Ne­ma­todes

The sur­vival and pro­lif­er­a­tion tricks of these par­a­sitic liv­ing be­ings are sim­ply amaz­ing.

Alive - - News - ■ by Maneka San­jay Gandhi

Ihave 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.

They range from 2.5 mm to me­ters long. The long­est known ne­ma­tode is 13 me­tres long and it re­sides in the sperm whale. One cu­bic me­tre of soil con­tains more than a mil­lion. They are found ev­ery­where: fresh and salt wa­ter, moun­tains, the bot­tom of the ocean, deserts and marshes.

Two species, Hal­i­cephalobus mephisto and Plec­tus aquatilis live as deep as 3.6 km be­neath the Earth’s sur­face and are the deep­est-liv­ing mul­ti­cel­lu­lar or­gan­isms known. 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. Their bod­ies have a flex­i­ble skin called a cu­ti­cle and they shed it of­ten.

They sur­vive heat, drought and snow, and sim­ply ride out bad weather by wrap­ping them­selves up in 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 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. You 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; es­pe­cially 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 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 twenty 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.

Vic­tim of ne­ma­todes

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 the sol­diers’ 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.

Change of be­hav­iour

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 cad­dis­flies, 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.

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. This is the only known ex­am­ple of a par­a­site that changes its host’s ap­pear­ance to keep its preda­tors away.

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