Can Parasitic Worms Control Snails' Minds?

A genus of flatworms called banded broodsacs have a lifecycle that seems to involve purposefully getting eaten first by snails and then by birds, using a combination of biomimicry and biohacking. Learn about Leucochloridium worms in this episode of BrainStuff, based on this article: https://animals.howstuffworks.com/insects/parasitic-worms-snails.htm

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Episode Transcript

Speaker 1 (00:01):
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

(00:22):
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

(00:44):
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

(01:05):
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

(01:26):
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

(01:49):
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

(02:13):
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

(02:34):
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

(02:57):
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

(03:17):
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.

(03:39):
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.

(04:00):
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

(04:21):
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

(04:42):
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

(05:04):
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

(05:25):
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

(05:50):
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

(06:12):
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

(06:36):
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,

(06:59):
visit the iheartrak a you ap Apple Podcasts, or wherever
you listen to your favorite shows.