ANNAPOLIS, Md. – Move over, bats. Halloween may have a new animal icon.
Meet the hammerhead worm, an invasive species of flatworm that downs its prey with the same neurotoxin employed by pufferfish and can reproduce an entirely new worm by detaching a piece of itself — no head required. It can be a gardener’s best friend because it feeds on invasive earthworms that degrade the soil. And, frankly, it’s just a weird-looking creature.
“Biologically, they are really cool, very fascinating organisms,” said Peter Ducey, an evolutionary ecologist at the State University of New York at Cortland. “They’re creepy looking for sure, and they do things somewhat differently than other animals.”
These transplants from East Asia have likely been around in the Chesapeake Bay region for decades. (The present theory is they have spread across the world in plant pots.) And they probably would have continued living their strange lives in quiet anonymity – if not for social media.
“In the past, if you saw something gooey, you killed it, threw it away and maybe told your friend,” Ducey said. “Now, you post about it.”
From there, it’s only a matter of time for that post to make the rounds across the social media landscape. Then the news stories begin to appear. And for a few weeks at least, hammerhead worms are the topic du jour. Think murder hornets, but with a far less menacing name.
“I see it as a social media phenomena,” Ducey said. “It’ll be a big deal for about two weeks, then people will move on to something else.”
Hammerhead worms found themselves in the spotlight this summer in the Bay region in the wake of a few viral posts. Then, those local stories went national, with outlets such as CNN and CBS News wriggling into the mix. Occasional stories were still cropping up into September.
Theresa “Tree” Dellinger, an entomologist with the Virginia Tech Insect ID Lab, said she can understand why hammerhead worms tend to grip the public’s imagination, especially after her own recent encounter.
“I saw one in my front yard over by my potting area where I start my seeds. I moved a pot and there was this thing on the ground, and it was undeniably weird looking,” Dellinger said. “They’re flat. They’re squishy. I think it’s the head that really bothers people.”
A hammerhead worm would make for an instantly recognizable Halloween costume. The head is shaped like a half-moon or crescent. (Yes, like a hammerhead shark.) Another striking feature is its length. The species most common in the Bay region, Bipalium kewense, grows to a snakelike foot in length. Its body tends to be tan or yellowish brown with five, neatly spaced dark stripes running along its entirety.
Some of the recent news stories have hyped the worm’s “poisonous” component.
Hammerheads can indeed subdue earthworms with tetrodotoxin, the neurotoxin found in blue-ringed octopuses, newts and pufferfish. It’s a good idea, therefore, to wear gloves while handling them. Exposure can lead to a rash. But they are incapable of injecting the venom into humans or pets, and there are no documented cases of people or pets being sickened by them, according to a fact sheet compiled by the Penn State Extension office.
“Unless you’re an earthworm, it’s probably not a big deal,” said Michael Skvarla, the entomologist who authored the fact sheet.
Still, Dellinger offers this advice: “Don’t lick the flatworms. Sometimes, I can’t believe we have to say that.”
From an ecological perspective, hammerhead worms don’t seem to be causing trouble, she said, adding: “I think if they were going to cause problems, it would have happened by now.”
The B. kewense species feeds almost exclusively on earthworms. Some varieties of earthworms can improve soil health, but others, most notably the jumping worms, can upend soil composition, making it drier and less capable of holding nutrients.
In that sense, the hammerhead worms are acting as a biological control, experts say. But they’re not quite ready to recommend introducing them into new landscapes to ferret out unwanted worms.
“I don’t know what I could say what is the better scenario,” Delliner said. “You’ll always have more gazelles than lions. You’ll never have enough predators to wipe out the prey item.”
When it comes to hammerhead worms, scientists have more questions than answers. It is a rare research paper indeed that attacks the hammerhead knowledge gap head on. Because the species doesn’t appear to pose issues with agriculture or human health, funders are unlikely to invest in its research.
“They’re an orphan group that nobody claims,” Skvarla said.
One of the most burning questions in the field is if the non-native hammerhead species that is prevalent in the Southeast U.S. is being drawn northward by a warming climate. That would be significant because the B. vagum species eats snails and slugs instead of earthworms. The potential impacts of a movement north of Virginia are unknown.
Certainly, the hammerhead worms don’t appear to be as destructive as fellow invaders, such as emerald ash borers or spotted lanternflies, said Bruce Snyder, an earthworm researcher with Georgia College and State University. But people shouldn’t let their guard down, he advised.
“We don’t know what the down-the-line impacts are going to be,” he said.
Are they good eatin?