Biologists are warning about an invasion of giant predatory worms in French territories across four continents. The infestation has gone underreported for almost two decades and poses a threat to fauna, according to researchers.

Biologist Jean-Lou Justine, amateur naturalist Pierre Gros, and three other colleagues conducted a five-year study of over 700 sightings of giant worms across French territory since 1999, and published their findings in the journal PeerJ on Tuesday. Five years ago, Gros began sending Justine photos of three separate and highly unusual worms he discovered in his garden.

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The alien worms are all clones that reproduce asexually. Native to Asia, the predatory creatures feed on earthworms. They possess a bioweapon known as ‘tetrodotoxin’ which immobilizes their prey. One of Justine's colleagues once reportedly put one of the flatworms in his mouth and described it as "one of the worst experiences of his life."

Throughout the course of the study, the team identified five species of alien worms in France and French overseas territories. Justine initially dismissed the threat posed by the exotic worms as nothing more than a minor pest - as, apparently, did other biologists.

"We were amazed that these long and brightly coloured worms could escape the attention of scientists and authorities in a European developed country for such a long time," the study notes. One of the worms, the Hammerhead flatworm, can reach up to one meter in length "in elongated state" and glides through the soil like their oceanic counterparts.

Recorded sightings of the alien worms date back as far as 1999 - one family sent the researchers a VHS tape recording they made as the worm was so bizarre. In 2013, a group of kindergarten children came across what they believed to be a huge, writhing mass of snakes in their playground. A total of 111 observations of the hammerhead flatworms were made between 1999 and 2017 alone, many of which were made in the south of France where conditions were both humid and temperate enough for the exotic worms to survive.

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The French researchers warn that the worms pose a serious threat in their new surroundings. Similar flatworm invasions in Scotland and Ireland reduced yields of agricultural grass by six percent.

"Recently, a tendency to deny the risks posed by non-native species has emerged; in opposition to this ‘denialism,’ we strongly believe that invasive flatworms, as active predators, constitute a danger to native fauna wherever they are introduced," the team concluded.

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