Speaker 1: Welcome to Brainstuff, a production of iHeartRadio, Hey brain Stuff
Speaker 1: Lauren Vogel bomb here. By one estimate, somewhere around forty
Speaker 1: percent of all known animal species are parasitic, from tapeworms
Speaker 1: that grow in fish up to thirty feet that's nine
Speaker 1: meters in length, to a cough drop sized crustacean that
Speaker 1: drinks shrimp blood to survive. A planet Earth is crawling
Speaker 1: with parasites. Many of them have evolved to find very
Speaker 1: specific hosts. A take a louse that happens to be
Speaker 1: named s Gary Larsnai the after cartoonist Gary Larsen, who
Speaker 1: created The Far Side. This louse spends its entire life
Speaker 1: cycle on the skin of an unsuspecting owl, where the
Speaker 1: stowaway feeds on feathers and other organic materials. No other
Speaker 1: animals are known to harbor this particular kind of louse.
Speaker 1: But sometimes one host isn't enough. Sometimes the only way
Speaker 1: for a parasite to reproduce and complete its own life
Speaker 1: cycle is by passing through multiple carriers. Such is the
Speaker 1: case of the banded brood sec, which is a genus
Speaker 1: of worm by the name of Leucochloridium that's been accused
Speaker 1: of turning snails into zombies. This behavior is said to
Speaker 1: be part of an elaborate scheme that also involves hungry
Speaker 1: birds and their poop. Supposedly, if things go according to plan,
Speaker 1: the worm's plan, that is, those poor snails get their
Speaker 1: eyes pecked out and banded brood secks aren't just weird,
Speaker 1: they're flukes. Literally. Flukes, also known as trematodes, are flatworms
Speaker 1: that use suckers to grab hold of various objects. There
Speaker 1: are around eighteen to twenty four thousand different species. All
Speaker 1: of them are parasitic, and most have complex life cycles
Speaker 1: that depend on different host species at different times. Usually,
Speaker 1: the parasites spend at least part of their lives investing
Speaker 1: some kind of mollusk, that is, the spineless animal group
Speaker 1: whose membership includes octopuses, muscles, and yes, snails. Depending on
Speaker 1: the species, a fluke might shack up inside the host's kidneys,
Speaker 1: digestive structures, or even as reproductive organs. As snails are
Speaker 1: a common target for trematodes, and without them, the zombifying
Speaker 1: banded brood sacks simply couldn't procreate as adults. They are
Speaker 1: long flat worms that infest bug eating birds. Their specific
Speaker 1: habitat of choice is the bird's cloaca, which is the
Speaker 1: orifice through which birds both poop and reproduce. Don't bother
Speaker 1: judging them before it dies, A grown banded broodsack may
Speaker 1: spend weeks or months living inside its avian host. The
Speaker 1: timeline isn't quite clear. At some point, though, the parasites
Speaker 1: lay their eggs, which get pooped out by the bird
Speaker 1: and you know what eats a lot of bird droppings,
Speaker 1: ground dwelling snails. If the right kind of snail happens
Speaker 1: to gorge itself on feces laced with the flukeat eggs,
Speaker 1: things get a little bit surreal. After a target snail
Speaker 1: gobbles the eggs up, they'll hatch into clear bodied newborns.
Speaker 1: In the next phase of their development, the sphoresist stage,
Speaker 1: the little guys may develop their titular brood sacks. These
Speaker 1: sacks are pulsating, colorfully banded tubes that are jam packed
Speaker 1: with larvae, and they look sort of like wiggly little caterpillars.
Speaker 1: Maybe they're supposed to. The thing about these brood sacks
Speaker 1: is they don't pop up just anywhere on the snail's body.
Speaker 1: The snails view the world through light sensitive eye spots.
Speaker 1: Each one is located on the tip of a tentacle
Speaker 1: or eyestalk connected to these snail's head. A healthy snail
Speaker 1: can withdraw its tentacles and pull them back into its
Speaker 1: head whenever it likes. You may have noticed this yourself
Speaker 1: if you've ever picked one up. But when a snail
Speaker 1: gets infected with these flatworms, the eye stalks become hampered.
Speaker 1: The fluke's swelling brood sacks invade the tentacles, which prevents
Speaker 1: the snail from retracting them. Then, adding insult to inconvenience,
Speaker 1: the sacks start to pulsate. They expand and contract in
Speaker 1: a sort of dance. They can pulsate dozens of times
Speaker 1: per minute, and their color schemes are eye catching bands
Speaker 1: and speckles in shades of green, orange, yellow, white, black,
Speaker 1: or brown. Thanks to the snail's ultra thin skin, the
Speaker 1: entire show is clearly visible and sort of like an
Speaker 1: extremely slow rave. There could be an evolutionary method to
Speaker 1: this madness, though since the early eighteen hundreds, naturalists have
Speaker 1: wondered if this performance is just a ploy designed to
Speaker 1: trick birds into mistaking these brood sacks for juicy little caterpillars.
Speaker 1: Any bird that plucked one off of a snail would
Speaker 1: get a mouthful of larvae ready to make a beeline
Speaker 1: for its Cloaca grow up into adult flukes and begin
Speaker 1: the cycle anew. But okay, we mentioned zombies. Here's where
Speaker 1: that comes in. During the nineteen twenties and thirties, a
Speaker 1: few scientists proposed that banded brood sacks actively manipulate the
Speaker 1: way that snails behave. The parasites allegedly force their hosts
Speaker 1: to deviate from their normal routine. Influenced by the flukes,
Speaker 1: the hapless snails are driven into exposed and well lit
Speaker 1: areas like leaftops, high up off the ground. Once they're
Speaker 1: in the open, the snails make easy targets. The caterpillar
Speaker 1: loving birds see the dancing sphoresists and hungrily rip them
Speaker 1: out along with the ice talks. Or so goes the hypothesis.
Speaker 1: The trouble is field researchers have never seen this happen
Speaker 1: in the wild. Experiments conducted in eighteen seventy four did
Speaker 1: find that captive birds were more than happy to attack
Speaker 1: the throbbing sporocysts of infected snails, But that doesn't prove
Speaker 1: that the same thing occurs in nature. Some animals have
Speaker 1: been known to change their habits in captivity. After all,
Speaker 1: research is actually ongoing. But when all said and done,
Speaker 1: there's still a lot we don't know about the relationship
Speaker 1: between the flukes and their hosts. If these parasites really
Speaker 1: do influence the snails, which seems likely, how the heck
Speaker 1: do they do it? And do the brood secks really
Speaker 1: fool wild birds into thinking that they're caterpillars? If not,
Speaker 1: then how to adult flukes find their way into a
Speaker 1: feathered host's kloeca. Maybe we'll have clear answers someday. In
Speaker 1: the meantime, we certainly have some nightmare fuel. Today's episode
Speaker 1: is based on the article do these nightmare parasites hack
Speaker 1: snail brains to survive? On how stuffworks dot Com? Written
Speaker 1: by Mark Nancini. Brain Stuff is production of by Heart
Speaker 1: Radio in partnership with how stuffworks dot Com and is
Speaker 1: produced by Tyler Klang. Four more podcasts my Heart Radio,
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