Thanks to Brilliant for supporting this whole
week of SciShow! Go to Brilliant.org/SciShow to learn more. [♪ 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
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