Mysterious neurotoxin may help flatworms kill prey

Tetrodotoxin, the deadly chemical in pufferfish, makes first appearance in land invertebrate

TOXIC ASSETS  Two terrestrial flatworms, Bipalium adventitium and Bipalium kewense (shown), may carry a deadly chemical called tetrodotoxin, which could help the invertebrates stun their earthworm prey.

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An infamous neurotoxin may be the secret weapon of goo-slurping flatworms. Tetrodotoxin, the deadly chemical that makes pufferfish a risky meal, has perplexed researchers for years. The poison shows up in a variety of organisms — many aquatic — but scientists haven’t unraveled which creatures make it or how. Adding to the puzzle, a new study says that two species of flatworm may be the first terrestrial invertebrates known to carry the toxin. The toxin may help the flatworms quell their prey, earthworms.

Bipalium adventitium and Bipalium kewense are flatworms with hammer-shaped heads that stalk many regions of the United States, although they are native to Southeast Asia. These aggressive hunters chase down earthworms, clamp onto their heads and inject enzymes that digest the prey whole. The result is a jellylike earthworm carcass that the flatworm sucks up.

“It really is just like a puddle of goo,” says chemical ecologist Amber Stokes of California State University, Bakersfield. With colleagues, Stokes noticed that before the mushy murder, the flatworms subdue their writhing prey with an odd head-to-head rub. The researchers wondered whether the rub wiped a paralyzing toxin, such as tetrodotoxin, onto the earthworms.

In animals, tetrodotoxin blocks signals between the body and the brain. Mild doses in humans cause tingling, numbness and difficulty moving. Larger doses cause paralysis, respiratory arrest and death.

Stokes and colleagues found evidence for tetrodotoxin in the flatworms, particularly in their heads. When the team tried to feed the flatworms to salamanders, many of the amphibians tried to wipe their mouths, a common animal response to the tingly effects of tetrodotoxin. The results appear June 25 in PLOS ONE.

The researchers speculate that tetrodotoxin could help the flatworms not only stun their prey but also avoid predators. But Stokes and her team don’t know how the flatworms get the toxin. Although some marine bacteria seem to make the toxin, researchers aren’t sure if animals can also produce it. Animals that carry tetrodotoxin may simply collect it by eating bacteria or bacteria-eating creatures, Stokes says.

Additionally, scientists don’t even understand how bacteria or any other organism would make tetrodotoxin. In the lab, chemists toiled for years before they could re-create the toxin — a remarkably complex organic molecule.

But it may be too early to try to tackle the bigger questions about tetrodotoxin’s origins in flatworms, says molecular biologist Rocky Chau of the University of New South Wales in Australia. He says that researchers still need to confirm that these flatworms carry tetrodotoxin. The biochemical assays that Stokes’ team used to identify the toxin are not always accurate, he says, because the assays are indirect measurements. The team may have merely detected chemicals with structures similar to that of tetrodotoxin.

“We should be very careful with our methods for identifying [tetrodotoxin],” he says, “especially in new sources.” 

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