2017, p. 18, and the longer version “Oysters can ‘hear’ the ocean even though they don’t have ears, https://www.newscientist.com/article/2151281-oysters-can-hear-the-ocean-even-though-they-dont-have-ears/, citing Mohcine Charifi, Mohamedou Sow, Pierre Ciret, Soumaya Benomar, Jean-Charles Massabuau, “The sense of hearing in the Pacific oyster, Magallana gigas”, PLoS ONE, October 25, 2017, https://doi.org/10.1371/journal.pone.0185353.
[8] Andy Coghlan, “Shipping noise pulps organs of squid and octopuses”, New Scientist, no. 3328, April 3, 2011, https://www.newscientist.com/article/dn20364-shipping-noise-pulps-organs-of-squid-and-octopuses/, citing Frontiers in Ecology and the Environment, https://doi.org/10.1890/100124.
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