This "nemesis" of the African giant snail was found in inland China for the first time

On October 19, 2024, when the friends in Guangdong and Guangxi were still nauseated by the huge and slimy African snails on the roadside, a seemingly ordinary incident quietly made the news:

New Guinea flatworms were discovered in Zigong, Sichuan. This is the first time in Sichuan Province and also the first time that this world-famous vicious invasive alien species has been discovered in inland China.

After being first discovered in Hong Kong in 2012 and in Guangdong in 2018, the New Guinea flatworm has once again refreshed its distribution area in China.

Part 1 Primitive invertebrates - flatworms

The New Guinea flatworm (Platydemus manokwari) , also known as the Manokwari flatworm, belongs to the class Turbellaria of the phylum Platyhelminthes. It is a local flatworm (also called turbellarian) native to New Guinea.

New Guinea flatworm, with its head at right

Image source: wikipedia

Flatworms are a relatively primitive class of invertebrates, located near the base of the species evolution tree. They have soft, bilaterally symmetrical, unsegmented bodies, no body cavity, and no specialized circulatory and respiratory organs, so they can only maintain a flat shape, thereby obtaining oxygen from the outside world through osmosis and diffusion. Their digestive cavity has only one opening, which is the so-called mouth without anus, and they use the same mouth to eat and poop.

A classic species evolution tree. The flatworm is framed near the base in the lower left corner.

There are not many species in the phylum Platyhelminthes, but there are not too few either. The number of species in the entire phylum is about 20,000, and the class Turbellaria, where the New Guinea flatworm belongs, has about 4,500 species. However, due to their simple body structure, flatworms cannot survive without water. Most flatworm species live in seawater and freshwater, and only a few live on land. These few flatworms that live on land can only survive in humid environments, such as inconspicuous moist soil and leaf litter. So for most people, flatworms are neither familiar nor common.

Warm reminder: The pictures may cause discomfort, so readers who are afraid of insects should click carefully~

New Guinea Flatworm

Image source: Reference [1]

If you only look at the appearance, flatworms look a bit like earthworms in the Annelida phylum or slugs in the Mollusca phylum, but their taxonomic status is very different. It can only be said that there are similarities in morphology, but they are thousands of miles apart in terms of kinship.

As such a rare and primitive little animal with little presence, the New Guinea flatworm would have been unknown - if there were no African giant snail.

Part.2 Nemesis of the African giant snail

Without the giant African snail, the New Guinea flatworm might not have spread and would still be confined to its own area today; but at the same time, it might not have become as notorious as it is today.

Giant African snail

Image source: wikipedia

The Giant African Land Snail (Lissachatina fulica) is a giant snail native to East Africa. This giant mollusk has been widely introduced around the world for various reasons, including the pet trade and food demand. It not only destroys local vegetation and crops, but also carries various parasites and bacteria that are harmful to humans. It is one of the notorious "Top 100 Invasive Species in the World".

Friends in southern China should be familiar with the giant African snail. After a refreshing rain, watching many giant African snails slowly crawling beside the flower beds and roadsides, I'm afraid everyone's scalp tingled.

Many people living in other parts of the world, such as those living on Pacific islands, are also troubled by the giant African snail. After the giant African snail invaded these Pacific islands, it not only destroyed crops and spread diseases, but also posed a huge threat to the rare and endemic land snails on those islands through competition.

In order to stop these devilish African giant snails, people came up with a "brilliant" idea - to introduce New Guinea flatworms to control African giant snails!

New Guinea flatworms that eat snails

Image source: wikipedia

If you only look at the appearance, it may be hard to imagine that the seemingly weak New Guinea flatworm is actually an efficient predator. The New Guinea flatworm mainly feeds on land snails and also eats other land invertebrates. They will track snails along the mucus left by snails. They are silent but extremely efficient snail killers.

The giant African snail is right on the New Guinea flatworm's diet. When the New Guinea flatworm was introduced to the Pacific island of Guam in 1978, it eliminated more than 95% of the giant African snails in many areas of the island in less than five years.

Similar things are happening on many islands in the Pacific. Giant African snails that invade islands, destroy crops and take over rare local snails are being wiped out by New Guinea flatworms. People take it for granted that this will be another classic case of successful biological control.

Part.3 Disaster of biological invasion

Unfortunately, although people can decide when to start a war, once the war starts, they have no say on when it will end.

After the invasive African giant snails were eradicated, the next in line were the rare and endemic snails on the islands.

Island-specific acorn snails (Euglandina sp.) preyed on by New Guinea flatworms

Image source: Justin Gerlach et al., 2020

Whether it's the giant African snail or the much smaller native land snail, they are just lumps of delicious meat to the New Guinea flatworm. In the eyes of the New Guinea flatworm, there is no distinction between high and low meat.

They eat giant African snails and rare land snails unique to the island. They were introduced by humans and brought equal extinction to all species of snails on the island.

The New Guinea flatworm quickly went from being an excellent example of biological control against the invasion of giant African snails to a vicious invasive species that threatens rare and endemic island snails, and the New Guinea flatworm has since joined the list of the "100 most invasive species in the world."

New Guinea flatworms feed on recently reintroduced endemic snails (Partula varia) on the French Polynesian island of Huahine.

Image source: Gerardo Garcia et al., 2023

Once Pandora's box is opened, it will be difficult to close it.

After being taken out of its native habitat in New Guinea, it is no longer possible to stop the spread of the New Guinea flatworm. In addition to being introduced to the Pacific islands, the New Guinea flatworm has gradually spread to all parts of the world, including China, through soil (such as seedlings, gardening soil trading, etc.), especially in coastal areas.

Thailand, Singapore, Malaysia, China... they can be found in the coastal areas of southeast Asia. Now, they have even crossed the mountains and rivers and appeared in the inland Sichuan Basin.

At the same time, New Guinea flatworms currently have almost no known predators, and pesticides are generally ineffective against them. Therefore, the difficulty of eradicating the already invasive New Guinea flatworms has long exceeded people's imagination.

Some known distributions of New Guinea flatworms

Image source: Jean-Lou Justine et al., 2014

Fortunately, the geographical conditions of islands and continents are different. Compared with rare island-specific snails that have nowhere to escape, snails on the mainland at least have places to escape and are less likely to be caught by New Guinea flatworms.

So at least the ecological threat posed by New Guinea flatworms on the mainland is much lower than that posed by rare and endemic island snails trapped on isolated islands with nowhere to escape.

Part.4 Potential threats to humans

Unfortunately, the New Guinea flatworm is also a potential threat to humans. As a snail-eating creature, the New Guinea flatworm, like snails, can act as an intermediate host for a variety of parasites, including Angiostrongylus cantonensis (also known as Guangdong blood nematode).

New Guinea flatworms like to hide under vegetable leaves, so they are often picked up together with the vegetables when they are harvested. If the harvested vegetables are eaten raw or undercooked, people who accidentally eat New Guinea flatworms may develop angiostrongylus disease, which affects the brain and spinal cord.

A 2018 study of New Guinea flatworms found in Thailand showed that 12.4% of the New Guinea flatworms collected in Thailand carried parasites of the genus Angiostrongylus.

Angiostrongylus parasites in New Guinea flatworms collected in Thailand

Image source: Reference [5]

New Guinea flatworms have only recently arrived in China, and as a silent killer that is both good and evil, all the changes they bring about are silent and unnoticed.

We still don’t know where the dominoes pushed down by the New Guinea flatworm will eventually fall. Will the giant African snails that have long been entrenched in southern China encounter their lifelong enemies again, or will the New Guinea flatworm create new parasite problems as it gradually spreads?

No one knows. Just like a butterfly flapping its wings in the Amazon River Basin in South America, it has no idea where the hurricane it accidentally stirs up will eventually blow to.

The New Guinea flatworm continues its silent killing.

References:

[1] The invasive land planarian Platydemus manokwari (Platyhelminthes, Geoplanidae): records from six new localities, including the first in the USA. Jean-Lou Justine et al. PeerJ. 2015.

[2] Status of Tree Snails (Gastropoda: Partulidae) on Guam, with a Resurvey of Sites Studied by HE Campton in 1920. Hopper, David R et al. Pacific Science. 1992.

[3] Potential impacts of the invasive flatworm Platydemus manokwari on arboreal snails. Shinji Sugiura et al. Biological invasions. 2009.

[4] Prey preference and gregarious attacks by the invasive flatworm Platydemus manokwari. Shinji Sugiura. Biological invasions. 2010.

[5] Distribution of the newly invasive New Guinea flatworm Platydemus manokwari (Platyhelminthes: Geoplanidae) in Thailand and its potential role as a paratenic host carrying Angiostrongylus malaysiensis larvae. Kittipong Chaisiri et al. Journal of Helminthology. 2018.

[6] Negative impacts of invasive predators used as biological control agents against the pest snail Lissachatina fulica: the snail Euglandina 'rosea' and the flatworm Platydemus manokwari. Justin Gerlach et al. Biological invasions. 2021.

[7] The invasive New Guinea flatworm Platydemus manokwari in France, the first record for Europe: time for action is now. Jean-Lou Justine et al. PeerJ. 2014.

Source: Science Institute