Nematoda

M. Hodda
Nematode Biosystematics & Ecology
National Research Collections Australia, CSIRO, GPO Box 1700, CANBERRA ACT 2601, Australia

Introduction to the Phylum Nematoda

Nematodes are the most abundant and ubiquitous multicellular organisms on earth. They are found from the bottom of the deepest ocean to near the tops of the highest mountains, from the tropics to polar regions, and from every conceivable habitat. Nematodes are also found in or on most other types of organisms as parasites, commensals or phoretics: everything from earthworms, insects, molluscs, fish, reptiles birds, mammals to humans. In fact it is said that if the everything on the earth were to disappear except the nematodes, the outlines of everything would still be visible: the mountains, lakes and oceans, the plants and the animals would all be outlined by the nematodes living in every habitat.

Nematodes are also amongst the most diverse taxa on earth, with an estimated 500 000 to 1 000 000 species. Only about 20 000 species have been described, and the systematic literature is widely dispersed. It is hoped this key will fill some of this void.

Nematodes range in size from nearly 10 metres long to only a fraction of a millimetre. They are mostly worm-like in shape, but some are spherical, lemon-shaped, sausage-shaped and a host of other variations. In diameter, they range from a few millimetres to about 5 microns.

Although they are small, nematodes are incredibly abundant. If all the nematodes in the Murray River were combined into one huge animal, it would be over 20 metres long (see image left). Laid end-to-end they would encircle the earth’s equator. Nematodes are even more abundant in terrestrial, estuarine and marine substrates.

Nematodes impact on human economies in many ways: from loss of agricultural production, to pasture and turf damage, to invasion of forest trees, to adverse effects on the health of wild and domestic animals and humans. In addition to causing losses, there are many beneficial effects of nematodes: controlling soil nutrient cycling and controlling harmful species to name just two. The total value of these economic effects are difficult to quantify, but must be very substantial indeed if one considers that the lost production to just one crop (wheat) in just one country (Australia) is estimated at $200 million per annum. Illustrated at right is the effect of plant parasitic nematodes on a canola crop.

Because of their ubiquity and diversity, nematodes are also used in measuring the impacts of various perturbations on ecosystems, such as pollution, organic enrichment, and physical disturbance. Nematodes are intimately involved in many parts of the soil ecosystem, so they can be used as indicators of sustainability for soils.