Nematodes are highly abundant, non-segmented worms that are present in a variety of habitats. Many are free-living (such as those that live in the soil), while others are parasitic and must attach themselves to a plant, insect, animal or human host to survive and reproduce. Some common nematodes include roundworms, Ascaris, hookworms and Trichinella. There are a number of characteristics shared by members of this large phylum.
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Most nematodes have the same simple body plan. Their bodies are bilaterally symmetrical (one half is a mirror image of the other), and while most are microscopic, they can grow to as long as 8 meters. Many have a “tube-within-a-tube” body plan comprised of a long, cylindrical body that encloses a hose-like canal (called an alimentary canal). Food enters the alimentary canal on one end, and waste is expelled through the anus on the tail end. While nematodes have digestive, reproductive, nervous and excretory systems, they do not have a distinct circulatory or respiratory system. Adults are made up of roughly 1,000 somatic cells, and hundreds of those cells are typically associated with the reproductive system.
Basic Life Cycle
There is diversity within the life cycles of different nematodes; however, all nematodes follow the same basic pattern of growth and reproduction. The nematode life cycle has seven stages, including an egg, four larval stages and two adult stages. Sexual reproduction is generally initiated by adult female nematodes that have attached themselves to a host organism. The female lays eggs that are passed by the host to the external environment where the eggs then pass through three developmental stages before becoming larvae.
Abundant and Diverse in Habitat
Nematodes are the most abundant group of multicellular organisms on earth. Approximately 20,000 different species of nematodes have been identified, although it is thought that there could be as many as one million, many of which have eluded identification because of their microscopic size.
Nematodes can be found in diverse habitats, including the ocean, soil, roots, stems and leaves of plants, fresh water, and tissues of animals and insects. In a single cubic foot of soil, as many as one million nematodes may be present.
Nematodes play an important role in the ecosystem. Some nematodes have the potential to harm the ecosystem by killing plants, insects and animals. For example, each year, nematode parasites cause billions of dollars in lost production to plant and vegetable growers in the United States. On the other hand, some insect parasitic nematodes are effectively used as biological control agents to control the population of pest insects such as mosquitoes.
Nematodes are also used as indicators of soil health. A study published in a 2008 issue of “Applied Soil Ecology” found that the composition of nematodes in the soil was an indicator of the biological properties of the soil used by banana growers. When used with other indicators such as measures of soil density and acidity, information about nematode composition could help improve the sustainability and environmental accountability of the banana industry in Australia.