Absurd Creature of the Week: The Parasitic Worm That Turns Snails Into Disco Zombies

Those are parasitic worms dancing in the snail’s eyes. Dancing in the snail’s eyes. It’s like Saturday Night Fever, only with more blindness and less chest hair.

One of the crueler tortures to bestow on a snail is the salt shower, as the behaviorally challenged kid who grew up down the street from you could attest. It’s a horrible death: The salt draws water out of the creature until it perishes from dehydration. Even if you live in Florida, which is overrun with giant foot-long snails that are devouring houses, please do not dispatch them with salt. Snails have enough problems as it is.

Mother Nature, you see, has cooked up an even more sadistic punishment for the humble snail. It’s called Leucochloridium, and it’s a parasitic worm that invades a snail’s eyestalks, where it pulsates to imitate a caterpillar (in biology circles this is known as aggressive mimicry—an organism pretending to be another to lure prey or get itself eaten). The worm then mind-controls its host out into the open for hungry birds to pluck out its eyes. The worm breeds in the bird’s guts, releasing its eggs in the bird’s feces, which are happily eaten up by another snail to complete the whole bizarre life cycle.

It’s an existence that’s as brilliant as it is strange. But while science has known about Leucochloridium for more than a century, it was only in 2013 that biologist Tomasz Wesołowski of Poland’s Wrocław University confirmed the worm is indeed capable of manipulating its snail hosts. (Specifically, amber snails—like many other mind-controlling parasites, it’s highly species-specific, that is, it’s unable to manipulate the behavior of more than one species.)



 Inside the snail, Wesołowski says, the whole grand show begins as the ingested egg develops into what is known as a sporocyst, “which looks like a bunch of whitish tissue, seated mostly in the liver of the snail. And then it grows like a tumor, more or less.” It doesn’t have a mouth, so like many parasitic worms, such as the horsehair worm that infects and mind-controls crickets, it simply sits around soaking up the snail’s hard-earned nutrients through its skin. Like a clubber downing vodka Red Bulls, it’s gonna need energy if it’s gonna dance.

As if it weren’t enough of a meany-head, Leucochloridium also castrates its host. This makes good evolutionary sense: Energy normally spent producing eggs and sperm (snails are hermaphrodites) goes toward sustaining the worm. So, pumped full of the requisite nutrition, the sporocyst sends out branches that tunnel through the snail’s body and into its eyestalks, also known as tentacles, where it forms a brood sac full of larvae. It’s these larvae that eventually go all disco.

Now, it’s worth talking for a moment about the physiology of snail peepers. At the tip of the tentacle sits a rudimentary eyespot, which is really only good for discerning light and dark. The snail can’t see color, and the eyestalk doesn’t have the muscles required to focus. But what the snail does have are muscles that retract the tentacles, which it can then redeploy by pumping them full of fluid.

Sketches of Leucochloridium. Notice the thin trailing bit, which would lead back to the sporocyst in the snail’s liver.

 Not so fast, says the Leucochloridium. It so greatly swells the tentacle that the snail is no longer capable of retracting it, so the host is left with a massive strobing eye of larvae that looks mighty delicious to passerine birds. (These are the kinds of birds with three toes facing forward and another facing backward, what you’d typically find in your backyard, unless you live in Antarctica and your neighborhood is lousy with penguins.) And what the world looks like to a snail with wormy eyes is anyone’s guess, but I’m willing to bet it’s somewhat dizzying.

The worm, though, has a problem: Snails are largely nocturnal, and passerine birds, which hunt by sight, most certainly are not. So once the Leucochloridium has sufficiently developed in the eyestalk, it begins manipulating the behavior of its host, forcing it out into the many dangers of the light of day, where predators swarm and the sun rapidly desiccates. It’s probably using chemicals, but how it’s able to pull off this incredible feat remains a mystery, as do the chemical secrets of any number of other zombifying parasites (though scientists are making progress in decoding the compounds that the Ophiocordyceps fungus uses to assume control over ants).

So quite weirdly the Leucochloridium worms must themselves know the difference between night and day. “What is most amazing is that these brood sacs are pulsating only in daylight,” said Wesołowski. “They have no photosensitive anything—no trace of, say, any nervous system, no sense organs. Nothing. Still they recognize when it’s worth pulsating and when it’s not worth pulsating. So that’s very, very unusual. Nobody knows how it’s achieved.”

Wesołowski also found infected snails are up to three times as active as their non-zombified peers: He even observed one traveling a full 3 feet in just 15 minutes. That may not sound impressive to you, what with your fancy legs and all, but “for a snail, that’s a race,” he said. In addition, he found that the worms convinced their host to stay “on the upper parts of plants and higher elevated places. So all of this combined made them easier to be spotted by foraging birds.”

An amber snail wondering why it even bothers to get out of bed in the morning.

 And when the reckoning comes, the snail ends up with its eyes plucked out. But because birds don’t typically go after snails—only when their eyeballs look like caterpillars—they’ll take off without eating the rest of the body. (If the eyestalk happens to rupture on its own, the faux caterpillar will spill out onto a leaf and pulsate for some time before drying out. The worms really, really want to get eaten.) Mercifully, or perhaps horrifically, the snail will not only survive, but will regenerate the lost tentacles and eyespots and regain the ability to reproduce. That’s actually quite beneficial for these parasites, for the wounded snail eventually becomes another potential host capable of producing many more potential hosts.

And so the cycle begins anew as the worms grow and reproduce in the bird’s gut. Strangely, though, Leucochloridium and other so-called trematode worms (all of which are parasitic, though not necessarily zombifiers) seem to have figured out how to skip a step. They belong to the flatworm phylum, whose members typically go through two intermediate hosts on their way to their primary host, according to Wesołowski—the former for Leucochloridium just being the snail and the latter the bird. The intestinal worm Metagonimus yokogawai, for instance, starts in snails, which are eaten by fish, which if not cooked properly by humans ends up in our guts.

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