Meet a Real Life Body-Snatching Parasite
This is the parasitic flatworm, Curtuteria australis. He lives in three separate hosts during his convoluted lifecycle. Best of all, he's a card-carrying body-snatcher who drives his host to suicide.
Like most worm species, Curtuteria is a hermaphrodite, with both male and female sex organs. This suits Curtuteria better when there are few potential mates around, which is quite a lot when you're a highly mobile internal parasite.
In a life that can last just a few weeks, Curtuteria australis must sequentially infiltrate the bodies of two molluscs and one bird out on the mudflats of New Zealand. If he fails, he'll die.
If he succeeds, he'll also die. But he'll die a smidge happier.
Part I: Hatching and Cloning
Curtuteria australis, or Curt if you will, starts life as a fluke egg in the gut of an oystercatcher bird.
When the bird defecates, Curt and his fellow eggs make a gentle landing on the coastal mudflats.
This is where the easy ride ends. Having been shat out by a bird, Curtuteria australis and his friends are about to be consumed by whelks; predatory marine snails with pointed spiral shells.
To humans, whelks are apparently delicious. Perhaps somewhat less so once you know they're riddled with parasites. To Curt, whelks are at once terrifying and appealing hosts.
Deep inside the digestive gland of the whelk, Curtuteria hatches into a larval worm. Without a moment to lose, he then goes through a type of asexual reproduction called budding.
Budding is more than a bit weird. It involves growing new body parts, then tearing them off so they can develop into entire clones. Corals, flatworms, jellyfish and sea anemones are very good at this.
When you and your clones find yourselves in the intestinal cavity of a whelk, there's really only one exit strategy.
For the second time in his short, miserable life, Curt is expelled from an anus. This time he ends up in the ocean.
This mass die-off is typical for young Curtuteria. Survival is a numbers game when you reproduce on mass and don't bother taking care of your babies.
Curt is lucky to survive as a juvenile flatworm, and now can look forward to being consumed by his next host.
Part II: Body-Snatching
Next, Curtuteria australis is siphoned up by a cockle; a bi-valve shellfish that's closely related the clam.
These marine molluscs eat and breathe by siphoning water over their gills, and filtering out the phytoplankton and oxygen. When cockles accidentally filter our body-snatching parasites, it effectively sentences them to death.
Curtuteria instinctively makes his way deep into the cockle's muscular tissue, where he can control the mollusc's movements.
But this is yet more risky business for Curt; cockle flesh tastes delicious. At least, according to the Spotty fish, who's always on the lookout for a tasty bit of protruding cockle.
Curt must take care to hide deep within the cockle's tissues or die a virgin and fail to pass on his frankly ridiculous genes.
When the Spotty fish moves on, Curt can resume his plan to convince the cockle to die in a more appropriate manner.
How? Cockles move in two ways. Underwater, they're generally carried by tides and ocean currents. And on land, they extend their muscular bodies out of their shells to burrow into the sand.
Curtuteria is about to ruin it all. He migrates deep into the muscular body and takes up residence in the cockle's foot.
Curtuteria forms hard cysts inside the cockle's foot. The muscle atrophies and shrivels, making it useless for digging into the sand.
When the cockle becomes beached at low tide, it can no longer dig itself underground to evade predation. It just lies there pitifully on the mudflats.
What's Curt thinking? Rule number one in Parasitism for Dummies is don't kill your host—isn't it?
Actually, if you're done with your present host, killing it is a very convenient way of finding your next host—by driving it into the chops of a predator.
Part III: Cross-Fertilisation and Egg Laying
The dragon is of course Curt's primary host—an oystercatcher bird. It finds the stranded cockle lying helplessly on the mudflats and feasts on the infected flesh.
Inside in the bird's gut, Curtuteria australis matures and reproduces sexually. Ideally, he achieves cross-fertilisation by teaming up with another hermaphrodite flatworm to exchange sperm and fertilise each other's eggs.
Curt then lays a fresh batch of eggs inside the bird, so the whole cycle can begin anew.
This is how the lifecycle of Curtuteria australis exists today. It's pretty hellish for all parties involved, but by god it works.
Real sneaky, evolution. Real sneaky.