Hammerhead worm. Photo: നവനീത് കൃഷ്ണന് എസ്/Wikicommons
The creatures in question are brightly coloured, muscular giant hammerhead flatworms — so-called because of their hammer shaped head.
Usually found in the tropical climates of Asia, the invaders, which can reach up to a sizeable 40 centimetres in length, have somehow wormed their way into the ecosystems of French gardens.
But this isn't a recent phenomenon.
In fact, they've been at it for 20 years but have managed to carry out their invasion almost unnoticed.
That is until amateur naturalist Pierre Gros photographed the unidentified worm in 2013 and sent it on to scientists. It eventually made its way to Professor Jean-Lou Justine, a zoologist at the French National Museum of Natural History.
“Blue” hammerhead worm found in Mayotte. Photo: Laurent Charles
“This photograph was sent from email to email to email and finally it came to me,” Justine told The Independent this week.
“I looked at it and said 'Well, this is not possible – we don’t have this kind of animal in France'.”
Justine then decided to investigate the species and over four years has amassed a collection of recorded sightings in France, as well as tropical French territories including Guadeloupe, Martinique and French Guiana, dating back to 1999.
A hammerhead worm killing an earthworm. Photo: Pierre Gros
This led the professor and his team to draw the conclusion that hammerhead worms, which eat earthworms and other invertebrate prey and pose a threat to the ecological processes in general, are present almost everywhere in metropolitan France.
The study also showed that two of the flatworm species found — one black, spotted in France, and one a blue colour, found on the island of Mayotte, a French territory in the Indian Ocean — are probably newly discovered.
However what he still doesn't understand is how the creatures managed to become so prolific without the general population or the scientific community at large noticing.
“I am still amazed – I don’t understand how this is possible in a developed country,” he said.
A species of hammerhead flatworm now discovered to be living in France. Photo: Pierre Gros
So, how did they get here?
Well, it seems most of the Asian worms currently living in France are likely to have originated from populations transported over in tropical plants.
And while a system has now been put in place to control their movements between countries — the UK also has its own population of hammerhead worms — they are an incredibly robust species.
In fact when bits of them are amputated, they can regrow into complete worms and in one known case, a flatworm sent into space grew a second head after its tail was cut off.
They can also reproduce asexually and produce chemicals that make them taste unpleasant, making them unappealing to predators.