Photograph by Carola Petersen, Hinrich Schulenburg, Kiel University
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This slug in the Arion genus, found in northern Germany, unwittingly swallows worms and carries them in its gut.


Photograph by Carola Petersen, Hinrich Schulenburg, Kiel University

"Huge Surprise": Worms Hitchhike in Slug Guts

Unable to travel far on their own, some worms catch a ride inside slugs to reach new territory, a new study says.


Just when it seemed that travel couldn't get more uncomfortable than flying coach, scientists have discovered that worms hitch rides inside slugs.

Arion slugs accidentally swallow the worms—including the popular research species Caenorhabditis elegans—while eating decomposing plants. Instead of being digested, however, the tiny worms somehow hunker down in slug intestines until they are pooped out, often far away from where they were first eaten. (Also see "Weasel Rides Woodpecker in Viral Photo—But Is It Real?")

This strategy transports the tiny worms much farther than they would get on their own, giving them access to more food sources, according to a study published July 12 in the journal BMC Ecology.

"These results are a huge surprise for us, because we never expected it to survive inside another organism," says study leader Hinrich Schulenburg, a zoologist at the University of Kiel in Germany.

Well-Known Worm

In the 1960s, scientists discovered C. elegans is perfect for laboratory research. It's tiny—just a millimeter long—and has a short life span, making it ideal to breed in captivity. Since then, researchers have decoded the worm's genome and overall know more details about its biology than nearly any other organism on Earth.

What was missing was any idea of what the worm—which lives in temperate areas around the world—did outside the lab, says Christian Braedle, a biologist at the University of Nice Sophia Antipolis in France.

"It was embarrassing how little people knew about the ecology of C. elegans," says Braedle, who was not involved in the new study.

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The worm Caenorhabditis elegans is perhaps the most thoroughly studied organism on Earth.

That inspired Braedle and other scientists, including Schulenburg, to study them in Europe in their natural habitat.

The results were eye-opening: Instead of living in the soil as previously thought, the worms live on the surface, eating bacteria, fungi, and yeast on decomposing plants, Braedle discovered.

Gutsy Moves

Decomposing leaves are a good home and food source, but eventually the grub runs out—and the worm has to search elsewhere.

Watch a video of parasitic worm hijacking a snail brain.

Halloween Special: Real-Life Zombies In a spooky coup, a parasitic worm hijacks a snail's brain and makes the snail sacrifice itself to a hungry bird. Carl Zimmer, a contributor to National Geographic's Phenomena science salon and author of the book Parasite Rex, explains how the snail's death helps the parasite perpetuate its sneaky species.

When you're as small as C. elegans, it's tough to travel far—which is why Braedle and Schulenberg were surprised to find the worms living in temperate zones throughout the world. (Also see "What's the Giant, Slimy Worm That Horrified the Internet?")

To figure out how the worms were getting around, Schulenberg and colleagues began searching gardens and compost heaps for creatures that might be offering them a ride.

The team collected over 600 slugs, centipedes, and other invertebrates, along with 400 worms of various species. They found many instances of Caehorhabditis worms and Arion slugs living together.

But the worms weren't hitchhiking on the slugs: When the researchers dissected the wild slugs, they discovered the worms living inside. The worms did not appear to harm the slugs in any way. (Also read "Mindsuckers" in National Geographic magazine.)

Braedle praised the work and said it was the first time scientists had actually measured the interactions of Caenorhabditis with other invertebrates, an important step to understanding the worm's ecology.

To show his discovery wasn't a freak occurrence, Schulenberg paired 79 slugs with more than 15,000 fluorescent-tagged worms in the lab. (See "New Hot-Pink Slug Found in Australia.")

After the slugs ingested the worms, the researchers then dissected the slugs and analyzed their feces. The researchers found healthy worms in both the intestines and in the slug poop.

"Somehow they possess the means to protect themselves from being digested," Schulenburg said.

You could call it worming their way out of a bad situation.

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