Nematoda

Nematodes, or roundworms, are among the most numerous of the multicellular organisms found in any ecosystem. As with the protozoa, they are primarily inhabiters of water films, or water-filled pore spaces in soils. Nematodes have a very early phylogenetic origin, but as with many other invertebrate groups, the fossil record is fragmentary. They are classified among the triploblastic pseudocoelomates (three body layers: ectoderm, mesoderm, and endoderm). In other words, nema-todes have a body cavity for the gastrointestinal tract, but it is less well-differentiated than that for the true coelomates, such as annelids and arthropods.

The overall body shape is cylindrical, tapering at the ends (Fig. 4.7). In general, nematode body plans are characterized by a "tube within a tube" (alimentary tract/the body wall). The alimentary tract, beginning at the anterior end, consists of a stoma or stylet, pharynx (or esophagus), intestine, and rectum, which opens externally at the anus. The reproductive structures are quite complex, as shown in Figure 4.7. Some species are parthenogenetic, reproducing without sex. It is possible to view the internal structures of most nematodes because they have virtually transparent cuticles. The nematodes can be keyed out fairly readily to family and/or genus under a moderate magnification (about 100x) binocular microscope or in a Sedgwick-Rafter chamber on an inverted microscope (Wright, 1988), but species-specific characteristics must be determined under high magnification, using compound microscopes.

Nematode Feeding Habits

Nematodes feed on a wide range of foods. General trophic groupings include bacterial feeders, fungal feeders, plant feeders, and predators and omnivores. For the purposes of our general overview, one can use anterior (stomal or mouth) structures to differentiate general feeding, or trophic, groups (Fig. 4.8) (Yeates and Coleman, 1982; Yeates et al., 1993; Yeates, 1998). The feeding categories are a good introduction, but the feeding habits of many genera are either complex or poorly known. Thus immature forms of certain nematodes may be bacterial feeders, then become predators on other fauna once they have matured (Allen-Morley and Coleman, 1989). Some of the stylet-bearing nematodes (e.g.,

FIGURE 4.7. Structures of a Rhabditis sp., a secernentean microbotrophic nematode of the order Rhabditida. (Left) Female. (Right) Male. ST = stoma; C = corpus area of the pharynx; N = nerve ring; E.p = excretory pore; B.b = basal bulb of the pharynx; I = intestine; T = testis; E = eggs; V = vulva; Va = vagina; U = uterus; O = ovary; SP = sperm; V.d = vas deferens; R.g = rectal glands; R = rectum; A = anus; S = spicules; G = gubernaculum; B = bursa; P = phasmids; G.P = genital papillae; CL = cloaca (courtesy of Proceedings of the Helminthological Society of Washington) (from Poinar, 1983).

FIGURE 4.7. Structures of a Rhabditis sp., a secernentean microbotrophic nematode of the order Rhabditida. (Left) Female. (Right) Male. ST = stoma; C = corpus area of the pharynx; N = nerve ring; E.p = excretory pore; B.b = basal bulb of the pharynx; I = intestine; T = testis; E = eggs; V = vulva; Va = vagina; U = uterus; O = ovary; SP = sperm; V.d = vas deferens; R.g = rectal glands; R = rectum; A = anus; S = spicules; G = gubernaculum; B = bursa; P = phasmids; G.P = genital papillae; CL = cloaca (courtesy of Proceedings of the Helminthological Society of Washington) (from Poinar, 1983).

Dorylaimus

FIGURE 4.8. Head structures of a range of soil nematodes. (a) Rhabditis (bacterial feeding); (b) Acrobeles (bacterial feeding); (c) Diplogaster (bacterial feeding, predator); (d) tylenchid (plant feeding, fungal feeding, predator); (e) Dorylaimus (feeding poorly known, omnivore); (f) Xiphinema (plant feeding); (g) Trichodorus (plant feeding); (h) Mononchus (predator) (from Yeates and Coleman, 1982).

FIGURE 4.8. Head structures of a range of soil nematodes. (a) Rhabditis (bacterial feeding); (b) Acrobeles (bacterial feeding); (c) Diplogaster (bacterial feeding, predator); (d) tylenchid (plant feeding, fungal feeding, predator); (e) Dorylaimus (feeding poorly known, omnivore); (f) Xiphinema (plant feeding); (g) Trichodorus (plant feeding); (h) Mononchus (predator) (from Yeates and Coleman, 1982).

the family Neotylenchidae), may feed on roots, root hairs, and fungal hyphae (Yeates and Coleman, 1982). Some bacterial feeders (e.g., Alaimus) may ingest 10-|im width cyanobacterial cells (Oscillatoria) despite the mouth of the nematode being 1-|im wide, indicating that the cyanobacterial cells can be compressed markedly by the nematode (Yeates, 1998). Recent laboratory studies (Venette and Ferris, 1998) have confirmed that population growth of bacterial-feeding nematodes is strongly dependent on the species of bacteria ingested. The six nema-tode species used in the study of Venette and Ferris (1998) reached maximal population growth rates when ingesting from 104 to 105 colony-forming units (CFUs) per nematode. Population growth rates (L) under these controlled conditions ranged from 1.1 to greater than 12 d-1, making these organisms ideal for detecting rapid changes in the soil environment.

Although specialized in nature, the feeding habits and impacts of entomopathogenic nematodes are quite marked in several soil environments worldwide (Hominick, 2002). The nonfeeding "infective juveniles," or third instar dauer larvae of nematodes in the family

Heterorhabditidae, live in the soil and search for hosts and disperse. An infective juvenile enters the insect host (which it senses along a CO2 gradient) (Strong et al., 1996) through a spiracle or other opening, punctures a membrane, then regurgitates the symbiotic bacterium Photorhabdus luminescens, which kills the host within 48 hours. A rapidly growing bacterial population then digests the insect cadaver and provides food for the exponentially growing adult nematode population inside. The symbiotic bacteria produce antibiotics and other antimicrobial substances that protect the host cadaver and adult nema-todes inside from invasion by alien bacteria and fungi from the soil (Strong et al., 1999). When the cadaver is exhausted of resources, reproduction shunts to infective juveniles, which break through the host integument and disperse into the soil. For example, as many as 410,000 Heterorhabditis hepialus infective juveniles are produced in a large ghost moth caterpillar (Strong et al., 1996). In pot experiments, Strong et al. (1999) found that Lupinus arboreus seedlings, whose seedling survival decreased exponentially with increasing densities of root-feeding caterpillars, had virtually the entire negative effect of the herbivore cancelled upon the introduction of the entomopathogenic nematode into the system. For more information on dynamics of entomopathogenic nematodes in soil food webs, see Strong (2002).

For identifying fungal-based food chains, Ruess et al. (2002) have shown that the measurement of fatty acids specific to fungi can be traced to the body tissues of fungal-feeding nematodes. Although still in early stages of development, this technique shows considerable promise for more detailed biochemical delineation of food sources of specific feeding groups of nematodes.

Because of the wide range of feeding types and the fact that they seem to reflect ages of the systems in which they occur (i.e., annual versus perennial crops [Neher et al., 1995], or old fields and pastures and more mature forests), nematodes have been used as indicators of overall ecological condition (Bongers, 1990; Ettema and Bongers, 1993; Yeates, 1999; Ferris et al., 2001). This is a growing area of research in soil ecology; one in which the intersection between community analysis and ecosystem function could prove to be quite fruitful. We discuss some of these concepts further in Chapter 5 on decomposition and nutrient cycling.

Nematode Zones of Activity in Soil

As noted in Chapter 2, the rhizosphere is a zone of considerable metabolic activity for root-associated microbes. This extends also to the soil fauna, which may be concentrated in the rhizosphere. For example, Ingham et al. (1985) found up to 70% of the bacterial and fungal-feeding nematodes in the 4-5% of the total soil that was rhizosphere, namely the amount of soil 1-2 millimeters (mm) from the root surface (the rhizo-plane). In comparison, Griffiths and Caul (1993) found that nematodes migrated to packets of decomposing grass residues, with considerable amounts of labile substrates therein, in pot experiments. They concluded that nematodes are seeking out these "hot spots" of concentrated organic matter, and that protozoa, also monitored in the experiment, do not.

Nematodes are very sensitive to available soil water in the soil matrix. Elliott et al. (1980) noted that the limiting factor for nematode survival often hinges on the availability of soil pore necks, which enable movement between soil pores. In recent studies, Yeates et al. (2002) measured the movements, growth, and survival of three genera of bacterial-feeding soil nematodes in undisturbed soil cores maintained on soil pressure plates. Interestingly, the nematodes showed significant reproduction even when diameters of water-filled pores were approximately 1 Mm. This information should prove useful when determining biological interactions under field conditions, and indicates that soil nematodes may be more active over a wider range of soil moisture tensions than had been thought to be the case previously.

Nematode Extraction Techniques

Nematodes may be extracted by a variety of techniques, either active or passive in nature. For more accuracy in determination of populations, the passive or flotation techniques are generally preferred. The principal advantage of the oldest, active method, namely the Baermann funnel method, is that it is simple, requiring no fancy equipment or electricity. It is based on the fact that nematodes in soils will move about in the wetted soil and fall into the funnel itself. Thus samples are placed on coarse tissue paper, on a coarse mesh screen, and then placed in the cone of a funnel and immersed in water. Once they crawl through the moist soil and filter paper, the nematodes fall down into the neck of the funnel. Because nematodes have only circular and not longitudinal muscles, they do not stay in suspension in the water and fall to the bottom of the funnel stem, which was closed off with a screw clamp on a rubber hose. At the conclusion of the extraction (typically 48 hours), the nematodes in solution are drawn off into a tube and kept preserved for examination later. One drawback to the technique is that it allows dormant nematodes to become active and be extracted, so it may give a slightly inflated estimate of the true, "active" population at a given time. Other methods include filtration, or decanting and sieving, and flota-tion/centrifugation (Christie and Perry, 1951, Coleman et al., 1999) to remove the nematodes from the soil suspension. When handling larger quantities of soil (up to 500g) to recover large amounts of nema-todes, various elutriation (extraction using streams of air bubbles in funnels) methods are employed. For details, see Gorny and Grum (1993) (Fig. 4.9).

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