Nematodes are small, pale, unsegmented roundworms, sometimes called threadworms or eelworms. Under the microscope, they appear as very thin tubes with a definite blunt shaped "head"and a thin tail. They grow up to about 1 mm in length although some can reach over 5 mm in length. There has been one report of a nematode living inside a sperm whale and being 8 m in length. Over 28,000 different species have been described but as they are a very diverse group of organism the number of species may be much higher with some estimates for the number of species of nematodes range from 100,000 to 100,000,000 (Luc et al., 2010). Molecular biology tools are now being used to support traditional species descriptions using morphological characteristics (Van Megan et al., 2009).
The number of nematodes in a soil does vary with temperate soils tending to have more diversity than those in the tropics although this could be a reflection on the amount of research which is carried out in the tropics (Powers et al., 2009). In forest litter and soil there are about 10 million individuals per m2 and over 400 species (Yeates, 2007).
Many nematodes live on decaying organic matter (saprophyes) or are predatory on other micro-organisms. Some, such as the parasitic genus Heterodera, infest the roots of plants and if the resulting wounds become heavily infested by secondary pathogens this results in stunting and decreased yield especially to soyabeans, solonaceous crops and fruit trees. Damaged roots, through drought for example are prone to nematode attack. Some species are parasites of man and other animals.
Nematodes have been controlled by long rotations (e.g. 5 years) with non-host crops, including for example canola (rapeseed) which produce root exudates with nematocidal properties, use of genetically resistant varieties and soil fumigation. In recent years and through international efforts, nematocides such as methyl bromide are being strictly restricted because of toxic environmental effects.
Moderate effects on soil have only transitory effects on nematode populations although major disturbance for example slash and burn management of forests which disturb the understory layers can reduce nematode population numbers.
Needing a film of water to move in, nematodes prefer moist, coarse textured soils, but some species favour sandy soils, whilst others more clay-soils. Soil pH appears crucial to nematode survival with pH 6.0 being about the optimum. Acid or alkaline soils tend not to favour nematodes. During unfavourable conditions they form survival structures called cysts.
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