Nematodes: not well understood

Having recently discovered a few of the wonders of what goes on in our soils, and writing a piece about mycorrhizae, I met one of our very few experts on soil life – Dr Michael Wilson, a recent import from the UK and working at AgResearch, Ruakura.


Nematodes are the most numerous multicellular animal on Earth.

After an hour with Mike, my new fascination is nematodes, which are his speciality. So what are nematodes, and what do they do in the scheme of things?

Apparently there are many thousands of different ones, varying in size between 0.1mm in soil, to a huge 8m long one sometimes found in the innards of sperm whales. It’s claimed they are the most numerous multicellular animals on Earth.

Because most of them are tiny, only those researchers with strong microscopes and
a lot of patience have been able to study even a few nematodes.

As with many microscopic creatures, there are ‘goodies’ and ‘baddies’, meaning the goodies eat up bad stuff like nasty bacteria, and the baddies are parasitic on roots, plants, and are known as ‘roundworms’ when they turn up inside animals and humans.

Free-living
There has been a lot of research done on the baddies, mainly because their efforts affect both health and the economy.

On the goodies we have some knowledge of some of the soil living ones, often referred to as ‘free living’. And apparently the goodies species outnumber the baddies by about 10:1.

No matter what their size, nematodes have a recognisable body shape and structure. Basically they are unsegmented worms, which consist of a multipurpose digestive and
egg-laying tube within a tougher outer cuticle like a very thick skin.

All are pointed at the tail end, and have mouthparts which vary according to what they eat.  

The bacteria-eating ones have a sucking tube up front, while the parasitic ones have a spike which allows them to break into cell walls in whatever host they select. But like snakes and worms, they can wriggle, which allows them to move, mainly sideways. And like earthworms, they improve what they eat, and what comes out the tail end is more useful for the relevant bacteria and for plant roots to absorb.

Some species have males, which do what males do to help make more, while others –particularly in plants and soils – have so few males they mostly manage with parthenogenetic reproduction. After the egg stage there can be up to four juvenile stages where the outer cuticle is ‘moulted’ but there is no metamorphosis.

Fungi munching
In most soils there are a variety of nematodes eating different things. Some eat only bacteria, while others munch on fungi.

Both are needed in decomposition. There are predators which feed on other soil nematodes and on other animals of comparable size, while others seem to eat anything.

Some species load up on bacteria, then wriggle their way into any aperture in an insect.  Then they exude the bacteria, which multiply madly and either kill the insect by poisoning, or just cause it to explode.

While the bacteria do their stuff, the nematode produces eggs galore, which then go through three of the juvenile stages before moving outside to find a new insect to move into. They can cause significant reductions in harmful insect populations.

Pasture slugs
At present research is being done to see if nematodes of a certain species can assist in controlling slugs in pasture and maize crops at the time of shoot emergence. However, first they have to be able to prove this particular nematode was already in New Zealand, as the Hazardous Substances and New Organisms Act doesn’t want any new ones moving in, in case they develop a taste for something we value.

One interesting experiment carried out here using superphosphate to stimulate growth, found the nematode population ballooned after application while shoot growth stalled, because the extra nematodes were eating the roots and the plants were trying to keep up root growth to cope.

While chemicals have allowed farmers to boost pasture growth, after 30 years this has slowed, and we are only starting to realise just what we do to disrupt the complexity of soil life which we ultimately rely on for all of our food.

For more information from a New Zealand source go to http://soilbugs.massey.ac.nz/nematoda.php


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