You know liver flukes? Those leaf-shaped worms that can infect your liver if you drink bad water or eat undercooked freshwater fish?
Source: Wikipedia
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| Source: CDC |
Those creatures have a pretty complex lifecycle to exploit two kinds of hosts, as shown above.
- Eggs hatch in water into "miracidia". Miracidia are little swimmers.
- Miracidia find a snail and somehow get into the snail. The two popular methods are being eaten, and tunneling through the skin of snail.
- Miracidia develops into sporocysts. Sporocysts are basically egg bags.
- Little eggs in sporocysts develop into rediae. Rediae are basically hungry hungry little worms. They not only eat food from the snail's body, but eat other parasites too, to reduce the competition.
- Rediae give birth to cercariae, which look like tadpoles. They get out of the snail somehow (by pooping or by tunneling through), and swim to the shore.
- Cercariae climbs onto a plant and builds a hard, waterproof shell called a metacercariae. It hibernates there until it's eaten, usually by a cow or sheep.
- The metacercariae bursts out of the shell, tunnels through the digestive tract, and somehow finds the liver. At this stage, they are called "flukes", and they can cause all kinds of nasty diseases.
- The flukes grow up and have sex in the bile duct, then the female flukes lay eggs in the river of bile. The eggs would be washed into the intestine, then pooped out, starting the cycle again.
Parasites are known to have very complex life cycles, some involving 4 hosts (Life cycle evolution in the digenea: a new perspective from phylogeny, 2003)! It's entirely unclear why they evolved complex life cycles. Some possibilities include
- Access to more resources: by passing through both fish and cattle, the parasite can exploit both water-food and grass-food.
- A bigger dating scene: In some parasites, they get into the next host by being eaten, until they reach an apex predator, where they are finally ready to have sex. Just like how mercury from all over the world concentrates in apex predators, so do parasites. And a bigger dating scene means more genetic diversity, which keeps the species strong. (How a complex life cycle can improve a parasite’s sex life, 2005) Sounds like why some humans go to college.
How would you find the liver, if you were a fluke?
Sukhdeo proposed that, for animals of earth, there are three kinds of environments: aquatic, terrestrial, and inside-the-body.
- Aquatic: surrounded by water, hard to see far, gravity not very important.
- Terrestrial: surrounded by air and hard ground, gravity extremely important, easy to see far (except in jungles).
- Inside-the-body: mostly inhabited by parasites.
In aquatic and terrestrial environments, to find a distant target, one usually follow the scent floating in water or air (or on the ground, if you are a hound).
However this can't work inside the body. If you are inside a body cavity, it would smell the same, kind of like you can't smell your way towards the farter on a plane, because it smells equally stinky everywhere.
From (Sukhdeo, 1997):
Orientation up or down chemical gradients is a strategy that is well known in free-living environments (see Fraenkel and Gunn 1961), but this strategy may not work in the parasite's world... First, the abdominal cavity is completely filled with intestines and mesenteries, which, because of the normal processes of digestion and propulsion of ingesta through the guts, are in a constant state of motion. The turbulence from this mechanical mixing in the abdominal cavity makes it difficult to establish a consistent chemical gradient. Second, gradients require an open end, or a sink of some sort, to prevent gradient saturation. Saturation evens out the difference between high and low intensities, effectively eliminating the gradient.
Instead, parasites use action scripts to navigate, because the bodies of individuals in a species are so similar.
The parasite's environment is also topologically identical in every member of the host species. This means, for example, that a gastrointestinal parasite being passively propelled through the gut will pass through the mouth, the esophagus, the stomach, the small intestine, and so on, always in exactly this order. Moreover, each region or area will have its own predictable set of con- ditions. Some intraspecific variation is expected among individual hosts, but to a lungworm crawling in the abdominal cavity, for example, the lungs are always located on the other side of the diaphragm.
A sketch: Parasite Simulator
You might have played Goat Simulator, but it's not actually accurate.
The main inaccuracy is that it does not accurately simulate what a goat can feel. Nogoat ever sees themself from the back. The viewpoint is wrong.
A first-goat-view would help, but even that isn't good enough. If you take a close look at goat eyes, you can see that they have those 一 shaped pupils.
Besides, goat eyes are on the sides of the head, not in front. So an accurate goat-vision simulator should be super-wide screen, giving something like a 300-degree vision, with screen ratio 10:1.
Fortunately goats do have color-vision similar to humans, so the color of the goat-vision can look the same as human-vision. Some goats can see two colors (like mildly colorblind humans), and some three (like most humans).
Source: Fedigan et al. 2014
Of course, for a full-goat-experience, one should also take care to make accurate soundscape. Fortunately, goat hearing is also similar to human hearing. Their hearing range is 78 Hz - 37 kHz, most sensitive at 2 kHz (Heffner and Heffner, 1990).
Okay, so that's my critique of Goat Simulator. If you speak the jargon, I was just saying that a real goat simulator must make sure to replicate the umwelt of a goat.
With that in mind, what should a Liver Fluke Simulator be like?
The answer, surprisingly enough, is a visual novel in several chapters. Just like a visual novel, the life of a liver fluke is completely scripted. Almost everything is scripted. It's a long sequence of fixed action pattern (FAP) triggered by fixed events.
Of course, a real liver fluke has no brainpower to generate all the monologues and psychological drama essential in a visual novel, but the visual novel is a great metaphor for the umwelt of a liver fluke. And who knows, maybe the the liver fluke really has a complex inner life (I don't believe it, but it's a fun thought), except that the complex inner life is entirely scripted, much as how humans, for all their complex thought decisions and "free will", are scripted to decide to keep living.
The Hero's Journey: The liver fluke (visual novel)
Disclaimer: this story is humanized version of the life-cycle of a generic liver fluke. I base this mainly on the description in Parasite Rex (2001), itself based mainly on Schistosomes: Development, reproduction, and host relations (1992).
Chapter 1: Miracidia
In this chapter, our hero (miracidia) hatches from an egg in water. The hero is humanoid, but covered with hairs that beat rhythmically, allowing fast swimming. The hero swims fervently all day, until they (at this point in the story, the hero has no sex) encounter a soft, moist titan. The hero pulls out a great drill and burrows deep.
The hero falls into a massive blue river (the snail's blood), and starts swimming until they reached a calm place, where they built a small house.
Chapter 2: Sporocyst
In this chapter, an ancient song awoke in our hero's head, and they remembers their destiny. The hero starts dredging the blue lakes for all kinds of material, and transforms those into tools and building blocks, until they have made a massive factory. The hero walks into the central office of the factory, and locks the door. The great factory (sporocyst) comes alive, constructing thousands kid-adventurers (redia) who walks out of the factory, ready to continue the destiny. The hero dies from exhaustion inside the inner sanctum.
Chapter 3: Redia
Our viewpoint shifts to one of the kid-adventurers. They (sex won't appear until chapter 6!) are armed with a small sword, a grapple hook. Looks kinda like Link from the Zelda series. They wander through the body of the titan, gathering food from the blue rivers, and slowly grow stronger.
One day, the adventurer wakes up to find some black, worm-shaped monster (rival parasite infection) outside their camp. They draws the sword and stabs the monster in the heart sevenfold, then cooks the monster for food (parasite competition inside the snail).
Chapter 4: Cercaria
Years passed, and the adventurer has grown old, waiting for a sign. Finally, the sign has come from the air and the water. They pushed the sword in the ground and leaned hard on the sword, turning into a peach tree.
On the tree, a hundred peaches mature, falling into the river. These peaches split open, revealing little tadpoles armed with... drills again. They swam until they reached the surface vessels of the titan, and pushed their drills into it.
Several hours of hard drilling later, in the deep darkness, our hero contemplates about the circle of life, about the outer darkness where no life can penetrate, and the ancient song in the head, guiding their destiny. Just as the hero is losing hope of ever reaching the other side, the darkness breaks.
Chapter 5: Metacercaria
The hero struggles to the great green waters outside, floats to the surface, and swims fervently in one direction, until they bumps into something tall and stalky. "The legendary tower of heaven!" the hero smiles, and climbs up, using the grapple hook.
The hero climbs until the tower has tapered to no more than three embraces in circumference. At that point, they open up their backpack and slathers a tube of glue on the tower, then inflates a resin balloon, attached to the glue, then hides into the balloon, and enters hibernation.
Chapter 6: Adolesent fluke
"The deliverance at the tower of heaven..." mutters the hero, as a great earthquake awakens them. Everything is black, and the hero pulls out their sword, waiting for the sign.
The balloon falls violently into a reedy lake (first stomach of a cattle), and the hero waits. The dark lake felt oppressive, menacing (because it's acidic), but the hero is calm, trusting the resin shell would protect them. The hero goes back to sleep and dreams of a great country, with red earth and red skies.
The hero is awakened by a horrible stench (the smell of bile). The hero starts slashing against the balloon with their great sword. Bit by bit, the resin prison falls apart, and the hero emerges.
The hero has been carried by the current into what looked like a long, doomed cave, filled with the stench. The hero leaves behind their shattered resin chamber, and pushes the sword deep into the cave wall, their mind concentrated on just one thing: escape from this stench. Stab by stab, the hero tunnels through, until they emerges on the other side.
The hero stands unsteadily on the outer wall of the tunnel, and comes face to face with a red cliff (inner wall of the abdomen). The stench is gone, and they smiles. The promised land is near.
With their grapplehook, the hero journeys across the red cliff in a line as straight as they could manage, passing by many strange and wonderful places. The hero ignored them all, for they were not the promised land.
Chapter 7: Adult fluke
Many days later, the hero, thirsty and hungry, comes upon a land of red earth and red skies (the liver), and they rejoices, for they have reached the promised land. The time of strife is behind and the time of rest is ahead.
At this point, the hero finally found out their sex . Male/Female. This is literally the only point where you get to choose in the story.
The hero walks across the land of red and builds a house (a cyst) there, and lives a quiet life, until one day a male of her kind meets her, and it was love at first sight.
(Liver flukes don't do courtship. As soon as two different-sex flukes meet, the female would hug the male forever, locking into a posture that wouldn't change even if one side dies. It's extremely monogamous.)
The last step in the ancient song plays in their minds, and they journey through the red rivers (blood) again, with the male carrying the female on his back. As they swim through the river, they drink from the river, and their bodies change to become sexually mature.
After three cycles though the house of thunder (the heart), they finally reached their destination, the Nupitial Chamber (different for different flukes, but usually somewhere along the intestine). There they settle in a new house near a branch of the red river.
The years pass, and the couple has sex everyday, drinks from the red river, and once in a while, the female lays eggs in the river. The pair eventually dies, still hugging each other.
Most of the eggs are carried by the red river back into the intestines, where the eggs start glowing and sink through the intestine walls (the eggs cause an immune response that locally inflames the intestine walls, allowing them to fall in). Then the eggs are carried through the intestines and pooped to the outside world.
(Yeah, I'm tired of humanizing parasite life-history...)
The end
Get ready for the thrilling companion piece, The Hero's Journey: Tapeworm!
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