At first, Justine thought the gardener who sent him the photo, Pierre Gros, was playing some kind of prank. But the two eventually paired up for the study and discovered several species of flatworm in metropolitan France.
They studied 111 records and observations of the worms from 1999 to 2017.
"The invaders are giant hammerhead flatworms — brightly colored specimens that look like earthworms on steroids," Live Science wrote about the species, two of which are part of the Diversibipalium genus. According to the study, these are probably newfound species.
The land flatworms, which produce unpleasant-tasting chemicals that keep predators at bay, can have an effect on soil ecology and plant life cycles by preying on organisms that live in soil.
The muscular worms measure about 1 foot or 40 centimeters in length and typically consume earthworms and other invertebrate prey.
“As invasive predators, [giant flatworms] are likely to be a threat to the abundance and biodiversity of the soil invertebrates,” Justine told Live Science.
While they're not the most exciting worms creatures out there, flatworms are capable of regenerating, "even from snipped-off fragments that represent 1/300th of the worm's body," according to Live Science. They also reproduce asexually and can quickly produce many offspring.
The five non-native species identified in Justine's study were distributed in mainland France, as well as the Caribbean French islands, French Polynesia and French Guiana.
According to entomologist Archie Murchie of Britain's Agri-Food and Biosciences Institute, who was not involved in the research, the worms are likely to continue to spread "with increased global trade," he told the Washington Post. "The species are cryptic and soil-dwelling so can be easily overlooked, which often explains their inadvertent shipment round the world."
In Ireland and Scotland, Murchie said invasive New Zealand flatworms ate so many earthworms that it “yields of agricultural grass in affected areas shrank by about 6 percent,” the Post reported.
“The authors are rightly cautious about the potential impact of the hammerhead flatworms,” Murchie said.