What are nematodes?
Take a handful of soil from almost anywhere in the world, from the Arctic to the Tropics, from the tops of mountains to the depths of seas, from deserts to swamps, extract the living organisms in water, and among the other forms of life you will find elongate, threadlike, active animals – these are nematodes (or eelworms, or roundworms). Many of them will just be visible without magnification, but others will only be seen with a good magnifying lens or microscope.
Or, if you catch a fish, bird, or mammal, dissect out its stomach or intestine, in most cases you will find some nematodes living there.
Nematodes (the name is derived from the Greek word for thread) are elongate, tubular organisms that move like snakes or eels. They are aquatic and live in marine or fresh water, and in films of water within soil/compost/forest litter/moss etc. They are one of the most successful and adaptable of animal groups, being rivalled only by insects as regards range of habitats or number of species.
Most species are free-living, with their food consisting of micro-organisms – fungi, bacteria, and algae, and these play an important role in decomposition and recycling of nutrients. Nematodes occur in soil in great numbers-rich arable soil may contain 3 billion (3,000,000,000) nematodes per acre (0.4 hectares), calculated to a depth of 20 cm.
Many nematodes are highly successful parasites. The first recorded ones were, not surprisingly, human parasites. Passages in the “Papyrus Ebers” (dated 1550 BC) are said to contain reference to the human intestinal parasite, Ascaris lumbricoides, and the tissue parasite, Dracunculus medinensis, (the Guinea Worm), both of which are large parasites which may exceed a foot in length. The Guinea Worm is believed to have been the fiery serpent of the bible which plagued the Children of Israel after their journey through the swampy regions near the Red Sea. Certainly this pest is still found there today.
Many of these animal parasites have intricate life cycles during which the parasites alternate between their vertebrate hosts and invertebrates such as arthropods and crustacea.
Although nematodes have undoubtedly been associated with plants for thousands of years, the first plant-parasitic nematode was reported in 1743 by an English clergyman, Needham. He was examining some dry, diseased wheat kernels in a drop of water, and after a time, to his amazement, he saw a mass of fibres that began to twist. He wrote:
“I am satisfied that they are a species of aquatic animal and may be denominated worms, eels, or serpents, which they much resemble.”
Whereas animal parasitic nematodes may be large, (the largest of all being Placentonema gigantissima, a monster nine metres long found in the placenta of sperm whales), the plant parasites are mainly microscopic. The largest, the longidorids, are 5-10mm long, but many nematodes are shorter than 1 mm. Being slender and transparent, they cannot often be seen by the naked eye.
Other groups of worms may be confused with nematodes. These include flatworms, (Platyhelminthes), e.g. liver flukes, which are flattened and oval in shape; tapeworms (Cestodes), most of which are adapted for living in the gut of vertebrates and have a body of rectangular flattened segments with suckers and hooks at the anterior end; and earthworms, which are long and segmented. The white, wriggling worm-like organisms, commonly seen in rotting plant material or compost, are seldom nematodes. With a few exceptions, if you can see an organism, with the naked eye, it is not a plant-parasitic nematode.
Although nematodes are typically worm-shaped, in one particular group of plant-parasitic nematodes the cuticle of the females develops into a tough wall creating a spherical, or near-spherical, cyst protecting the female’s fertilised eggs. These cyst nematodes are the most economically important nematode pests of temperate agriculture. The potato cyst nematodes, Globodera pallida and Globodera rostochiensis, introduced into Europe with potatoes from South America, have subsequently spread throughout most of the potato growing areas of the world. They are treated as quarantine pests because of the economic damage they cause and the difficulties faced when eradication is attempted.