In retrospect, there were signs.

In 2013, a bunch of kindergartners were scared by hundreds of “small snakes” on the grass at their school. A woman, no doubt horrified to find a long worm on the fur of her cat, took it to a local veterinarian only to be told it was a tapeworm.

Other people found strange worms they showed to local universities, but were brushed off with identifications of leeches or other “plain, uninteresting animals.”  And way back in 2005, one scientist actually published an account reporting a strange new group of worms in France, but did so in an obscure mycological journal. Invertebrate biologists don’t read those (in their defense, neither does anyone else).

Yet there are certain things you’d expect scientists to notice pretty quickly. Say … foot-long toxin-secreting predatory hammerhead flatworms in a place where there were no hammerhead flatworms before.

Exhibit A:

Credit: Justine et al. 2018

This is not an animal that is easy to mistake for any other.

Yet somehow just such a creature crawled under the radar of French scientists for more than two decades, in spite of the aforementioned numerous reports by citizens who were pretty darn sure they were seeing something new.

“We are still amazed by the complete lack of response from scientific authorities at the presence of these worms,” the authors of a new study in PeerJ wrote.

How this happened is a sad commentary, in my opinion, but more on that later.

First, the juicy details.

Terrestrial flatworms are, as the name implies, flat-ish and possess a unique “creeping sole” on their underside. On this sole is a thick carpet of powerful cilia that beat back and forth on a mucus strip laid down by special glands. They may sometimes use this mucus to lower themselves to the ground on a string as slugs do.  

They hunt and track prey by smell and use sticky, probably toxic secretions to subdue them. Once a meal is acquired, they consume their prey by injecting digestive enzymes and then sucking up the resulting juices. Their preferred entrees are soil-dwelling animals like earthworms, slugs, protists, roundworms, and insect larvae.

There are terrestrial flatworms native to Europe, but they are usually less than one centimeter long. Invasive southeast Asian hammerhead flatworms, on the other hand, are measured in inches, not millimeters. A few – even in France, apparently -- are measured in feet.

In a new paper in the journal PeerJ, a team of French and Australian scientists sifted through four years of worm reports by citizen scientists. They also compared collected worms’ cytochrome oxidase I genes to see how many species were present and how diverse each species was.

Putting it all together, they determined there are at least five species of invasive hammerhead flatworm in France, three from named species and two unnamed.

Within species, the flatworms appear to be clones, as their cytochrome oxidase sequences showed 0% intraspecies variability. The way hammerheads clone themselves bears the delightfully fancy name ‘scissiparity”. Behind that pompous name, the process boils down to the worm gluing its tail down, pulling, and breaking off a still squirming piece which proceeds to sprout a head and get on with it. You know, like you do.

About 1 cm from the tail end the worm's body constricts. The worm glues the tail down and pulls and, voilà! New worm. Big worm can do this once or twice a month. Credit: Justine et al. 2018

In their home in tropical and subtropical southeast Asia, hammerhead flatworms do reproduce sexually, although for reasons I have not been able to discover they do not in places they have invaded. This lack of sex explains the lack of genetic diversity, but it does not seem to bother the flatworms at all (interestingly, the same thing is true for the King of Invasive Species, the dandelion*). The zero genetic diversity within species also seems to imply that each speices was introduced to France a single time.

How did the worms enter France? That part’s easy. They got there the same way they got everywhere else in the world, through that great enabler of invasive species, the nursery trade. They traveled in the soil of potted plants shipped between countries.

Previously, scientists had thought the invasive flatworms could only survive in Europe in greenhouses, where they were first documented in 1878 in a greenhouse at the Royal Botanic Garden at Kew. But a video filmed in the French city of Urcuit in 1999 and obtained by these scientists clearly shows Bipalium kewense flushing that theory down the toilet. In the years since, reports multiplied.

Though the worms have now been documented throughout much of France, they seem to favor the southwest region called Pyrenées-Atlantiques, which the authors describe as “a small paradise for invasive land planarians”. This region has flatworm-friendly mild winters and wet weather. One resident observed multiple hammerhead flatworm species in September 2017, December 2017 and January 2018 alive outdoors in garden soil. Clearly, the European winter is no impediment to them there.

In retrospect, it shouldn’t have been surprising to find the worms in France given that four species have been known for decades to have invaded the subtropical  U.S. states of Florida, Georgia, North and South Carolina, Louisiana, Texas, and even California. Southern France is also considered subtropical.

I saw a hammerhead flatworm myself when I lived in Texas a few years ago, and puzzled by its odd head and distinctive appearance, I was able to figure out what it was with a quick google search. No one in France did this for 20 years, or if they did, didn’t think or know it was worthy of notice or publication?

The failure of the French science community to recognize these worms for more than two decades is a sad commentary on the state of natural history and diversity education among biologists not just in France -- a historic stronghold of diversity studies (one of my favorite prairie flowers, Gaillardia, was named after a French patron of botany) -- but everywhere.

The authors attribute it to “ignorance”, but I will go further. I have strong feelings on this topic.

For 20 years, biologists repeatedly lacked sufficient natural history knowledge to even know they were seeing something new and different, a turn of events I find shameful. It’s not the experts who should be ashamed, however. It is the educational institutions and funding agencies who have sacrificed diversity education and research on the altar of molecular biology.

I will be the first to acknowledge molecular biology is important – vital, even. I was a teaching assistant for introductory biochemistry in college, and I include molecular and biochemical information in posts at this blog all the time. Biology cannot be comprehended without it.

But diversity and natural history are vital too. I have written at length here before about why, but suffice it to say that when biologists cannot register that a large, toxic predatory flatworm is something shockingly new, we have arrived at a sad state of affairs. Room must be made in biology education and research funding for this important topic. It must.

The arrival of the flatworms in France is worrisome, the authors say, because as we have seen, hammerhead flatworms are large predators. They may be the largest invertebrate predators in the areas they invade, which suggests their impact on native invertebrates is “not negligible”, although no one yet knows for sure. Ominously, the French team photographed a flatworm attacking a native earthworm (in North America, about a third of earthworm groups are themselves invasive, so ... small silver lining). What other native invertebrates are they feasting upon with abandon?

In the end, France’s salvation may be the flatworms themselves. They “are their own worst enemy”, the University of Florida IFAS Extension reports, for a very simple reason: they are enthusiastic cannibals.


*a strong argument can also be made for humans, although in dandelions’ favor we have not yet acquired Roundup resistance or wind-borne progeny


Justine J, Winsor L, Gey D, Gros P, Thévenot J. (2018) "Giant worms chez moi! Hammerhead flatworms (Platyhelminthes, Geoplanidae, Bipalium spp., Diversibipalium spp.) in metropolitan France and overseas French territories." PeerJ 6:e4672