Hammerhead flatworms—so named for their weird, broad heads—are predatory critters that feast on earthworms and, sometimes, on one another. They can grow to more than a foot in length and are native to the warm climates of Asia. But as Ben Guarino reports for the Washington Post, these voracious flatworms have been stealthily making their way through France for the past two decades, evading scientists’ attention until recently.
Researchers first became aware of the planarian invasion when Pierre Gros, an amateur French naturalist, snapped a photo of a hammerhead flatworm in 2013 and sent the image to local experts. The photo eventually made its way to Professor Jean-Lou Justine, a zoologist at the French National Museum of Natural History, who assumed Gros was playing some sort of strange prank.
“I looked at it and said 'Well, this is not possible—we don’t have this kind of animal in France,’” Justine tells Josh Gabbatiss of the Independent.
His suspicions were heightened when Gros sent him a second picture of an entirely different hammerhead flatworm species, and then a third of yet another species. But once Justine confirmed that the images were in fact authentic, he felt compelled to find out if the invasion was widespread.
Through radio, television, newspapers and social media, Justine and his fellow researchers let France’s citizen scientists know that they were on the lookout for images of hammerhead flatworms. The reports came pouring in.
According to a study published in PeerJ (the title of which includes the phrase: “Giant worms chez moi!”), researchers were able to record a total of 111 sightings not only in France, but also in overseas French territories. The earliest observation came from a VHS tape made in 1999; a family had filmed hammerhead flatworms and held on to the video for nearly two decades because the creatures were just so strange.
Justine believes the worms were transported to Europe on tropical plants from Asia. “What we know now is that there are invasive flatworms almost everywhere in metropolitan France,” he tells Gabbatiss.
There were other surprises in store for the researchers. One of the worms, a vibrant blue creature spotted in Mayotte Island off the coast of southeast Africa, is likely a new species.
“[W]e 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 is certainly necessary,” the study authors write.
Scientists were not only shocked by their findings—they were also concerned. Invasive hammerheads have been known to wreak havoc in their new homes. They eat earthworms, which are “important constituents of the soil fauna,” the study authors write. Previous research has shown that in Scotland and Ireland, hammerheads from New Zealand have reduced yields of agricultural grass by around six percent.
Hammerheads kill and consume their prey in a nightmarish way. As the Encyclopedia of Life explains, the creatures attach themselves to earthworms using their muscles and “sticky secretions.” When they are ready to chow down, hammerheads secrete enzymes that start to digest prey outside of their bodies. They then suck up the liquefied tissues.
It’s a gruesome process—and because hammerheads’ secretions taste foul, they don’t have many natural predators, allowing them to proliferate largely unchecked. “Because of their cannibalistic habit, land planarians may be their own worst enemy,” notes the University of Florida’s Institute of Food and Agricultural Sciences.
Justine and his colleagues do not yet know how the influx of hammerhead flatworms is impacting France’s soils. But they aren’t particularly optimistic.
"[W]e strongly believe," the study authors write, "that invasive flatworms, as active predators, constitute a danger to native fauna wherever they are introduced."