Last Updated on November 12, 2019

Although often called “nematode worms,” members of the Phylum Nematoda are only distantly related to earthworms in the garden and marine worms in the sea. They are among the most diverse of all animals, though most people in developed countries are only ever likely to encounter them in the form of the pinworm (threadworm), a common childhood infection. They occupy an astonishing range of habitats, but to the casual observer one nematode looks much like the next.

Nematodes Among the Most Diverse of Animals

If you were to study a nematode closely, you would see that it has a slender, tubular body like an earthworm, but without segments. You would have to look very carefully, though, as most measure just a couple of millimeters and the smallest are visible only through a microscope. At the other end of the scale, the largest parasitic species can grow to 30 cm or more, and the longest nematode ever recorded was an 8 meter specimen found in the placenta of a sperm whale.

Experts on nematodes – known as nematologists – have traditionally distinguished one species from another on the basis of differences in the shape of the head and reproductive organs, and the presence or absence of various bristles, ridges and other structures on the surface of the body. However, with the advent of molecular techniques and DNA sequencing has come a glimpse of the extraordinary diversity of the Nematoda.

More than 20,000 species have been identified to date, though it is clear that many more remain to be described. Some nematologists estimate the total number of nematode species to be as high as one million, or even 100 million. To put this into perspective, there are fewer than 5000 species of mammal worldwide.

Free-living Nematodes on (and Under) the Earth

Nematodes are found in a greater range of habitats than any other multicellular organism. Although only a minority of the known species are free-living, they are found all over the world, from the poles to the tropics, on mountains and in deserts, in lakes, oceans, rivers and seas.

The only limiting factor seems to be water, and even here nematodes’ needs are minimal. A thin film of moisture in the gap between two barren rocks is enough to support a thriving population of many thousand individuals. Other species have been described that live exclusively in vinegar, in the glue used to bind books and in German beer mats.

Nematodes have been found living in cracks in the Earth’s crust, 3.6 km below the surface – far deeper than any other known animal. Here, withstanding a temperature of 48°C and a pressure around 1000 times greater than we surface-dwellers experience, these tiny creatures survive by feeding on bacteria. Among the nematodes found in this extreme environment was a previously unknown species, now named Halicephalobus mephisto after Mephistopheles, the Faustian Lord of the Underworld. The scientists who discovered H. mephisto suggest that, when searching for extraterrestrial life, we should look beneath the surface before declaring a planet dead.

Parasitic Nematodes

Most nematodes are parasitic, living on or inside the bodies of other animals and plants. Individuals are often parasitized by several different species that have evolved to live in different parts of the body – another example of the remarkable diversity of these simple animals. Writing in 1914, N.A. Cobb of the US Bureau of Plant Industry poetically suggested that:

“[I]f all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable…its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes…The location of the various plants and animals would still be decipherable… in many cases even their species could be determined by an examination of their erstwhile nematode parasites.”

Why Are There So Many Nematode Species?

It used to be thought that all nematodes were essentially the same, which raised the question of how such homogeneous animals could evolve into so many different species in so many different ecological niches. However, more recent genetic studies have revealed that nematodes are also among the most genetically diverse of all animals. For example, looking at the genes that encode part of the cell’s protein-producing machinery – the ribosome – two nematode species can be as distinct from one another as a monkey is from a mouse. Given these newer findings, the diversity of the nematodes is rather less surprising.

Nematodes Are Everywhere

The phylum Nematoda may contain more species than any group of animals on Earth, and they are almost certainly the largest group numerically. A single handful of soil contains thousands of these microscopic creatures, but due to their small size humans are generally unaware of their presence around – and inside – everyone of us.

Sources

Borgonie, G., García-Moyano, A., Litthauer, D. et al. Nematoda from the Terrestrial Deep Subsurface of South Africa. Nature. Accessed 16-08-11.

Cobb, N. Nematodes and their Relationships. Accessed 02-09-11.

Darwin Nematode Project. Nematode Structure. Accessed 02-09-11.

Ferris, H. The Beer Mat Nematode, Panagrellus redivivus: A Study of the Connectedness of Scientific Discovery. Journal of Nematode Morphology and Systematics. Accessed 02-09-11.

Paul’s Bizarre Worm Bazaar. Frequently Asked Questions. Accessed 02-09-11.

Society of Nematologists. Information on Nematology. Accessed 18-08-11.