Introduction to Nematodes

Nematodes are invertebrate roundworms that inhabit marine, freshwater, and terrestrial environments. They comprise the phylum Nematoda (or Nemata) which includes parasites of plants and of animals, including humans, as well as species that feed on bacteria, fungi, algae, and on other nematodes. Four out of every five multicellular animals on the planet are nematodes (Platt, 1994). Cobb (1914) calculated that if the nematodes resident in a single acre of soil near San Antonio, Texas, USA, were to proceed in head-to-tail procession to Washington D.C., some 2000 miles away, the first nematode would reach Washington before the rear of the procession left San Antonio!

The majority of nematodes are microscopic, averaging less than a millimeter in length, but some of the animal parasites are quite large and readily visible to the naked eye. The animal and plant parasites are of direct importance in agriculture, the environment, and in human health; however, most nematodes in the environment are not parasites. Nematodes that feed on other organisms are important participants in the cycling of minerals and nutrients in the ecosystem that is fundamental to other biological activity. Some of these nematodes may have major roles in decomposition, including biodegradation of toxic compounds. In fact, the incidence of certain nematode species is sometimes used as an indicator of environmental quality. Insect-parasitic nematodes can be of importance in regulating insect populations, and are being used in the biological control of insect pests.

The management of plant-parasitic nematodes has been fundamental to advances in agricultural production in the United States and worldwide. However, the use of certain pesticides, such as 1,2-Dibromo-3-chloropropane (DBCP) to control nematodes, has resulted in contamination of soil and groundwater in California. Current research seeks integrated and sustainable approaches to the management of nematode
populations through genetic manipulation of host plants, design of cropping systems, development of suppressive chemicals of botanical origin, and biological control.

There is currently a major effort to shift from nematicide-based nematode management to alternative methods. Some 15 million pounds of 1,3-Dichloropropene have been used annually on approximately 200,000 acres in California. Alternatives to this approach are critical, particularly following the April 13, 1990 suspension by the California Department of Food and Agriculture (CDFA) of the use of 1,3-Dichloropropene. Since alternative approaches are more target-specific and system-specific, priorities will have to be established for research and development in applied nematology. At the same time, strong linkages to more basic areas of nematode biology must be fostered and maintained to promote the development of new technology.

Of the nineteen Orders in the phylum Nematoda, seven contain nematodes that are parasites or associates of invertebrates, and six include species that are parasites of vertebrate animals. Nematodes are reported as parasites and associates of many invertebrate animals, especially in the Annelida, Mollusca, and Arthropoda. In some cases, the invertebrate functions as the intermediate host in a life-cycle that includes parasitism of a vertebrate. In other cases, the invertebrate, usually an insect, functions as a vector between vertebrate hosts, or the nematode is passively transported by the insect. Several interesting plant-
parasitic nematodes fall into this latter group and, significantly, they are closely related to nematode species that are parasites of insects. A considerable research effort has been applied toward using nematode parasites of insects as biological control agents, e.g., for mosquitos and blackflies (Maggenti, 1981).

Some of the nematode associates of insects are important because they vector bacteria that kill the insect. The nematode invades (or is consumed by) the insect, and bacteria are released into the insect hemolymph. When the insect is dead or near death, growth and subsequent development of nematodes occur as they utilize essential steroids supplied by the insect (Maggenti, 1981). These nematodes are also used extensively in the biological control of insects and are particularly effective against those insects that pass through at least one life
stage in the soil.

Some 5,000 species of nematodes are estimated to be parasites of vertebrate animals and humans. These species are often characterized in a larger group of worm parasites as helminths. Nematode parasites of domestic vertebrate animals are managed by strategies that include control of secondary hosts or vectors and the use of chemical anthelminthics. Helminth infections of wild animals are, of course, not managed, except by attrition of infected individuals. As human demography patterns change in California, and throughout the world, the interface between the ranges and habitats of wild and domestic animals change and overlap. Consequently, the pattern of exposure of domestic animals to helminth infections is also changing, and new associations continue to be reported; for example, the incidence of heartworm (Dirofilaria immitis) infection in dogs is currently increasing in California.

Both freshwater and marine fish are subject to nematode infections. The impact of the infections on the health and longevity of fish in nature is generally unknown. Frequently, nematodes are observed in the tissues of fish purchased by consumers. The nematodes are usually killed during cooking, but certainly the transfer of live fish parasites to humans can occur during consumption of sashimi and other raw fish products. Generally, these nematodes will not establish a permanent infection in humans, but they may cause intestinal disorders in
attempting to do so.

Other helminth infections are vectored as filarial worms by insects such as mosquitos, or the filaria may penetrate directly through the skin from water or soil. Filarial worms cause such diseases as river blindness (Onchocerca volvulus) and elephantiasis which are major health problems in some third-world countries. In the United States, most helminth infections of humans are controlled by public health programs, public education,
vector control, intermediate host control, and anthelminthic drugs. However, changing demographic patterns, including the immigration of new California residents from third-world countries, has resulted in the introduction of unfamiliar helminth infections into the state. Frequently, the faculty in the Departments of Nematology are consulted by public health officials for identification of unfamiliar nematodes.

In California, forestry is important for lumber, lumber products, and recreation. Assessment of the nematode impact on lumber yield and quality in forestry, however, is especially difficult since harvest cycles may be as long as 75 years, and land geography and soil composition are extremely diverse. Nematode impact may be via plant-parasitism; fungal feeding by nematodes, resulting in destruction of mycorrhizae essential to forest growth; and above-ground parasitic nematode parasites vectored by insects, e.g., pinewood nematode.
Preliminary investigations have indicated that there is an entirely new array of nematode pests indigenous to California forests, in addition to known agricultural pests, which feed on native forestry species. It is important that we begin to accumulate baseline information on nematode pests of California forests, particularly since no such programs exist in the western United States.

Recent plans for shipment of raw lumber from Siberia for processing in northern California may assuage public concerns regarding deforestation in the U.S.; however, analyses of Siberian logs have revealed a high potential for introduction of new pests. Included among the pests detected in the lumber is the nematode, Bursaphelenchus mucronatus, and its associated insect vectors. B. mucronatus is not reported from the U.S. and is a close relative of the pinewood nematode that has devastated forests in Japan. Although the pinewood nematode already occurs in the U.S., its detection in wood chips exported to Scandinavia has resulted in a ban on those products. Certainly there are approaches to mitigating the potential for introduction of new pests, including debarking and fumigation of logs prior to shipment.

    With respect to rangeland, it is necessary to consider a different array of nematodes than those which impact agriculture and forestry. The nematode problems in range include not only root and foliar parasites, but also diseases resulting from nematode-microorganismal associations.   The latter aspect is perhaps best illustrated by a nematode-bacterial association in which the grass forage becomes toxic to livestock; this situation constitutes a major problem in Australia where large numbers of sheep and, in some areas, entire unattended
flocks have been killed.   This nematode (Anguina agrostis) and the associated bacterium (Clavibacter sp.) also exist in northern California and southern Oregon.    Because the livestock and dairy industries constitute a major component of agricultural production in California, and because rangeland and forests constitute approximately two-thirds of California land area, the significance of nematode problems on rangeland, and the need for additional research in this area, becomes clear.

Zullini and Semprucci compared the characteristics of soil inhabiting and freshwater-inhabiting nematodes. They noted that aquatic (=open water) and semiaquatic species are, on average, longer and slimmer than soil species, they have a longer tail, greater body weight, smooth cuticle and larger amphids. Usually, but not always, nematodes living in and on freshwater sediments are characterized by:

i) Long cephalic and somatic setae. Setae are essentially useful sensory devices. The restrictive thickness of the water film around soil particles would inhibit their function in soil. Consequently,, they are generally reduced or absent in soil-inhabiting species. However, they are not always present in freshwater species. Freshwater species lacking setae, such as Dorylaimida, Mononchida and Rhabditida, are closely related to soil species

ii) Large amphids. Chemoreceptor organs such as amphids perform a different role in soil solution which is rich in salts and dissolved organic matter and where the chemical information travels a very short distance. In open fresh water, usually less rich in dissolved substances, the chemical signal travels long distances and quite rapidly. The signal strength changes slowly in soil but dissipates faster in open water. Soil species, and freshwater species related to soil species, usually have small, sometimes punctiform, amphids.

iii) Ocelli. Light receptors are fairly common in marine nematodes but are rare in freshwater species. In soil species, they are generally absent

iv) Caudal glands and spinneret. Glands secreting a sticky substance through the spinneret, for anchoring the tip of the tail, are useful for nematodes living at the surface of the sediment to avoid the effect of water currents. Not all aquatic species have caudal glands and spinnerets,

Analyses of nematode communities in aquatic environments reveals that the incidence and prevalence of species in the community reflect the nature and quality of the environment. Not surprisingly, the types of species present (and the resultant community structure) differ in marine, brackish, and freshwater environments. Recent observations indicate that various nematode species respond differently to degradation of environmental quality. Thus, the degree and nature of change in the community structure of aquatic nematodes may be an excellent indicator of water quality or pollutant levels.

Nematodes in freshwater aquatic systems also serve as a nutrient source for invertebrates, small vertebrates, and fungi. The source of food for these nematodes is primarily bacteria, but algae and fungi are also consumed. A considerable number of plant-parasitic nematodes in aquatic systems are associated with higher plants, although the impact of their parasitism on those plants is generally unknown. Preliminary research indicates a potential for management of these nematodes in the biological control of aquatic weeds (Gerber and Smart, 1987). Genera that parasitize crop plants grown in immersed culture (e.g., rice) are well-characterized and are extremely important crop pests worldwide.

The marine environment provides habitat for an enormous diversity of nematodes, from surface, littoral and estuarine zones to the ocean depths. One interesting group of deep sea nematodes are the Rhaptothyridae, which have no mouth and a very reduced alimentary tract. The digestive tract is filled with symbiotic chemoaototrophic bacteria . A similar relationship exists in the mouthless genus Astomonema. A Darwin Initiative Project between 199 and 2002 focused on Nematode Biodiversity and Worldwide Pollution Monitoring.

Nematodes are the most abundant metazoans in marine (littoral, estuarine, coastal and oceanic) sediments, from the high-water mark into the deepest oceanic trenches (Nicholas, 1975). All marine free-living nematodes are considered to be members of the meiobenthos; small organisms, mainly metazoans, which are separated from the larger macrobenthos in that they will pass through a 1 mm or on a taxonomic basis as belonging to specific groups of small animals that include Nematoda, Harpacticoida, Gastrotricha, Kinorhyncha, Tardigrada, Foraminifera etc. Usually, the nematodes are the dominant group of the meiofauna, often comprising more than 90% of the metazoan fauna.

The ecological success of marine nematodes is also demonstrated by their high species diversity. The number of species present in anyone habitat is usually an order or magnitude greater than for any other major taxon. In 0.25m² of marine bottom their densituy can be >10⁶ individuals. (Mokievsky et al. 2004, 2007). About 4,000–5,000 species of marine nematodes have been described and more are discovered and described from ongoing marine exploration projects. Most species have been described from relatively accessible shallow waters, but shallorw waters constitute only 9% of the world's oceans; 91% of the ocean bottom is in the "deep sea"and studies thus far have reported over 600 species at depths ranging from 400-8350 meters deep (Miljutin et al., 2010). Clearly there is an enormous volume of ocean yet to be explored.

    Individuals create a personal environment involving trees, shrubs, other annual and perennial ornamentals, ground covers, lawns, indoor plants, and vegetable gardens.    Frequently, the plants are placed directly into soil already containing plant-parasitic nematodes from a previous planting; nematodes may also be introduced with the current planting in associated soil and roots. Obviously, the layout and plant species   involved   in   urban   gardening   are   not   selected   for incompatibilities of their associated nematode species.    Nematodes
introduced or supported by one plant species can be very damaging to neighboring or companion plants. Such individual plants have aesthetic value or generate emotional attachments and, therefore, have a high individual value.   Nematode damage to lawns and groundcover often results in poor growth, bare spots, and chlorotic appearance which the individual attempts to remedy by increasing the application of both fertilizer and water.

Societal components of urban nematology include nematode effects in parks, recreation areas, landscape plantings, highways, schools, etc. Nematode problems of turf constitute an area of concern for recreational and landscape industries. Again, all these plantings have tremendous value in terms of management input and public perception. Damage and lack of plant vigor due to nematode parasitism result in unsightly areas, increased weed competition, and increased water and fertilizer usage. Commercial components of urban nematology include the costs of nematode management-or of lack of nematode management-by producers of plants in nurseries, turf farms, and greenhouses. The economics of urban nematology also include the activities of agrichemical industries, wholesalers, retailers, advisors and consultants.

As with other areas of nematology, we should consider the positive and negative aspects of nematodes in the urban environment. Nematodes can act primarily as pests or pathogens, or can also participate in decomposer food chains by using dead organisms as sources of carbon and energy. Often, the most productive soils contain high populations of non-parasitic nematodes participating in the cycling of minerals and nutrients. In fact, high population levels of such nematodes may be an indicator of productive soils with a healthy biological status.

As knowledge of nematode problems in urban situations increases, so will the potential to design spatial patterns and sequences of backyard gardens to provide the ultimate in nematode management options. Planting sequences and companion plantings can be selected with nematode susceptibilities, or even allelopathic effects, in mind. Enhancing the biological activity of soil to improve nutrient and moisture status, as well as the biological antagonism of nematodes, can be accomplished with minimal cost through the intensive efforts of avid gardeners in small-scale areas. The backyard garden offers the possibility of creating a microcosmic "sustainable agriculture" situation. Here is an environment that the homeowner can control and manipulate experimentally without the overriding concerns of marketability or profitability of crops. Experimentation with soil manipulation, incorporation of organic matter, companion plants, sequences, and spatial patterns of plants may be a rich source of innovations and provide important biological insights. Biological antagonists of nematodes can be introduced and tested; beneficial nematodes for the biocontrol of insect pests can also be introduced and conserved in the system. The opportunities for observation, formulation, and testing of hypotheses are more readily provided by the backyard garden than by commercial agriculture. To capitalize on those opportunities, many research questions must be addressed; these include plant species compatibilities (in terms of host status or effects on various nematode species), and knowledge of the nematodes present at a given site and their virulence on ornamental plants. Conceivably, increased regulation may be required for industries that impact urban nematology in order to minimize nematode introductions and spread in the urban environment.

Introduction to Nematodes

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