Are nematodes disgusting? You should thank them for your sushi

In the once popular movie "Invasion of the Iron Maidens", Koreans infected by mutated iron maidenhair worms were in chaos. When the larvae matured, the iron maidenhair worms were excreted from the human body and escaped into toilets and sewers. The horror scene lingered in the minds of the audience, which also raised questions in real biology: Why are iron maidenhair worms so cruel? Can iron maidenhair worms parasitize the human body?

A parasitic nematode Chordodes formosanus living inside a praying mantis | Fang-Yu, Shen / inaturalist

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Messy Gordian Knot

To solve this problem, let's first understand what wireworms are. Wireworms belong to the phylum Nematomorpha (Calendar Girl: Note that it is a "line" worm, not a phylum Nematoda). They are called differently in different places, and wireworms are a more common name in Chinese. In English, wireworms are called horsehair worms and hairsnakes. These names are quite accurate for these brown-black nematodes that are 1 mm thick and tens of centimeters long.

Stenopelmatus infected with wireworms | jkirkhart35 / Wikimedia Commons

They actually have a more interesting name - Gordian worm, which I personally think is the most vivid. According to legend, Alexander came to the city of Gordium, where there was a tangled ball of headphone thread called the Gordian worm. Whoever could untie it would be the king of Asia Minor. As a result, Alexander split the worm with a sword, solved the problem, and conquered the place. Whether inside the host or excreted from the body, the iron wire worm usually entangles with each other like a coil, just like the Gordian worm.

The wireworm can parasitize many arthropods, but the most famous one is the wireworm that crawls out of the praying mantis. There is a reason for this. The wireworm has multiple parasitic processes in its life. Some insects, such as mayflies and mosquitoes, are their intermediate hosts (paratenic hosts), and some are their definitive hosts (definitive hosts). Because the mantis is the "top boss" in the insect food chain, it often accidentally preys on intermediate host insects infected with wireworms. Therefore, we tend to think that the mantis is a host "tailor-made" for the wireworm. In fact, not only mantises, but also katydids, crickets, locusts, and even spiders may become their definitive hosts.

The life cycle of a typical wireworm. The intermediate host is a small aquatic insect, in which the larvae parasitize. The final host is a large terrestrial insect. After the intermediate host is eaten, the wireworms are transferred to the final host. | Ben Hanelt et al. / PLoS ONE (2012)

The most chilling feature of the iron wire worm is that "the host will automatically jump into the water to commit suicide." The iron wire worm waits for an opportunity to "burst" out of the host, mate and lay eggs, completing its life cycle.

There is no scientific conclusion on how the wireworm controls the behavior of its host. One speculation is that the protein molecules secreted by the wireworm can affect the behavior of insects, such as making the host move to a lower place. Another view is that the wireworm may not control the host to "commit suicide" in the water, but use the host's vision to make them attracted by the light reflected from the water surface. Mantises infected with wireworms will also be attracted by horizontally polarized light similar to the reflection of the water surface.

But no matter what the reason is, the core factor is that the iron wire worms must find a water source because their eggs must grow and develop in the water and parasitize the new host.

Insects in amber excrete a wireworm, Paleochordodes protus | George Poinar / Advances in Biology (2014)

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How the Iron Worm Contributes to Sushi

At this point, I'm sure many of you will sigh: "It's disgusting, why does this creature exist on Earth?" First of all, the evolution of any species is not aimed at being "useful". The iron wire worm is part of the ecosystem, and it controls the number of some insects and stabilizes the ecological balance to its current position. This is easy to understand.

But the wireworms have a more direct benefit to us. When young Japanese char Salvelinus leucomaenis japonicus live in wetlands along freshwater rivers, 60% of their diet consists of drowned insects. Most of these insects jump into the water to commit suicide after being infected by the wireworms. Insects that are not infected will retreat from the water. Autumn is the time when young salmon are fattening, and it is also the time when wireworms infect arthropods in large numbers. Without wireworms, young salmon simply cannot get enough nutrition to cope with future marine life. The disappearance of salmon will also bring about some chain reactions - for example, the native habitat cannot get enough nitrogen brought by salmon migration.

A nematode expelled from a praying mantis | Frost Entomological Museum / Wikimedia Commons

Salmon is just one example, and there are many other animals that prey on drowned insects. Although many salmon are now farmed, before the salmon farming industry developed, when people dipped their salmon sushi in soy sauce, should they also thank the iron worms?

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The Storage Expert in the Mantis's Belly

The question that everyone is most concerned about is, can the iron worm parasite humans? There are some cases that show that when humans swim and play in rivers or accidentally drink water with iron worms, they may indeed be infected with iron worms (usually more women than men). But most iron worms can only stay in the human body for a short time. Mammals are not suitable hosts for them, and they cannot control human minds by infecting humans. This can only happen in arthropods. Occasionally, there are cases (mainly women) that iron worms can cause urinary tract infections, but there will be no serious harm. In any case, the horror scenes in the movie will never happen!

There are many other bizarre characteristics of the wireworms. For example, how can so many wireworms fit into such a small stomach of the praying mantis? If there are a bunch of wireworms living in the same host body, they will fight each other in this narrow "snail house" and finally occupy the host above the chest. Some will even squeeze out from the head and thorax! If there are only one or two wireworms in the host body and there is enough space, they will coil up neatly in the host's abdomen like small springs. The wireworms are storage experts and can arrange the small space in the host's stomach very well.

Chordodes formosanus, a nematode obtained from the mantis fish Hierodula formosana | Ming-Chung Chiu et al. / Parasitology (2015)

Sometimes you will see immobile "small white wireworms" in the wild. These may not be adult wireworms, but their "egg strips". Some wireworms mature early or encounter hosts that are reluctant to enter the water. They will squeeze out their "egg strips" one by one in the host's body like a noodle machine to deal with emergencies.

Eggs of the nematode | Cleo Szmygiel et al. / Zootaxa (2014)

Human beings’ knowledge of wireworms is still very limited, and wireworms still have many little secrets. Through understanding and observing wireworms, we can also better understand the survival code of parasites.

This article comes from the Species Calendar, welcome to forward

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