Carnivorous Hammerhead Flatworms Discovered in France

carnivorous hammerhead flatworm

Story at-a-glance -

  • Native to Southeast Asia, hammerhead flatworms are considered to be invasive species in other parts of the world, including in Europe, where the worms have apparently been living for years without attracting scientists’ attention
  • Between 1999 and 2017, 111 observations of hammerhead flatworms were reported in Metropolitan France and French overseas territories; the worms appear to be permanent visitors, infesting the same gardens year after year
  • Their harm comes in their voracious appetite for earthworms (and possibly other beneficial organisms), which provide immeasurable benefits to the soil
  • Hammerhead flatworms produce tetrodotoxin, the same neurotoxin produced by pufferfish, to paralyze its prey; it then secretes enzymes that liquefy the earthworm, allowing it to easily suck up the meal; they have no known predators

By Dr. Karen Shaw Becker

With a head the shape of a hammerhead shark’s and bodies that can grow to be more than 12 inches long, hammerhead flatworms are the stuff many gardeners’ nightmares are made of. They’re also carnivorous, but though they don’t devour human flesh they do enjoy earthworms. This is what makes the sighting of these unusual worms in a place they don’t typically belong — like France — rather alarming.

Native to Southeast Asia, hammerhead flatworms are considered to be invasive species in other parts of the world, including in Europe, where the worms have apparently been living for years without attracting scientists’ attention. A study published in the journal PeerJ, however, which used data from a collaboration of citizen scientists, reveals the worms are not only making a home in France but also that their numbers may be increasing.1

Hammerhead Flatworms Invade France

According to a report by The Washington Post, the worms may have first come to light courtesy of Pierre Gros, a gardener who took a photo of the strange-looking creature and sent it to Jean-Lou Justine, Ph.D., a professor at the National Museum of Natural History in Paris.2 Justine first thought it was a prank, but the discovery led to a research collaboration between the pair, which called upon the efforts of other citizen scientists to send in photos of large worms with broad heads.

Sightings of the worms dated back to 1999 and were found as recently as 2017. Altogether, 111 observations were reported in Metropolitan France and French overseas territories. The worms are also known to be invasive species elsewhere in the world, including in the U.S. in states including California, Florida, Georgia, Louisiana and Texas.

It’s believed that they can only survive mild winters, but temporary populations may also exist as far North as Maine and in greenhouses in Illinois and Ohio (it’s thought that some species of flatworms came to the U.S. via potted plants).3 The French study revealed that the worms appear to be permanent visitors, infesting the same gardens year after year.

The worms are hermaphroditic and, researchers believe, capable of reproducing and laying eggs, but although egg cases have been found, sexual reproduction has not been observed. It’s thought that in temperate regions, the worms may reproduce asexually, by leaving a small rear portion or fragment behind, which begins to form a head within 10 days. The worms may fragment in this way a few times a month, according to the Texas Invasive Species Institute (T.I.S.I.).4

Another curious feature about the worms is how they eat. Although they’re large, they’re not that much bigger than the earthworms found in a typical garden. So how do they ingest their large victims? Hammerhead flatworms produce tetrodotoxin, the same neurotoxin produced by pufferfish, that’s used to paralyze its prey. It then secretes enzymes that essentially liquefy the earthworm, allowing it to easily suck up the meal.

If you’ve never spotted one, it doesn’t necessarily mean they aren’t there. Hammerhead flatworms are nocturnal, feeding and moving about mostly at night. But, according to T.I.S.I., “During the day hammerhead flatworms spend their time under leaf litter, rocks and logs and such, and under shrubs, out of the sun; under dripping garden faucets. They may be found out on the soil, driveways, patios, sidewalks after heavy rains.”5

Why Are Hammerhead Flatworms a Scary Sight?

The sight of a foot-long worm may give you shivers, but they’re not dangerous to humans, at least not directly. Their harm comes in their voracious appetite for earthworms (and possibly other beneficial organisms), which provide immeasurable benefits to the soil.

In some areas, earthworms contribute between 40 and 90 percent of soil macrofaunal (invertebrates that live in sediment) biomass and 8 percent of total soil biomass.6 As earthworms burrow through the soil they eat decomposing plant matter, including leaves and roots. In return, they leave behind nutrient-rich excrement, full of nitrogen, phosphorous and calcium that plants need to grow.7

Meanwhile, the tunnels they make through the soil provide aeration and improve its ability to absorb water, which not only helps plants to grow but also reduces flooding. Indeed, writing in the journal Reviews in Agricultural Sciences, researchers defined earthworms as “the most valuable animals that influence the functioning of soil ecosystems” and compiled a list of their many gifts to soil, including:8

Increase bulk density

Increase pore size

Increase water content

Increase water infiltration rate

Increase water-holding capacity

Increase litter decomposition

Increase soil organic matter dynamics

Increase nutrient cycles

Promote plant growth

Reduce some soil-borne diseases

Produce organo-mineral biogenic structures

Influence gaseous composition in the atmosphere

Restoring ecosystems, especially when soil is degraded or after mining

Enhanced microbial activity

Increase of nutrient availability in soil

Increase of mineral absorption by plants

Increase crop productivity

What’s more, although the exact numbers of earthworms aren’t known, it’s possible they’re already dwindling due to threats like agricultural chemicals and plowing. Adding in another enemy in the form of hammerhead flatworms is a concerning proposition, one that’s likely to affect not only earthworms but also other plants and soil ecology.

Already, research conducted on the New Zealand flatworm, which is native to New Zealand but found its way to the U.K., Ireland and the Faroe Islands, suggests the worms ate so many beneficial earthworms that agricultural grass declined by 6 percent in affected areas.9 Along with their ability to produce offspring immediately, hammerhead flatworms have such an unpleasant taste (due to the chemicals they secrete) that other animals don’t eat them. As such, they have no known predators.

For now, researchers like Justine are trying to spread the word that the creatures are here, and likely here to stay. “As scientists, we were amazed that these long and brightly colored worms could escape the attention of scientists and authorities in a European developed country for such a long time; improved awareness about land planarians [flatworms] is certainly necessary,” they wrote10 … but what to do about the invasion is another matter entirely, one that has yet to have a viable solution.