Nematode

The nematodes () or roundworms (phylum Nematoda) are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode species might be approximately 1,000,000. Unlike cnidarians or flatworms, roundworms have a digestive system that is like a tube with openings at both ends.

Habitats

Nematodes have successfully adapted to nearly every ecosystem from marine to fresh water, from the polar regions to the tropics, as well as the highest to the lowest of elevations. They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as mountains, deserts, oceanic trenches, and within the earth's lithosphere. They represent, for example, 90% of all life forms on the ocean floor. Their many parasitic forms include pathogens in most plants and animals (including humans). Some nematodes can undergo cryptobiosis.

One group of carnivorous fungi, the nematophagous fungi, are predators of soil nematodes. They set enticements for the nematodes in the form of lassos or adhesive structures. Nematodes have even been found at great depth (0.9 – 3.6 km) below the surface of the Earth in gold mines in South Africa.

Nathan Cobb (from p. 472 of Cobb, 1914) described very well the ubiquitous presence of nematodes on Earth as follows:

"In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a correspond-ing massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites."

Taxonomy and systematics

The group was originally defined by Karl Rudolphi in 1808 under the name Nematoidea, from Ancient Greek νῆμα (nêma, nêmatos, 'thread') and -eiδἠς (-eidēs, 'species'). The vernacular word "nematode" is a corruption of this taxon, reclassified as family Nematodes by Burmeister in 1837 The first differentiation of roundworms from horsehair worms, though erroneous, is due to von Siebold (1843) with orders Nematoidea and Gordiacei (Gordiacea). They were classed along with Acanthocephala in the new phylum Nemathelminthes (today obsolete) by Gegenbaur (1859). Then the taxon Nematoidea has been promoted to the rank of phylum by Ray Lankester (1877) including the family Gordiidae (horsehair worms). In 1919, Nathan Cobb proposed that roundworms should be recognized alone as a phylum. He argued that they should be called nema(s) in English rather than "nematodes" and defined the taxon Nemates (Latin plural of nema). For ITIS, the taxon Nematoda is invalid. Since Cobb was the first to exclude all but nematodes from the group, the valid taxon should be Nemates Cobb 1919 or Nemata Cobb 1919.

Phylogeny

seem to hold "the key to the Ecdysozoa debate", but they have been little studied.Whether they are relatives of the nematodes is still unknown.]] The relationships of the nematodes and their close relatives among the protostomian Metazoa are unresolved. Traditionally, they were held to be a lineage of their own, but in the 1990s it was proposed that they form a clade together with moulting animals such as arthropods. This group has been named Ecdysozoa. However, the monophyly of the Ecdysozoa was never unequivocally accepted: while most researchers consider at least the placement of arthropods as more distant relatives of annelids — with which they were formerly united — to be warranted, the presumed close relationships of the nematodes and relatives with the arthropods has been a major point of contention.

Even though the amount of data since accumulated in regard to this problem is staggering, the situation seems if anything less clear these days. DNA sequence data, initially strongly supporting the Ecdysozoa hypothesis, has become rather equivocal on ecdysozoan monophyly, and is simply unable to refute either a close or a more distant relationship between the arthropod and nematode lineages. That the roundworms have a large number of peculiar apomorphies and in many cases a parasitic lifestyle confounds morphological analyses. Genetic analyses of roundworms suggest that — as is also indicated by their unique morphological features — the group has been under intense selective pressure during its early radiation, resulting apparently in accelerated rates of both morphological and molecular evolution. Furthermore, no distinctive apomorphies of Ecdysozoa are known; even moulting has recently been confirmed to occur outside the presumed clade.

Dorylaimia

Enoplia

Spirurina

Tylenchina

Rhabditina

As it seems, the Secernentea are indeed a natural group of closest relatives. But the "Adenophorea" appear to be a paraphyletic assemblage of roundworms simply retaining a good number of ancestral traits. The old Enoplia do not seem to be monophyletic either but to contain two distinct lineages. The old group "Chromadoria" seem to be another paraphyletic assemblage, with the Monhysterida representing a very ancient minor group of nematodes. Among the Secernentea, the Diplogasteria may need to be united with the Rhabditia. while the Tylenchia might be paraphyletic with the Rhabditia.

The understanding of roundworm systematics and phylogeny as of 2002 is summarised below:

Phylum Nematoda

Basal order Monhysterida

Class Dorylaimea

Class Enoplea

Class Secernentea

* Subclass Diplogasteria (disputed)

* Subclass Rhabditia (paraphyletic?)

* Subclass Spiruria

* Subclass Tylenchia (disputed)

"Chromadorea" assemblage

Anatomy

Nematodes are a slender type of worm, typically less than long. The smallest nematodes are microscopic, while free-living species can reach as much as and some parasitic species are larger still. The body is often ornamented with ridges, rings, warts, bristles or other distinctive structures.

The head of a nematode is relatively distinctive. Whereas the rest of the body is bilaterally symmetrical, the head is radially symmetrical, with sensory bristles and, in many cases, solid head-shields radiating outwards around the mouth. The mouth has either three or six lips, which often bear a series of teeth on their inner edge. An adhesive caudal gland is often found at the tip of the tail. Some nematodes, such as Heterorhabditis spp., undergo a process called endotokia matricida: intrauterine birth causing maternal death. Some nematodes are hermaphroditic, and keep their self-fertilized eggs inside the uterus until they hatch. The juvenile nematodes will then ingest the parent nematode. This process is significantly promoted in environments with a low or reducing food supply.

of soybean cyst nematode (Heterodera sp.) and egg]] Plant parasitic nematodes include several groups causing severe crop losses. The most common genera are Aphelenchoides (foliar nematodes), Ditylenchus, Globodera (potato cyst nematodes), Heterodera (soybean cyst nematodes), Longidorus, Meloidogyne (root-knot nematodes), Nacobbus, Pratylenchus (lesion nematodes), Trichodorus and Xiphinema (dagger nematodes). Several phytoparasitic nematode species cause histological damages to roots, including the formation of visible galls (e.g. by root-knot nematodes), which are useful characters for their diagnostic in the field. Some nematode species transmit plant viruses through their feeding activity on roots. One of them is Xiphinema index, vector of GFLV (Grapevine Fanleaf Virus), an important disease of grapes.

Other nematodes attack bark and forest trees. The most important representative of this group is Bursaphelenchus xylophilus, the pine wood nematode, present in Asia and America and recently discovered in Europe.

Agriculture and horticulture

Depending on the species, a nematode may be beneficial or detrimental to plant health. From agricultural and horticulture perspectives, there are two categories of nematode: predatory ones, which will kill garden pests like cutworms, and pest nematodes, like the root-knot nematode, which attack plants and those that act as vectors spreading plant viruses between crop plants. Predatory nematodes can be bred by soaking a specific recipe of leaves and other detritus in water, in a dark, cool place, and can even be purchased as an organic form of pest control.

Rotations of plants with nematode resistant species or varieties is one means of managing parasitic nematode infestations. For example, marigolds, grown over one or more seasons (the effect is cumulative), can be used to control nematodes. Another is treatment with natural antagonists such as the fungus Gliocladium roseum. Chitosan is a natural biocontrol that elicits plant defense responses to destroy parasitic cyst nematodes on roots of soybean, corn, sugar beets, potatoes and tomatoes without harming beneficial nematodes in the soil. Furthermore soil steaming is an efficient method to kill nematodes before planting crop, but indiscriminately eliminates both harmful and beneficial ones.

CSIRO has found that there was 13- to 14-fold reduction of nematode population densities in plots having Indian mustard (Brassica juncea) green manure or seed meal in the soil.

Hundreds of Caenorhabditis elegans were featured in a research project on NASA's STS-107 space mission (which ended in the Space Shuttle Columbia Disaster).

Epidemiology

for intestinal nematode infections per 100,000 inhabitants in 2002.]] A number of intestinal nematodes affect human beings. These include ascariasis, trichuriasis and hookworm disease.

See also

Ascariasis: A human disease caused by the parasitic roundworm Ascaris lumbricoides

Chitosan (Natural Biocontrol for Agricultural & Horticultural use of nematodes)

Caenorhabditis elegans: An important model organism often used to study cellular differentiation, sometimes simply referred to as "worm" by scientists.

List of parasites (human)

Toxocariasis: A helminth infection of humans caused by the dog or cat roundworm, Toxocara canis or Toxocara cati

List of organic gardening and farming topics

Biological pest control

Footnotes

References

(1973): The Respiratory Physiology of the Marine Nematodes Enoplus brevis (Bastian) and E. communis (Bastian): I. The Influence of Oxygen Tension and Body Size. J. Exp. Biol. 59(1): 255–266. PDF fulltext (2003): Worms survived Columbia disaster. Version of 2003-MAY-01. Retrieved 2008-NOV-04. (1982): The Masterpiece of Nature: The Evolution and Genetics of Sexuality. University of California Press. (1998): A molecular evolutionary framework for the phylum Nematoda. Nature 392: 71–75. (HTML abstract) (2007): Deep-sea species' loss could lead to oceans' collapse, study suggests. Version of 2007-DEC-27. Retrieved 2008-NOV-04. (1951): "Giant nematoda from the placenta of Cetacea; Placentonema gigantissima nov. gen., nov. sp.". Proc. USSR Acad. Sci. 77(6): 1123–1125 [in Russian]. (2001): Biodiversity in helminths and nematodes as a field of study: an overview. Nematology 3: 199-208. (HTML abstract) (1999): Endotokia matricida in hermaphrodites of Heterorhabditis spp. and the effect of the food supply. Nematology 1(7–8): 717–726. (HTML abstract) (2006): Giant Kidney Worm Infection in Mink and Dogs. Retrieved 2007-FEB-10. (2002a): Bilateria. Version of 2002-JAN-01. Retrieved 2008-NOV-02. (2002b): Nematoda. Version of 2002-JAN-01. Retrieved 2008-NOV-02. (1976)): The Structure of the Ventral Nerve Cord of Caenorhabditis elegans. 275(938): 327–348. PDF fulltext

External links

Harper Adams University College Nematology Research

nematoda knol Nematodes/roundworms of man

http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html

European Society of Nematologists

Nematode.net: Repository of parasitic nematode sequences.

http://www.nematodes.org/

NeMys World free-living Marine Nematodes database

Nematode Virtual Library

International Federation of Nematology Societies

Society of Nematologists

Australasian Association of Nematologists

Research on nematodes and longevity

Nematode worms in an aquarium

on the UF / IFAS Featured Creatures Web site

soil nematodes

Illustrated Key to the Genera of Free-living Marine Nematodes in the Superfamily Chromadoroidea

Belonolaimus longicaudatus, a sting nematode

Bursaphelenchus cocophilus, red ring nematode

Dolichodorus spp., awl nematodes

Hoplolaimus galeatus, lance nematode

Paratrichodorus minor, a stubby-root nematode

Rotylenchulus reniformis, reniform nematode

Steinernema scapterisci, mole cricket nematode

Trichodorus obtusus, a stubby-root nematode

Nematode on BBC

Category:Parasites Category:Organic gardening Category:Composting Category:Garden pests Category:Pest control