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WARNING! This episode will freak you out. Did you know that almost half of all life on earth is parasitic? Join us for an eerie video hosted by Hank Green and learn about four of the creepiest parasites on Earth.

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Hosted by: Hank Green

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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 engagement. 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? 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.

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[♪ OUTRO].