There are >20,000 morphotypes of free-living interstitial nematodes that are found in terrestrial habitats and along a continuous gradient, into the deep-sea sediments. Many more species remain to be described, particularly from marine sediments. To these we must add about 2000 plant parasites, that can devastate agricultural productions. Nematodes are ubiquitous in soils and are an integral component of decomposition ecology. For general reference on nematode structure and function, students are referred to Grasse's encyclopaedia (1965) and Perry and Wright (1998), and several general nematology texts are available as an adequate introduction.
The general body plan of nematodes is simple, consisting of a small number of circular cell layers (Figs 1.24 and 1.25). The cuticle is <0.5 ^m in most soil species and covers the organisms like a protective skin. It also lines the anterior and posterior portion of the digestive tract (stoma and pharynx, rectum and anal pore). The cuticle is secreted by the epidermis cells (or hypodermis). It is elastic but strong, so as to allow the body to bend, but provides an exoskeleton against which the musculature can work. The cuticle permits gas exchange and osmoregulation (water balance and excretion of soluble by-products such as urea and ammonia), and provides a physical protection against the habitat. It is subdivided into four sublayers. The epicuticle is the outermost sublayer and consists of a glycocalyx 6-45 nm thick. It could be required for species and mate recognition but does not have a known function. The exocuticle (<200
nm) resembles keratin proteins in structure and is particularly thick in parasitic forms. The mesocuticle resembles collagen fibrous proteins in structure. It is the thickest layer of the cuticle and the most varied in structure between species. It is a compressable layer of fibrous proteins that becomes thickened in the anterior cephalic region. It is presumed to func tion as a mechanical buffer, or bumper, as the organism moves through soil or forces its way into tissues. The endocuticle is a thin sublayer of thin irregular protein fibres that resembles a basal membrane. It chemically links the epithelium to the cuticle. Nematodes grow from the juvenile form to the adult form by four consecutive moults. Each time the new cuticle is secreted by the epithelium, the old (outer) cuticle is shed.
The longitudinal muscle cells present in a single layer are striated, branched and have extensions to the nerve cells. Their contractions bend the body in sinusoidal waves that are characteristic of nematode locomotion. There is no circular muscle layer. The nerve cell extensions form a net throughout the body, with cell bodies concentrated into bundles (nerve cords). The larger nerve cord is ventral, with a smaller dorsal cord and several smaller ones across the periphery. The nerve cells are poorly integrated, so nematodes lack a true central nervous system. The sensory cells that extend between epithelium cells, through extensions of the cuticle and contact the outside environment are chemosen-sory and participate in chemotaxis. These sensory extensions are called papillae. Some do not break through the cuticle and probably detect touch and temperature. Several papillae occur about the anterior cephalic region and guide in food detection. Specialized sensory cells and secretory cells occur inside extensions of the cuticle around the reproductive organs, and sometime elsewhere on the body. They help in sensing and holding the partner during sex. Caudal secretory cells (the caudal gland cells), when present near the posterior ventral surface, allow the organism to attach its posterior end on to the substratum. This seems to permit more control over movement and direction. The caudal gland cells are secondarily lost in some species.
The variety of sensory cells along the nematode body permits chemotaxis, mate recognition, pheromone response and, at least in some species, CO2 level sensing. This is not an exclusive list, but demonstrates the extent of directional locomotion possible. One important sensory response is to temperature (Dusenbery, 1989). Nematodes can respond to temperature gradients as low as 0.001°C cm (Pline et al., 1988).
The intestinal tract is a central tube, extending from the pharynx to the rectum, composed of a single layer of cells, bearing microvilli on the absorptive surface. Digestion occurs mostly in the middle intestine, although secretory cells also occur around the upper intestine. Some cells participate in secretion of digestive enzymes and contain lysosomes, but all are absorptive. As older intestinal cells are lost, they are replaced by cell division with new cells. It seems that newly divided cells are only absorptive, and develop the ability to secrete digestive enzymes with age. Endocytosis has also been reported. These cells are capable of storage, and can accumulate protein granules, glycogen and lipids. Undigested material and whatever has not been absorbed by the intestinal cells reaches the rectum and is excreted through the anal pore back into the habitat.
One or more cells (renal cells) may participate in the accumulation and excretion of soluble metabolic wastes from the internal body cavity, through a narrow pore. The internal cavity of the nematode, the pseudocoel, is filled with a serous solution that contains absorbed nutrients from digestion and metabolic by-products from cells, as well as some free haemoglobin. The fluid participates in distributing nutrients and gas exchange, as contractions of the organism contribute to moving and mixing this solution. The older literature describes free-moving phagocytic cells sometime reported in the pseudoceolom. These were speculated to engulf invasive bacteria and to have a defensive role in protecting the organism. The modern literature admits there are 1-6 of these cells depending on the species, called pseudocoelocytes. However, these cells are claimed by some authors to be immotile and non-phagocytic, though branched with granular inclusions (Meglitsch and Schram, 1991). One can doubt the verity of these claims, based on comparisons with other related invertebrates, but their role remains uncertain in nematodes.
The pseudocoel and renal cells participate in osmoregulation and permit survival during soil desiccation. Soil water solutions of 15 mM NaCl equivalent (hypotonic) can increase to >300 mM NaCl equivalent (hypertonic). As water in the habitat becomes limiting, water is lost from the body and volume decreases. Water also becomes limiting inside the body, so that excretion of ammonia and diffusion of metabolites and gases become difficult. Urea and purine then become alternative modes of nitrogen excretion. The increase in internal osmotic pressure initiates dehydration and dormancy, called anhydrobiosis (see Perry and Wright, 1998).
The reproductive cells of adults can account for half of the body weight, and spermatogenesis in the males, or oogenesis in the females, diverts a large portion of the digested and absorbed nutrients. Male spermatozoa do not have a cilium but are amoeboid, and vary greatly in shape. Males deposit spermatozoa inside the female, and fertilization is internal. After fertilization, the egg cell wall acquires a protective chiti-nous mid-layer and the eggs are deposited outside. Development from a fertilized egg to completion of oogenesis in the new adult requires several days and varies with temperature. As an example, in well-fed conditions, a species may require 14 days at 14°C, 8 days at 20°C and 4 days at 28°C to complete the cycle from egg to egg. These values vary between species, as each has different durations of development and different optimal growth temperatures. In a small number of species, male spermatozoa are required to initiate fertilization, but the spermatozoa are not functional and do not fertilize the egg cell. The egg proceeds with development by parthenogenesis. In a few species, females are strictly parthenogenic, in that males are not required at all for fertilization, or development of the egg to embryo and adult.
Morphologically identical isolates are not necessarily the same species. It has been known for a long time that identical organisms from geographically distant locations do not necessarily mate. These may not recognize each other as compatible, or may conjugate and produce sterile offspring (Maupas, 1900, 1919). In some case, isolates of identical morphotype are each restricted to a different environment (Osche, 1952). These observations may demonstrate divergence between isolated populations of one species, or different species within one morphotype. It is clear that there are limits to morphological descriptions of nematode species.
Functionally, and for ecological purposes, nematodes can be separated into groups based on the structure of the stoma and pharynx (see Yeates et al., 1993). The anterior region sensory extensions, as well as the details of the pharynx and stoma, are important morphological characters in species identification. The cuticle lining of the pharynx may be simple as in most species, but it can be reinforced or 'armoured' with thickened extensions. This armature may appear as rows of teeth (denticles, dentate pharynx), as larger tooth-like extensions ('mandibles') or a single spear-shaped stylet. The stylet is hollow in some species, like a syringe needle. The denticles, mandibles and stylet armature rub against ingested food particles with the contractions and suction of the pharynx. Thus, armoured species can ingest more diverse and tougher food particles than species with a simple pharynx. More than one form of pharynx armature is found in some species. In many armoured species, the stylet or mandibles are also extensible out of the stoma, and attached to muscle cells. These species can use more force to penetrate cells and tissues of prey. Functionally, the free-living nema-todes are divided into four basic groups based on what they can ingest.
1. Those with a simple and narrow stoma feed by suction alone and remove small particles from their habitat. These include bacteria but also microdetritus. Some taxa are the Oxystomatidae, Halaphanolaimidae, Draconematina and Desmoscolecidae.
2. Species with prism, conic or cupuliform stoma, with or without a denticle, create a suction accompanied by more powerful peristalsis of the oesophagus muscles. These species can feed on larger particles that include diatoms, cysts, spores, invertebrate eggs and non-filamentous protists in general. The more common taxa include the Rhabditidae, Axonolaimidae, Terpyloididae, Monhysteridae, Desmodorina, Comesomatidae, Chromadoridae, Cyatholaimidae and Paracanthonchinae.
3. Those species with a denticle (or stylet) can succeed in penetrating the cellulosic walls of fine roots, plant tissues and algal filaments, or the chitinous wall of fungal hyphae and small invertebrates. Once the physical barrier is penetrated, the cytoplasm of cells is sucked out. Some common taxa are found in the Paracanthonchinae, Camacolaimidae, Tylenchidae and Dorylaimidae.
4. Species with an 'armoured' stoma and more powerful denticles also depend on oesophageal peristalsis for suction. Although some may feed on bacteria and protists in film water, it would not be sufficient to maintain growth and reproduction. These species can penetrate the cuticle of other nematodes and small invertebrates and can be effective predators, or parasitic in roots and invertebrates. Some taxa include the Enoplidae, Oncholaimidae, Choanolaimidae and Eurystominidae.
However, it is worth noting that free-living species may switch their food preference as they grow from juvenile forms to adult and, though some species tend to be less specific about their food preferences (omnivorous), others may be very specific. This method of classifying nematodes is faster and requires less knowledge than species identification. It is not as accurate or informative, and for more serious studies one always needs to attempt genus or species identification. This should be supplemented with fixed specimens, photographs and DNA extract.
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