Nematodes are the most abundant multicellular animals on the face of the earth. They occur literally everywhere – in soil and decaying matter from the poles to the tropics, in all forms of plant life, in the bodies of almost all animals, including humans, and in insects.
Living in such diverse environments as the sand and mud of the ocean bottom, stony mountain soils, and arid polar deserts are thousands to millions of nematodes per square meter.
Nematodes are nonsegmented roundworms with complete sensory, digestive, excretory, and reproductive systems. Most, but not all, are microscopic. The variety of nematode forms and habitats is almost unbelievable: they range from the minute inhabitant of your favorite mushroom to the 27-foot-long parasite in the placenta of a sperm whale.
Nematodes are essential elements of ecosystems, but most have no direct effect on humans. Those that do, however, can be devastating. In many places, people still suffer from diseases such as elephantiasis, river blindness, and hookworm, caused by nematodes. In most places, the effect on humans is indirect. For example, in the United States, plant-parasitic nematodes cause more than 3 billion dollars’ worth of crop losses each year, and cause similar losses in cattle, sheep, and swine.
The study of these little-known but immensely important creatures is the life’s work of nematologists. Nematologists are rarely found in a “Department of Nematology,” but are found throughout various disciplines depending on the nature of the organisms they study.
Plant-parasitic nematodes occur wherever plants grow. These nematodes are referred to as plant-pathogenic when they cause crop losses by direct injury to roots, stems, leaves, and seeds. All plants are susceptible to attack by one or more species of plant-parasitic nematodes, and the associations can range from transitory grazing by root hair feeders to the highly complex host-pathogen interactions of gall-inducing nematodes and their hosts. Plant nematologists study the structure, function, ecology, molecular biology, physiology, and classification of plant-parasitic nematodes and the diseases they cause. Plant-pathogenic nematodes are the subjects of studies to determine control methods.
Protocol for identifying plant resistance to plant parasitic nematodes: Download File (PDF)
HUMAN AND ANIMAL PARASITES
Nematodes can sap energy, cause serious diseases, and even kill humans and animals. Sheep, dogs, and cats are especially vulnerable. Parasitic nematodes such as hookworm suck blood and cause anemia. Ascarids live in the intestines of people, pigs, and other animals, and compete for nourishment with the host. Young ascarids migrate through the hosts’s body and cause a type of pneumonia. Trichinella spiralis causes muscle pain and even death in people who eat insufficiently cooked, infected pork.
Insect-parasitic nematodes as a group have diverse array of host associations, ranging from those in which the nematodes merely uses the insect as a mode of transportation from one location to another, to those in which the nematodes weakens or even kills its insect host. Some entomophilic nematodes have become economically important as biological control agents.
DISTRIBUTION, ABUNDANCE, AND DIVERSITY OF NONPATHOGENIC NEMATODES
The structural diversity of nonparasitic nematodes reflects the variety of ecologic niches they occupy. Many feed on bacteria, and thus help soils retain the minerals and organic materials that might otherwise be lost, and even regulate the rate of nutrient cycling. Other nematodes feed on unicellular algae and other creatures, and are in turn the food of many higher animals. Substantial evidence shows the significant roles nematodes play in the ecology of soil and aquatic habitats.
For further information see the Ecology Manual.