Nematodes have successfully adapted to nearly every ecosystem from marine to fresh water, to soils, and 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 numerical dominance, often exceeding more than 1 million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity in lifestyles and their presence at various trophic levels point at an important role in many ecosystems. 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 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 corresponding 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
Eophasma jurasicum, a fossilized nematode
The group was originally defined by Karl Rudolphi in 1808under the name Nematoidea, from Ancient Greek νῆμα (nêma, nêmatos, 'thread') and -eiδἠς (-eidēs, 'species'). It was reclassified as family Nematodes by Burmeister in 1837[15] and order Nematoda by K. M. Diesing in 1861.
At its origin, the "Nematoidea" included both roundworms and horsehair worms. Along with Acanthocephala, Trematoda and Cestoidea, it formed the group Entozoa.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). The taxon Nematoidea, including the family Gordiidae (horsehair worms), was then promoted to the rank of phylum by Ray Lankester (1877). In 1919, Nathan Cobb proposed that roundworms should be recognized alone as a phylum. He argued they should be called nema(s) in English rather than "nematodes"[a] and defined the taxon Nemates (Latin plural of nema). Since Cobb was the first to exclude all but nematodes from the group, some sources consider the valid taxon name to be Nemates or Nemata, rather than Nematoda.
Phylogeny
The mysterious Gastrotricha 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 close relatives of annelids—with which they were formerly united—to be unwarranted, 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, have become rather equivocal on ecdysozoan monophyly, and are 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[citation needed] 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.
Conversely, the identity of the closest living relatives of the Nematoda has always been considered to be well resolved. Morphological characters and molecular phylogenies agree with placement of the roundworms as sister taxon to the parasitic horsehair worms (Nematomorpha); together they make up the Nematoida. Together with the Scalidophora (formerly Cephalorhyncha), the Nematoida form the Introverta. It is entirely unclear whether the Introverta are, in turn, the closest living relatives of the enigmatic Gastrotricha; if so, they are considered a clade Cycloneuralia, but there is much disagreement both between and among the available morphological and molecular data. The Cycloneuralia or the Introverta—depending on the validity of the former—are often ranked as a superphylum.
Nematode systematics
Due to the lack of knowledge regarding many nematodes, their systematics is contentious. An earliest and influential classification was proposed by Chitwood and Chitwood—later revised by Chitwood—who divided the phylum into two—the Aphasmidia and the Phasmidia. These were later renamed Adenophorea (gland bearers) and Secernentea (secretors) respectively. The Secernentea share several characteristics including the presence of phasmids, a pair of sensory organs located in the lateral posterior region and this was used as the basis for this division. This scheme was adhered to in many later classifications even though it was realized that the Adenophorea were not a uniform group.
Initial DNA sequence studies[verification needed] suggested the existence of five clades
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
Later work has suggested the presence of 12 clades. It appears that Secernentea—a group that includes virtually all major animal and plant 'nematode' parasites—arose from within the Adenophorea.
A major effort to improve the systematics of this phylum is in progress and being organised by the 959 Nematode Genomes.
