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  • 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

  • betweenand 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 calledtongue-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,

  • butbasihyal-biting isopodis 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 sexkind 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!