Hammerhead flatworms, which can grow up to 1 metre long, have been living in France and its oversees territories, uncharted by scientists for over 20 years, according to a new study.
Predators to local earthworms, the giant Platyhelminthes are potentially a danger to the country's biodiversity.
"I'm completely shocked that no one has worked on this subject before," Jean-Lou Justine from Paris' Museum National d’Histoire Naturelle, head of the international team that wrote the paper, told Euronews.
The number and impressive size of the invertebrates would've made them hard to miss — those found in the study were up to 40 cm long.
"These worms are enormous," said Justine. "It's almost impossible for nature to create a worm around 1 metre long."
Researchers collected information via "citizen science," creating a blog and Twitter account and taking recording sightings via phone and email.
They concluded that "the species present in Metropolitan France and overseas territories should be considered invasive alien species".
Their major finding was the proof that these worms have been in France for at least 20 years.
"They (the hammerhead flatworms) weren't hidden, they were very visible. People found them in their gardens but scientists overlooked them," said Justine.
Hailing from Asia, it is likely they originally came into Metropolitan France and its overseas territories on plants.
To estimate the real impact on earthworms and the local environment Justine said a further ecological study is necessary.
"We only published a small part of our research," said Justine.
The team also studied other species found in France, including over 10 other invasive Platyhelminthes, besides the hammerhead flatworms.