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Living off another

Maneka Gandhi |

I ’ve just finished reading Ten Million Aliens by Simon Barnes, a collection of articles that looks at animals in a new way. According to Barnes, 80 per cent of the living animals on Earth are nematode worms. There are 28,000 species of which 16,000 are parasites.

The longest known nematode is 13 metres long and it resides in the sperm whale. Intriguingly, one cubic metre of soil contains more than a million. They are found everywhere — fresh and salt water, mountains, bottom of the ocean, deserts and marshes. Two species, Halicephalobus mephisto and Plectus aquatilislive as deep as 3.6 km beneath the Earth’s surface and are the deepestliving multi-cellular organisms.

They are herbivorous, carnivorous or parasitic. Their common name is roundworms. They have a head, muscles, a mouth with teeth and an anus. Nematodes breathe across their entire body surface and they have a flexible skin called a cuticle, which they shed often. They survive heat, drought and snow, and simply ride out bad weather by wrapping themselves up into a cyst and shutting down, and then coming back to life when things are better.

The nematode Caenorhabditis elegans is famous because it was the only living being to survive the Space Shuttle Columbia disaster in 2003! One nematode can lay thousands of eggs. Nematodes are also characterised by an unusual feature called “eutely”, in which every individual of the same species has exactly the same number of cells.

They parasitise plants, animals and humans. We know them as hookworms, threadworms, lungworms, pinworms and whipworms. It is estimated that 25 per cent of humans are infested with nematodes, especially those that live in hot, overcrowded spaces with bad water — also those humans that don't wash their hands after they go to the bathroom. Most nematode infections come from meat and fish.

You can get stomach aches and diarrhoea from their eggs, and they are the second biggest reason for blindness. How many ingenious ways do nematodes invent to survive? One species infects a tropical ant and causes its abdomen to become bright red.

Those ants become sluggish. Finally, they get eaten by birds who mistake them for red berries. The worms develop in the birds and are then excreted. The faeces of the bird are gathered by the same species of ant to feed its larvae. And so the cycle goes on!

The Sphaerularia bombi nematode parasitises bumblebees. It moves into the bloodstream and then throws out its uterus. This swells into a huge long sac that is 20 times larger than the rest of the worm. The uterus becomes a giant feeding organ, taking in nutrients from the bee’s blood. In 1862, the American Civil War left 16,000 soldiers wounded in the battle of Shiloh, Tennessee.

The wounded soldiers sat in the mud for two cold, rainy, days waiting for medical help. The first night they noticed something very strange — their wounds were glowing with a faint light. It was discovered that those whose injuries glowed had a better survival rate and their wounds healed quicker. In 2001, Bill Martin and Jon Curtis researched the bacteria found during the Battle of Shiloh.

They learned that Photorhabdus luminescens bacteria live in the guts of nematodes. Nematodes hunt down insect larvae in the soil, or on plants, and burrow into their bodies to parasitise them. They vomit out these bacteria which start glowing with a soft blue colour and produce chemicals that kill the insect host and all the other organisms already inside it. The nematode and bacteria feed, grow and multiply till the insect corpse is hollow. Then the nematode eats the bacteria, which hitches a ride to the next insect. The next insect victim comes quickly because the glow of the bacteria attracts insects to the body.

The soldiers were saved because the chemicals used by the bacteria killed off other pathogens that might have infected their wounds. The Grasshopper nematode, Mermis nigrens, attaches its eggs to plants. When grasshoppers come to feed on the leaves they eat the eggs.

The nematodes feed and grow in the body and finally the grasshopper dies. The nematodes move into the soil, mate and the egg-bearing females emerge after the rains to lay their eggs on foliage to repeat the cycle. Horsehair worms, Gordius robustus, have very thin, brown bodies with a blunt head and cleft in the hind end. They lay their eggs in long masses in water. The larvae are eaten by insects, like grasshoppers, crickets, beetles and caddis flies, as they drink water.

The larvae feed on their tissues and blood. When the larva turns into an adult and needs to exit, it changes the behaviour of its host who seeks out water and throws itself into it. On the other hand, the nematode Pristionchus pacificus lives in the body of a dung beetle. It lays eggs only when the beetle dies, and the eggs live off the corpse. But how do the nematodes get into the beetle to begin with? Hundreds of larvae converge and glue themselves together into a single, squirming “worm tower,” which waits for a beetle to pass overhead. It is the only structure of its kind in nature. The nematode Heterorhabditis bacteriophora is even smarter.

It doesn’t feed directly on the moth caterpillars it infests. It uses the guts of the caterpillar to farm other bacteria, which it feeds on. They eventually become so numerous that the caterpillar dies before it becomes a moth. What does the nematode do to make sure the caterpillar is not eaten by birds?

It turns the caterpillar from almost colourless to a pinkish red, and it starts producing light, which birds and other insects can see and who quickly learn that the red, glowing bugs taste disgusting. This ensures that the infected insect will belong only to the worm. It is the only known example of a parasite that changes its host’s appearance to keep predators away