Welcome to Brainstuff, a production of iHeartRadio, Hey brain Stuff Lauren Vogel bomb here. By one estimate, somewhere around forty percent of all known animal species are parasitic, from tapeworms that grow in fish up to thirty feet that's nine meters in length, to a cough drop sized crustacean that drinks shrimp blood to survive. A planet Earth is crawling with parasites. Many of them have evolved to find very
specific hosts. A take a louse that happens to be named s Gary Larsnai the after cartoonist Gary Larsen, who created The Far Side. This louse spends its entire life cycle on the skin of an unsuspecting owl, where the stowaway feeds on feathers and other organic materials. No other animals are known to harbor this particular kind of louse. But sometimes one host isn't enough. Sometimes the only way for a parasite to reproduce and complete its own life
cycle is by passing through multiple carriers. Such is the case of the banded brood sec, which is a genus of worm by the name of Leucochloridium that's been accused of turning snails into zombies. This behavior is said to be part of an elaborate scheme that also involves hungry birds and their poop. Supposedly, if things go according to plan, the worm's plan, that is, those poor snails get their eyes pecked out and banded brood secks aren't just weird,
they're flukes. Literally. Flukes, also known as trematodes, are flatworms that use suckers to grab hold of various objects. There are around eighteen to twenty four thousand different species. All of them are parasitic, and most have complex life cycles that depend on different host species at different times. Usually, the parasites spend at least part of their lives investing some kind of mollusk, that is, the spineless animal group
whose membership includes octopuses, muscles, and yes, snails. Depending on the species, a fluke might shack up inside the host's kidneys, digestive structures, or even as reproductive organs. As snails are a common target for trematodes, and without them, the zombifying banded brood sacks simply couldn't procreate as adults. They are long flat worms that infest bug eating birds. Their specific habitat of choice is the bird's cloaca, which is the
orifice through which birds both poop and reproduce. Don't bother judging them before it dies, A grown banded broodsack may spend weeks or months living inside its avian host. The timeline isn't quite clear. At some point, though, the parasites lay their eggs, which get pooped out by the bird and you know what eats a lot of bird droppings, ground dwelling snails. If the right kind of snail happens to gorge itself on feces laced with the flukeat eggs,
things get a little bit surreal. After a target snail gobbles the eggs up, they'll hatch into clear bodied newborns. In the next phase of their development, the sphoresist stage, the little guys may develop their titular brood sacks. These sacks are pulsating, colorfully banded tubes that are jam packed with larvae, and they look sort of like wiggly little caterpillars. Maybe they're supposed to. The thing about these brood sacks is they don't pop up just anywhere on the snail's body.
The snails view the world through light sensitive eye spots. Each one is located on the tip of a tentacle or eyestalk connected to these snail's head. A healthy snail can withdraw its tentacles and pull them back into its head whenever it likes. You may have noticed this yourself if you've ever picked one up. But when a snail gets infected with these flatworms, the eye stalks become hampered. The fluke's swelling brood sacks invade the tentacles, which prevents
the snail from retracting them. Then, adding insult to inconvenience, the sacks start to pulsate. They expand and contract in a sort of dance. They can pulsate dozens of times per minute, and their color schemes are eye catching bands and speckles in shades of green, orange, yellow, white, black, or brown. Thanks to the snail's ultra thin skin, the entire show is clearly visible and sort of like an
extremely slow rave. There could be an evolutionary method to this madness, though since the early eighteen hundreds, naturalists have wondered if this performance is just a ploy designed to trick birds into mistaking these brood sacks for juicy little caterpillars. Any bird that plucked one off of a snail would get a mouthful of larvae ready to make a beeline for its Cloaca grow up into adult flukes and begin the cycle anew. But okay, we mentioned zombies. Here's where
that comes in. During the nineteen twenties and thirties, a few scientists proposed that banded brood sacks actively manipulate the way that snails behave. The parasites allegedly force their hosts to deviate from their normal routine. Influenced by the flukes, the hapless snails are driven into exposed and well lit areas like leaftops, high up off the ground. Once they're in the open, the snails make easy targets. The caterpillar loving birds see the dancing sphoresists and hungrily rip them
out along with the ice talks. Or so goes the hypothesis. The trouble is field researchers have never seen this happen in the wild. Experiments conducted in eighteen seventy four did find that captive birds were more than happy to attack the throbbing sporocysts of infected snails, But that doesn't prove that the same thing occurs in nature. Some animals have been known to change their habits in captivity. After all,
research is actually ongoing. But when all said and done, there's still a lot we don't know about the relationship between the flukes and their hosts. If these parasites really do influence the snails, which seems likely, how the heck do they do it? And do the brood secks really fool wild birds into thinking that they're caterpillars? If not, then how to adult flukes find their way into a feathered host's kloeca. Maybe we'll have clear answers someday. In
the meantime, we certainly have some nightmare fuel. Today's episode is based on the article do these nightmare parasites hack snail brains to survive? On how stuffworks dot Com? Written by Mark Nancini. Brain Stuff is production of by Heart Radio in partnership with how stuffworks dot Com and is produced by Tyler Klang. Four more podcasts my Heart Radio, visit the iheartrak a you ap Apple Podcasts, or wherever you listen to your favorite shows.