Predatory worms invading European soils may sound like something straight out of an Alien vs. Predator flick, but that's exactly what’s been going down in Metropolitan France for the last two decades. Thanks to a troupe of citizen scientists, researchers with the National Museum of Natural History in Paris have identified and cataloged the invasive creepy crawlies for the first time.  

Biologist Jean-Lou Justine knew something was up when amateur naturalist Pierre Gros sent him a photo of a flatworm from his garden in France. The invasive species' hammer-shaped head makes them easily distinguishable from other land flatworms. Native to Asia, giant hammerhead flatworms, or land planarians, grow to lengths of more than 40 centimeters (1.3 feet). Until now, their elusive nature and underground habitat have helped them evade the watchful eyes of biologists.

That’s when Justine put out a call on a dedicated blog and Twitter account for photos of other out-of-place buggers. Researchers sifted through reports of earthworms, leeches, and caterpillars (among other things) before identifying 111 contributions and more than five species dating back to 1999.

“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,” say the paper’s authors in the study published in the journal PeerJ. 

Reports of the flatworm intrusion extend beyond France to include French territories in the Caribbean, South America, Africa, and Oceania. Molecular studies of specimens sent to the researchers – both alive and preserved in ethanol – showed the species have no genetic variation and reproduce asexually.

It’s likely they were transported between and within countries through the export and import of plants. The authors fear the worms could be spreading, posing a serious threat to native fauna wherever they are introduced. Hammerhead flatworms are predators and eat soil animals like earthworms. The researchers say their addition to local ecosystems could pose a threat to the biodiversity of native animals and soil ecology. However, their ecological impact hasn’t been studied.

“This highlights an unexpected blind spot of scientists and authorities facing an invasion by conspicuous large invasive animals,” they said.