Do These Nightmare Parasites Hack Snail Brains to Survive?

By: Mark Mancini  | 
zombie snails
Leucochloridium paradoxum is a parasitic flatworm (or "helminth") that uses a gastropod, or snail, as an intermediate host in order to intice its ultimate target, a bird. Henri Koskinen/Shutterstock

Look, something had to inspire those "Alien" movies.

By one estimate, somewhere around 40 percent of all known animal species are parasitic. From 30-foot (9-meter) fish tapeworms to Orthione griffenis, a "cough drop-sized" crustacean that drinks shrimp blood to survive, planet Earth is crawling with parasites.


Many of them have evolved to find very specific hosts. Take Strigiphilus garylarsoni, a "chewing louse" named after cartoonist Gary Larson (you may remember him as the creator of "The Far Side"). It spends its entire life cycle on the skin of an unsuspecting owl, where the stowaway feeds on feathers and other organic materials. Besides owls, 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 for Leucochloridium worms, biological oddities that have 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 — for the worms — those poor old snails get their eyes pecked out.


Leucochloridium Parasites Are Flukes

The Leucochloridium parasites aren't just weird; they're literally flukes.

Flukes, also known as "trematodes," are flatworms in the class Trematoda. A key trait of these creatures is the suckers they use to grab hold of various objects.


All flukes are parasitic. To quote the Animal Diversity Web database, most trematodes "have complex life cycles, with larval stages parasitizing one or more species that are different from [the hosts] of adults."

If you're a trematode, you may need to find yourself a good snail.

There are around 18,000 to 24,000 species of fluke. Usually, the parasites spend at least part of their lives infesting some kind of mollusk (the spineless animal group whose membership includes octopuses, mussels and, yes, snails).

Depending on the species, a fluke might shack up inside the host mollusk's kidneys, digestive structures or even its reproductive organs. Snails are a common target for trematodes, and without them, the "zombifying" Leucochloridium flukes simply couldn't procreate.

zombie snails
A snail infected by the parasitical Leucochloridium paradoxum worm is ripe for the picking by a bird.


Early Life Stages of the Leucochloridium

Adult Leucochloridium are long, flat parasites that infest bug-eating birds. The cloaca, the orifice through which birds poop and lay eggs, is their habitat of choice. Don't bother judging them.

Before it dies, a grown Leucochloridium may spend weeks or months living inside its avian host; the timeline isn't quite clear. At some point, the parasites lay their own eggs — which get pooped out by the birdie.


Know what eats a lot of bird droppings? Ground-dwelling snails. If the right kind of snail, usually an amber snail in the genus Succinea, happens to gorge itself on feces laced with the fluke eggs, things get a bit surreal.

After a target snail gobbles the eggs up, they hatch into clear-bodied newborns. In the next phase of their development, the "sporocyst" stage, the little guys may develop broodsacs. These sacs are pulsating, colorfully banded tubes that come jam-packed with larvae. If you squint, the broodsacs sort of look like wiggly caterpillars.

Maybe they're supposed to.


How Do the Snails' Eyestalks Change?

The thing about Leucochloridium broodsacs is they don't pop up just anywhere. Snails view the world through light-sensitive eyespots. Each one is located on the tip of a tentacle, or "eyestalk," connected to the mollusk's head.

A healthy snail can withdraw its tentacles and pull them back into its head whenever it likes. You might've noticed this yourself if you've ever picked one up.


But when a snail gets infected with these flatworms, the eyestalks become hampered. The fluke's swelling broodsacs invade the tentacles, which keeps the snail from retracting them.

Then, adding insult to inconvenience, the sacs start to pulsate. In a rhythmic dance that would look right at home in a trippy prog rock music video, the bulbous things expand and contract with wild abandon. They can pulsate dozens of times per minute. Thanks to the snail's ultra-thin skin, the entire show is clearly visible to the outside world.

zombie snails
The parasitic Leucochloridium paradoxum causes snails to involuntarily become prey to birds by changing the eyestalks to look like colorfully banded caterpillars.
Vasilii Aleksandrov/Shutterstock

What makes the display even more flamboyant is the color scheme. Leucochloridium broodsacs can include eye-catching shades of green, orange, yellow, white, black or brown, all arranged in bands.

There could be an evolutionary method to this madness.

Since the early 1800s, naturalists have wondered if the performance is just a ploy designed to trick birds into mistaking these broodsacs for juicy little caterpillars. Any bird that plucked one off 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.


How the Worm Influences the Snails' Behavior

Now here's where the zombie talk comes in.

During the 1920s and 1930s, a few scientists proposed that Leucochloridium actively manipulates the way snails behave.


The parasites allegedly force their host mollusks to deviate from their normal routine. Influenced by the flukes, the hapless snails are driven into exposed, well-lit areas, like leaf tops high up off the ground. Once they're in the open, the snails make easy targets. Caterpillar-loving birds see the dancing sporocysts and hungrily rip them out — along with the eyestalks.

Or so goes the argument. The trouble is, field researchers have never seen this happen in the wild.

Experiments conducted in 1874 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. Just ask a wolf scientist.


New Research and Unanswered Questions

Researchers Wanda Weslowska and Tomasz Weslowski took a fresh look at the host manipulation hypothesis in 2013.

For their study, published in the Journal of Zoology, the pair observed wild snails in Poland's Białowieża National Park. Some of them were known carriers of Leucochloridium broodsacs, while others were free of the parasites.


"The infected snails with pulsating broodsacs behaved differently from their apparently non-infected counterparts," wrote the authors in their paper. "They moved farther, positioned themselves in more exposed and better illuminated places, situated higher in the vegetation."

Surely that visibility makes the parasites easier targets for birds. Also, by favoring higher perching spots, the snails — and by extension, the broodsacs — became more accessible to birds flying overhead.

Another study, published in the Journal of Environmental Biology in September 2022, found that Leucochloridium broodsacs pulsate faster under daylight conditions than they do in the dark. Many of the bird species that ingest their larvae hunt by day. So being more active when it's light outside could help the parasites attract some avian attention.

When all's said and done, though, there's a lot we still 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? Is there mind control at play here, or something else altogether? Do the broodsacs fool wild birds into thinking they're caterpillars? And if not, then how do adult Leucochloridium find their way to a feathered host's cloaca?

Maybe we'll have clear answers someday. In the meantime, there should be more than enough material here to set up a dozen new "Alien" flicks. Your move, Ridley Scott.