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4 Parasites Too Creepy to Exist

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Aug 11, 2019

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4 Parasites Too Creepy to Exist
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  • [♪ INTRO]
  • Of all the strategies life has developed for survival,
  • parasitism is among the most popular.
  • A parasite is an organism that makes its living
  • taking resources from another, called the host.
  • Parasites give nothing back and often
  • harm the host in the process.
  • Scientists have estimated that anywhere
  • between ⅓ and ½ of all life on Earth is parasitic,
  • although there’s still a lot we don’t understand.
  • What we do know is that the unique challenges
  • and opportunities that come with parasitism have led
  • to the evolution of some truly strange and
  • sometimes disturbing features.
  • Fair warning: some of what you’re about to hear
  • is pretty gross.
  • So if you’re on the squeamish side,
  • this may not be the list show for you.
  • Case in point, the first parasite on our list is
  • a crustacean that latches onto a fish’s tongue
  • and lives in its mouth.
  • They’re often called “tongue-biting isopods,”
  • and they belong to a group of crustaceans called cymothoids.
  • This group includes many species that attack fish,
  • usually infecting the skin, muscles, or gills and
  • feeding on the fish’s nutritious fluids.
  • But species in the genus Cymothoa have a
  • particularly disturbing habit of going for the mouth.
  • Since there’s limited space in a fish’s mouth,
  • this approach is strictly a first come, first served sort of engagment.
  • These crustaceans all start out as free-swimming, male larvae.
  • They wander around until they find a fish and settle in its gills.
  • There, they check to see if the mouth is unoccupied.
  • If so, they move in and bite down on the fish’s tongue!
  • Well, technically fish don’t have true tongues.
  • They have a bony structure called a basihyal,
  • but “basihyal-biting isopod” is not nearly as catchy.
  • Anyway, once the male parasite has affixed itself,
  • it transforms into a much larger adult female!
  • And that’s where the isopod lives its life,
  • sucking nutrients from the fish’s bloodstream
  • through the bottom of its mouth.
  • Meanwhile, any other males who show up
  • settle down in the gills and make only occasional trips
  • to the mouth to mate.
  • So a fish with an infection usually ends up with
  • 1 female in its mouth and as many as 5 males in its gills!
  • This may seem strange and convoluted,
  • but it is apparently a successful strategy.
  • These parasites are known to infect numerous
  • species of fish, from snappers to grunions to croakers.
  • It probably won’t surprise you to find out that having
  • a bug stuck permanently on your tongue has some side effects.
  • A 2013 study found that infected fish tend to be
  • smaller and less healthy, but not for the reason you might think.
  • Scientists expected that the infected fish would
  • have trouble eating, but that doesn’t seem to be the case.
  • Instead, they think the fish is having trouble breathing.
  • The female isopod can grow up to three centimeters,
  • taking up a good part of the volume of the fish’s mouth
  • and blocking the flow of water to its gills.
  • And the males stuck in the gills don’t help either.
  • So one nickname given to these parasites -
  • “snapper-choking isopods” - might be spot on!
  • But when it comes to making a host unhealthy,
  • this next parasite takes it to a whole new level.
  • In a 2002 study, a team of scientists noticed that
  • at certain sites in northern California there were unusually
  • high numbers of amphibians with deformed legs.
  • They spotted frogs and salamanders with extra legs,
  • legs that hadn’t formed properly, or even legs that were missing entirely.
  • The scientists wondered if pollution was to blame,
  • but a closer look revealed that the deformities
  • correlated not with pollutants, but with worms.
  • The ponds with the most malformed amphibians were
  • also harboring an infection of trematodes,
  • a type of flatworm, particularly one named Ribeiroia.
  • These little parasites have a complicated life cycle
  • that involves three hosts at different stages of the worms’ lives.
  • They start out as eggs inside the poop of birds or mammals.
  • When that poop ends up in water, the eggs hatch
  • and little swimming larvae infect snails,
  • reproducing asexually to create hundreds more of themselves.
  • These swarms of worms emerge from the snails
  • and swim around to find a tadpole or young salamander,
  • where they tend to infect the skin near the developing limb buds.
  • Larval amphibians like tadpoles start their lives
  • limbless and grow legs when they metamorphose into adults.
  • But if they’re infected, the parasites get in the way
  • of this development and the legs come out all wrong!
  • The study found a clear correlation:
  • the ponds with the highest Ribeiroia numbers
  • had the most leg deformities.
  • In the most extreme cases, the researchers saw
  • 2 tree frogs that had each grown 4 extra legs.
  • Unfortunately, these infections appear
  • to be increasing in recent years.
  • Scientists worry that as humans have been
  • modifying these habitats, the worms are doing
  • better at the amphibians’ expense.
  • They found, for example more mutated amphibians
  • in artificially-dammed ponds compared to
  • natural bodies of water.
  • These deformities might not just be a side effect
  • of the infestation, either -- some scientists think
  • they might be a key step in the worm’s life cycle.
  • See, the third and final stage of the worms’ life
  • needs to happen inside the body of a bird or mammal,
  • and to get there, they need to get eaten.
  • By crippling their hosts, the Ribeiroia might also
  • render them far less likely to successfully escape a predator.
  • It’s an amazing - and gruesome - life strategy.
  • But Ribeiroia worms aren’t the only parasites
  • that force their hosts to become dinner!
  • Another genus of trematodes, Leucochloridium,
  • invades the eyestalks of snails, turning each eye
  • into a pulsating broodsac that looks like a juicy caterpillar,
  • ready to be snapped up by the parasite’s next host: birds!
  • The parasite even somehow forces the snails
  • to be more active and stay out in the open
  • where they’re easier for predators to spot.
  • This phenomenon - parasites affecting the
  • behavior of their hosts - is called host manipulation.
  • And while the worms are good at it,
  • the next parasite on our list is a
  • macabre master manipulator.
  • Allow me to introduce you to the parasitic barnacles
  • that take over a crab’s body and force it to serve them.
  • That may sound creepy enough for you, but trust me,
  • it’s so much worse than you’re imagining.
  • These creatures belong to a group called
  • the Rhizocephala, and they start out life like most barnacles.
  • Mom gives birth to a bunch of little larvae,
  • which spend some time swimming through the sea
  • until one day they search for a place to settle.
  • But while most barnacles head for a nice solid surface
  • like the pier at your local marina or whatever,
  • parasitic barnacles seek out host bodies.
  • Females look to settle on crabs or similar crustaceans.
  • When they find a suitable host, they burrow inside
  • and develop into their parasitic adult form.
  • Soon, a sac-shaped barnacle body emerges
  • out of the crab’s exoskeleton like a horrible pimple.
  • This blister-like part of the parasite is called the externa,
  • and it’s basically just a chamber for the ovary
  • and developing eggs.
  • The externa also has two little receptacles
  • on the outside, which are for sex… kind of.
  • While the female larvae infect a host,
  • the male larvae seek to attach to those receptacles
  • on an implanted female.
  • When a male has successfully attached itself,
  • it shrivels into a very tiny, very simple adult form.
  • And that’s where the male lives,
  • essentially serving as just like a thing to produce
  • sperm for the female to produce babies.
  • But those babies need nutrients,
  • and the nutrients come from the crab.
  • The externa of the parasite has no guts and no mouth,
  • but it is attached to the interna,
  • which is a system of roots that infiltrates the body of the host.
  • The first written report of this root-structure,
  • from 1858, came from a scientist who spotted it
  • in an infected hermit crab, and described it as quote:
  • “an innumerable quantity of copper-coloured tubules,
  • which ramify through the whole body.”
  • Horrifying. But wait: it gets worse.
  • You may think the crab would be quick to ditch this
  • parasitic hitchhiker, but instead it takes care of it!
  • Why? Because the barnacle is telling it to.
  • See, the parasite-pimple doesn’t just pop up
  • anywhere on the crab’s body.
  • It emerges in the host’s brood chamber,
  • in the spot where the crab would
  • normally carry its eggs.
  • In some species, this is under the belly.
  • In others, it’s in a special brood pouch.
  • The parasite is essentially mimicking a clutch of eggs.
  • And those roots aren’t just going to the crab’s guts for food,
  • but also to its nervous system,
  • where the barnacle uses a variety of chemical signals
  • to stoke all the crab’s most parental tendencies.
  • The crab could easily destroy the parasite,
  • but instead treats it like a brood of its own eggs,
  • grooming and protecting it.
  • The changes that the parasite
  • forces on the host are incredible.
  • The host’s genitalia degenerate,
  • leaving it functionally sterilized.
  • This prevents the parasite from encountering
  • any competition for the host’s parental care.
  • The barnacle also interrupts the crab’s moulting cycle,
  • probably to prevent the parasite from falling off during a shed.
  • And if the host crab is male, the barnacle stirs
  • its most maternal instincts …
  • by literally making it more female.
  • A male host’s hormones are hijacked so that
  • its size, shape, and behavior become more like females,
  • since they are the ones that brood young.
  • In some cases, the host’s testes
  • will actually convert to ovaries!
  • All of this ensures that the host will take
  • the best possible care of its parasitic overlord.
  • And when the barnacle’s babies are ready to face the world,
  • the crab host flaps its abdomen,
  • a behavior meant to help its own
  • hatching young spread into the water.
  • Instead, it helps the parasite’s larvae start
  • on their own journey to find the next generation
  • of poor, unfortunate crabs.
  • So far, our list has focused on animals,
  • but if you take a trip to the islands of southeast Asia,
  • you might spot one of the world’s biggest
  • and most beautiful parasites.
  • Rafflesia can be more than a meter across,
  • making them the largest flowers on Earth.
  • They’re so impressive that they’ve appeared on
  • stamps and currency, been named a national flower of Indonesia,
  • and even inspired the design of a Pokemon!
  • But these are no normal flowers.
  • Get up close and you’ll notice that they smell
  • strongly of rotten flesh, a characteristic odor
  • that’s earned them the name corpse flowers.
  • And if you peek beneath the flower,
  • you’ll see there’s no stem, no roots, and no leaves.
  • Instead, a narrow strand of cells infiltrates
  • the body of a grapevine,
  • not unlike the roots of those parasitic barnacles.
  • The corpse flower is stealing nutrients from the vine,
  • using them to grow big and beautiful.
  • Grapevines tend to collect lots of water,
  • making them like living canteens for Rafflesia.
  • Because it steals sustenance,
  • the corpse flower doesn’t have to worry
  • about producing its own.
  • Unlike most plants, it apparently has no chloroplasts,
  • meaning it can’t photosynthesize at all.
  • This makes it an obligate parasite,
  • or one that couldn’t survive without a host.
  • But one thing it does have in common with
  • other flowers is that it spreads by pollination.
  • That rotten flesh smell attracts carrion flies,
  • which come looking for meat and leave
  • covered in corpse flower pollen.
  • So the flower not only steals from grapevines,
  • but tricks flies into working for it for free --
  • no meat for them!
  • But Rafflesia has another trick that
  • might make it the strangest parasite on this list.
  • It’s not just stealing nutrients from the grapevine -
  • it also appears to steal DNA!
  • Research has found that a significant portion
  • of the corpse flower’s DNA has been
  • swiped from grapevines.
  • This is a form of horizontal gene transfer,
  • where genes hop between two distantly related species
  • instead of being inherited in the usual way.
  • Scientists aren’t totally sure why the corpse flower
  • is a DNA thief, but they suspect it might allow the flower
  • to better infiltrate and manipulate the host.
  • It’s gaining entry to the grapevines by using
  • their own genetic tricks against them.
  • From tongue-biting to gene-stealing,
  • parasites have evolved an incredible variety
  • of methods for mooching off others.
  • That’s made them part of nearly every
  • ecosystem on Earth --
  • maybe even the ecosystem inside you!
  • And now, if zombie crab parasites have you grossed out,
  • we would like to propose an antidote.
  • The natural world can be pretty gross,
  • but it is also beautiful.
  • And the mathematical laws underlying it all
  • give it that elegance.
  • What better way to appreciate them than
  • with a course on the beauty of math?
  • Brilliant.org just released a new course called
  • “Beautiful Geometry”, which will take you on
  • an adventure through some of the most beautiful topics in math,
  • from tiling shapes to origami folding.
  • And that’s not all Brilliant has to offer.
  • With over 50 courses in science, engineering,
  • computer science and math,
  • they’ve got your curiosity covered.
  • Their hands-on, interactive courses will help you
  • sharpen your math and science skills.
  • And right now, the first 200 people to sign up
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  • So thanks for checking it out --
  • and thanks for supporting us.
  • [♪ OUTRO]

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Description

Warning! This episode is a little gross… Between 1/3 and 1/2 of all life on earth is parasitic - and here are 4 of the creepiest parasites we know about.

Go to http://Brilliant.org/SciShow to try their Beautiful Geometry course. The first 200 subscribers get 20% off an annual Premium subscription.

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Sources:
https://www.otago.ac.nz/parasitegroup/PDF%20papers/Poulin2014-IJP.pdf
https://www.researchgate.net/publication/300095919_Effect_of_the_parasitic_isopod_Anilocra_apogonae_Cymothoidae_on_the_growth_condition_reproduction_and_survival_of_cardinal_fish_Apogonidae
https://repositories.lib.utexas.edu/handle/2152/ETD-UT-2012-08-6285
https://www.researchgate.net/profile/Denham_Parker/publication/257377303_The_tongue-replacing_isopod_Cymothoa_borbonica_reduces_the_growth_of_largespot_pompano_Trachinotus_botla/links/0c960533d44dfbd0d1000000.pdf
http://aquaticcommons.org/8583/1/NG_62_2003_Williams&Bunkley-Williams_Fish-parasitic_isopods.pdf
https://www.researchgate.net/publication/319913453_First_record_of_the_snapper-choking_isopod_Cymothoa_excisa_Isopoda_Cymothoidae_parasitizing_invasive_lionfish_Pterois_volitans_Scorpaeniformes_Scorpaenidae
http://people.oregonstate.edu/~blaustea/pdfs/JohnsonBlausteinEcoMonographs.pdf
https://www.researchgate.net/publication/8211765_Review_of_the_Trematode_Genus_Ribeiroia_Psilostomidae_Ecology_Life_History_and_Pathogenesis_with_Special_Emphasis_on_the_Amphibian_Malformation_Problem
https://zslpublications.onlinelibrary.wiley.com/doi/full/10.1111/jzo.12094https://www.cambridge.org/core/journals/journal-of-the-marine-biological-association-of-the-united-kingdom/article/biology-and-life-cycle-of-the-rhizocephala-cirripedia/11C2E47CABCABB175F4C49B1393BE125
https://www.sciencedirect.com/science/article/pii/S138511011530037X
https://nph.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1469-8137.2002.00396.x
https://www.harvardmagazine.com/2017/03/colossal-blossom
https://bulbapedia.bulbagarden.net/wiki/Vileplume_(Pok%C3%A9mon)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3460754/

Image Sources:
https://commons.wikimedia.org/wiki/File:Amphiprion_clarkii_-_Cymothoa_exigua_(28999213532).jpg
https://commons.wikimedia.org/wiki/File:Cymothoa_exigua_parassita_Lithognathus_mormyrus.JPG

https://commons.wikimedia.org/wiki/File:Annual_report_of_the_Game_and_Fisheries_Department_of_Ontario,_1921-34%22_(19176931660).jpg
https://commons.wikimedia.org/wiki/File:P._regilla_with_parasite-induced_limb_malformation.png
https://commons.wikimedia.org/wiki/File:Leucochloridium.ogv
https://commons.wikimedia.org/wiki/File:Three-legged_frog_(Rana_temporaria)_(8137866910).jpg
https://commons.wikimedia.org/wiki/File:Greenfrog_life_stages.svg

https://commons.wikimedia.org/wiki/File:Sacculina_carcini.jpg
https://www.eurekalert.org/multimedia/pub/144794.php?from=363929
https://commons.wikimedia.org/wiki/File:Haeckel_Sacculina.jpg
https://commons.wikimedia.org/wiki/File:Wurzelkrebs-drawing.jpg
https://commons.wikimedia.org/wiki/File:Xantho_poressa_2009_G1.jpg

https://commons.wikimedia.org/wiki/File:Ah,_Rafflesia!_(11967075415).jpg
https://commons.wikimedia.org/wiki/File:Rafflesia_zollingeriana_Kds.jpg
https://commons.wikimedia.org/wiki/File:Rafflesia-Bud_on_Liana.jpg
https://commons.wikimedia.org/wiki/File:This_Rafflesia_is_already_showing_signs_of_wilting_(11967407133).jpg
https://commons.wikimedia.org/wiki/File:Stamps_of_Indonesia,_029-08.jpg
https://commons.wikimedia.org/wiki/File:Rafflesia_arnoldii_cycle.jpg