Parasites get all the press, but there are some life forms on the planet that play very nicely with one another, sharing symbiotic relationships in which both organisms benefit. It's a "you scratch my back, I'll scratch yours" kind of thing -- and any lichen, like the one shown above, is a prime example. A lichen doesn't have a mutualistic relationship with another species -- it simply is a mutualistic relationship. But if a lichen is just one thing, how can it involve two or more different organisms? Read on to learn more.
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Lichens -- which are part fungi, part algae -- are ridiculously tough little critters. They can survive extreme climatic conditions in the bitter cold north, arid rock-lands, or the chaotic littoral zone -- any of which would surely wipe out many higher-order organisms. No matter what the weather, these two life-forms are almost always able to offer each other a helping hand: The substrate of fungus provides structural protection to the alga, while the alga photosynthesizes sunlight to provide nourishing sugars to the fungus. Next, take a look at a pair of saltwater partners.
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The stinging fingers of the sea anemone are like electric knives swaying in the tide -- most creatures wouldn't dare go near them. This clownfish, however, has evolved a mucus coating that makes the anemone harmless to its scaly exterior. Clownfish and sea anemones make a great team: The forest of stingers protects the clownfish from its larger predators, and it also provides great leftovers for the clownfish after the anemone snags a meal. Meanwhile, the clownfish cleans the sea anemone, chases away predators and oxygenates the surrounding water.
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Yes, it looks like a purple boot, but this is actually a pitcher plant. Pitcher plants are primarily carnivorous. They lure insects into their fragrant pitcher-traps and then dissolve them for nutrition once they're stuck inside. The Low's pitcher plant, however, has developed a fascinating, if a little weird, symbiosis with the mountain tree shrew of Borneo. In this mutualistic arrangement, the tree shrew uses the pitcher plant as, well, a toilet. Tree shrews come to the plant to stand directly over the pitcher and lick the sweet nectar from the "lid." While they're at it, they usually defecate into the pitcher. In this way, both organisms obtain useful food from the other. To see another example of pitcher plant mutualism, check out the next page.
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Some pitcher plants survive with the help of mutualistic ants. Camponotus schmitzi -- a species of ant from Borneo -- survives in a mutualistic relationship with the pitcher plant Nepenthes bicalcarata. The ants make their nests within the plant, scavenging their meals from the plant's acidic solvent traps, and in turn helping the plant digest more efficiently. Next you'll see a beetle with some hangers-on.
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This is the American carrion beetle carrying a busload of phoretic mites. The carrion beetle is simply gaga for rotting flesh. Not only does it survive on a diet of recently dead animals, it lays its larvae in these carcasses as well. This is where the mites come in: Mites benefit from this relationship because the carrion beetles offer protection and take them where they need to go. The beetles may also benefit from decreased reproductive competition with flying insects, since the mites decamp and gobble up fly larvae upon reaching a carrion site.
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This is an example of the family Dromiidae -- the sponge crab. These industrious arthropods don't leave personal safety to chance. To protect themselves against ocean dangers, they make themselves plates of armor out of living marine sponges, constantly trimming the growth of the sponge to their needs. In return, of course, the sponge is cultivated and provided with the benefits of mobile filter-feeding, which the sessile sponge would not receive if it were growing on a stationary rock.
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This is perhaps the clearest, simplest, most well-known mutualism in the world: Flowers offer delicious nectar that bees love, and bees carry grains of pollen from one immobile blossom to another. Flower feeds bee; bee helps flower have sex. To quote the American poet Emily Dickinson on the subject: "To make a prairie it takes a clover and one bee,— / One clover, and a bee, / And revery. / The revery alone will do / If bees are few."
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This is a remora, also known as a suckerfish. It didn't get this common name because it's easily duped into a game of Three-card Monte. No, this fish literally sucks its way through life, clinging to the bodies of larger predators such as sharks, whales and sea turtles via a vacuum-like sucking disc on its head. See some remoras in transit with their frightening host on the next page.
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Remoras attach themselves to this lemon shark and hang on for the ride. On their side of the deal, the remoras get food and protection from the shark, surviving on scraps left over after the shark eats, as well as on dirt and parasites found on the shark's body. The shark, in return, gets a personal groomer.
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Though the mutualistic fungus/alga combo we know as lichen can look like a parasitic organism when it grows on trees, most people consider it basically harmless. After all, the lichen doesn't need to get nutrition from a host source -- the algae inside convert sunlight into useable energy.
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The big fish here has a name that sounds fake: It's called a "painted sweetlips." But really it's not the big fish we're looking at -- it's the small, striped cleaner wrasse, Labroides dimidiatus, following along to peck bits of dirt and junk off the big fish's body. It may sound like a pitiful existence, but it doesn't seem to bother the wrasse. Small cleaner fish like this have symbiotic relationships with many other species in the ocean.
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The relationship between hummingbirds and ornithophilous flowers is similar to the one between a honeybee and its local flower of choice. Hummingbirds are promiscuous pollinators, and the flowers contain sweet treats of nectar, available to the bird with the right beak shape and size. Next up, do you know what a treehopper is?
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Some look like mere thorns or leaves, while others are more ornate. Many species of treehoppers have friendly relationships with protective bodyguard ants that live alongside them. Treehoppers that look like thorns are able to provide discouragement to larger predators (like birds) who might want to land on a branch and make a meal of some insect workers, while aggressive battalions of ants are effective against smaller, ground-level enemies. Next, see another example of ant mutualism.
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The acacia ant, Pseudomyrmex ferruginea, is the territorial custodian of the bullhorn acacia tree, Acacia cornigera. The ant gains nourishment from the tree itself, drinking from precious nectar glands like the ones shown above, but it is also fiercely jealous, fighting off any other herbivores that try to approach the tree for a snack. "This is my tree, get it?"
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A space alien observing human behavior might characterize our pets as pure parasites: We feed them, protect them, spend our saved-up vacation money on veterinarian bills, yet they don't provide anything tangible in return. After all, when was the last time your puppy vacuumed the living room or sorted a load of laundry? Of course, any animal lover knows that a pet does give back plenty: They make us laugh, they lower our blood pressure, and they generally give our physical and mental health a boost. Check out a marine cleaner in the next photo.
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Loggerhead sea turtles also benefit from mutualistic relationships with small cleaner fish. That mossy, salty shell might not look very appetizing to you, but to a cleaner fish, it's a virtual buffet of grime, feces and delicious external parasites. Mmmm. On the next page, you'll see a famous bird of Africa.
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Oxpeckers are birds that play the same game on land that cleaner fish play in the sea. One often finds these fastidious flyers perched on the backs of African megafauna, searching out ticks, fleas and other scrumptious little creeps that large mammals are unable to pick off of themselves. To the red-billed oxpecker (shown above), there's no better meal in the world than a hot, blood-engorged tick. Large mammals such as this impala generally tolerate oxpeckers, but some ecologists have recently suggested that these birds could be less than 100 percent helpful to their host mammals.
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The bird in the previous photo was a red-billed oxpecker. This is a yellow-billed oxpecker -- another native of Africa, with essentially the same survival plan. While some ecologists still argue that the relationship between oxpeckers and their large tick-suppliers is primarily mutualistic, others have suggested that it is less benign than previously thought: Though oxpeckers eat the blood-sucking arthropods that plague large mammals, they also turn out to be hungry little vampires themselves. Oxpeckers love blood, and they have been known to peck at not just the bugs but the skin itself, drinking from open wounds and consuming dead skin, mucus and other body products. Next, see some creatures that mastered agriculture long before humans did.
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Intelligent as Homo sapiens can be (on our best days), humans have only practiced serious agriculture for about 10,000 years, give or take. Leaf-cutter ants, meanwhile, are the original farmers of the animal kingdom. This is an ant fungus garden. Leaf-cutter ants maintain fungus gardens as food crops inside their nests, fertilizing them with dead plant matter and harvesting for food the fungus that grows in the dark. Of course mushrooms are delicious (and a common favorite ingredient of gourmet chefs), but can you imagine eating nothing but fungi?
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OK, so humans don't really get anything out of this deal, but we don't sacrifice much either. Foxtail seeds spread themselves via epizoochory -- which means they stick to animals that pass by and are later shed in a different location, allowing the plant to spread its genetic material geographically. When you come in from a walk in the woods and your shoes look like this, you have just been accessory to the sexual reproduction of plants. Relationships where one organism is helped and the other is neither helped nor harmed are called commensalism.
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Here we have yet another cleaner species. Like the remora, the cleaner shrimp Lysmata amboinensis apparently has no fear, reaching into the deadly mouth of a moray eel like a happy-go-lucky dental hygienist. Fearsome predators tolerate the scraping and prodding perhaps because they somehow sense it's for their own good.
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Like the deep orchid blossoms that have evolved alongside long-beaked hummingbirds, the yucca plant has co-evolved with a mutualistic pollinator: the yucca moth. These moths rely on the plant as a place to lay their eggs, and the yucca flower can only be pollinated by this particular moth. When moth larvae hatch inside a yucca blossom, they feast on the seeds found there, but instinct compels the moths to always leave some seeds behind, so that the flower can continue reproduction.
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"I can see my house from up here!" The cattle egret provides another example of what we would probably call commensalism. Like oxpeckers, egrets are voracious insect-eaters, but the overall relationship between egrets and elephants is slightly more neutral than that between oxpeckers and their hot-blooded hosts. Apparently, the egrets are both less helpful and less harmful: No blood-drinking goes on here, but no ambitious purging of ticks and other tiny parasites does either. The egrets are mostly along for the ride, snatching up insects that the elephants stir from the grass as they lumber along.
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Remember the lichen? You're looking at another algae-based symbiosis -- this time, between algae and acoel flatworms. These flatworms perform a strange brand of endo-symbiosis, in which the algae actually live inside the clear flesh of the flatworms. The sun feeds the alga through the protection of the flatworm's transparent skin, and the alga feeds the flatworm the energy it needs to survive.
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Zooxanthellae and reef-building coral participate in a vital, life-giving mutualism. It can be hard to recognize the interaction because, frankly, both species look like weird little particles of mush, but each one of these species is very important to the other. Zooxanthellae are photosynthetic, meaning they convert sunlight into usable energy, from which they process sugars and other carbon-based compounds that they eventually share with their host coral polyps. Next, check out a squid that has a radiant relationship with bacteria.
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This is the bobtail squid -- an undersea creature with a built-in invisibility cloak. When a bobtail squid is squirting along through the waves at night and a predator peeks up from below, the predator doesn't see the silhouette of the little squid floating above it. How does the squid accomplish this? With the help of bioluminescent bacteria, of course! The bobtail squid participates in a mutualistic relationship with the light-producing bacteria Aliivibrio fischeri. The squid keeps the bacteria inside its body and feeds them sugars from its own metabolism. In return, the bacteria light up the underside of the squid's body to match the luminosity of the moonlight coming from above, hiding the tiny squid's shadow from anyone down below.
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Look kind of like cheese puffs, don't they? Actually, they're the mutualistic organisms that would help arm your body for battle if you were to subject your digestive system to a sudden onslaught of said cheese puffs. They're microorganisms (mostly bacteria) known as gut flora, and though they are extremely helpful, they are completely separate organisms from their hosts. They dwell inside our bodies and help us with both digestion and immune response.
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The snapping shrimp (shown above) is also known as the "pistol shrimp" for the extremely loud sonic weapon it deploys with its claw. The snapping noise it produces is one of the most deafening sounds of the ocean -- loud enough to interfere with manmade sonar. But the snapping shrimp has another ally in addition to its clap of death: the goby fish. Snapping shrimp and gobies have a wonderful arrangement worked out: The goby gets to share in the protection of the snapping shrimp's burrow, and the shrimp gets to rely on the goby as a "lookout," alerting the shrimp when a predator is approaching.
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Would you let a crab crawl all over your body? Apparently it's just second nature to the marine iguanas of the Galapagos Islands, which share a cleaning symbiosis with the Grapsus grapsus crab, also known as the "Sally Lightfoot" crab. The iguanas tolerate these creepy groomers because they're able to efficiently remove dead skin, parasites and other debris. Move over, pugs and bulldogs: This relationship officially breaks the record for the world's ultimate "ugly cute."
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