Nematode


The nematodes, roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms are the cause of soil-transmitted helminthiases.
They are classified along with arthropods, tardigrades and other moulting animals in the clade Ecdysozoa. Unlike the flatworms, nematodes have a tubular digestive system, with openings at both ends. Like tardigrades, they have a reduced number of Hox genes, but their sister phylum Nematomorpha has kept the ancestral protostome Hox genotype, which shows that the reduction has occurred within the nematode phylum.
Nematode species can be difficult to distinguish from one another. Consequently, estimates of the number of nematode species are uncertain. A 2013 survey of animal biodiversity suggested there are over 25,000. Estimates of the total number of extant species are subject to even greater variation. A widely referenced 1993 article estimated there might be over a million species of nematode. A subsequent publication challenged this claim, estimating the figure to be at least 40,000 species. Although the highest estimates have since been deprecated, estimates supported by rarefaction curves, together with the use of DNA barcoding and the increasing acknowledgment of widespread cryptic species among nematodes, have placed the figure closer to one million species.
Nematodes have successfully adapted to nearly every ecosystem: from marine to fresh water, soils, 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, and oceanic trenches. They are found in every part of the Earth's lithosphere, even at great depths, below the surface of the Earth in gold mines in South Africa. They represent 90% of all animals on the ocean floor. In total, 4.4 × 1020 nematodes inhabit the Earth's topsoil, or approximately 60 billion for each human, with the highest densities observed in tundra and boreal forests. Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on Earth, their diversity of lifecycles, and their presence at various trophic levels point to an important role in many ecosystems. They play crucial roles in polar ecosystems. The roughly 2,271 genera are placed in 256 families. The many parasitic forms include pathogens in most plants and animals. A third of the genera occur as parasites of vertebrates; about 35 nematode species are human parasites.

Etymology

The word nematode comes from the Modern Latin compound of nema- 'thread' + -odes 'like, of the nature of'. The addition firstly of '-oid' and then to '-ode' renders 'threadlike'.

Taxonomy and systematics

History

In 1758, Carl Linnaeus described nematodes of a few genera including Ascaris and Dracunculus, then included in the Vermes. The name of the group Nematoda, informally called "nematodes", came from Nematoidea, originally defined by Karl Rudolphi in 1808, from Ancient Greek νῆμα and -ειδής . It was treated as family Nematodes by Burmeister in 1837. At its origin, the "Nematoidea" erroneously included Nematodes and Nematomorpha, attributed by Karl Theodor Ernst von Siebold in 1843. Along with Acanthocephala, Trematoda, and Cestoidea, it formed the obsolete group Entozoa, created by Rudolphi in 1808. They were classed along with Acanthocephala in the obsolete phylum Nemathelminthes by Gegenbaur in 1859. In 1861, Karl Moriz Diesing treated the group as order Nematoda. In 1877, the taxon Nematoidea, including the family Gordiidae, was promoted to the rank of phylum by Ray Lankester. The first clear distinction between the nemas and gordiids was realized by František Vejdovsky when he named the group containing the horsehair worms the order Nematomorpha in 1886.
In 1910, Grobben proposed the phylum Aschelminthes, and the nematodes were included as class Nematoda alongside the classes Rotifera, Gastrotricha, Kinorhyncha, Priapulida, and Nematomorpha.In 1919, Nathan Cobb proposed that nematodes should be recognized alone as a phylum. He argued they should be called "nema" in English rather than "nematodes" and defined the taxon Nemates, listing Nematoidea sensu restricto as a synonym. In 1932, Potts elevated the class Nematoda to the level of phylum, leaving the name the same. Although Potts' and Cobb's classifications are equivalent, both names are used, and Nematode became a popular term in zoological science.

Phylogeny

The phylogenetic relationships of the nematodes and their close relatives among the protostomes are unresolved. Traditionally, they were held to be a lineage of their own, but in the 1990s, they were proposed to form the group Ecdysozoa together with moulting animals, such as arthropods. The identity of the closest living relatives of the Nematoda has always been considered to be well resolved. Morphological and molecular phylogenetics agree with placing the roundworms as a sister taxon to the parasitic Nematomorpha; together, they make up the Nematoida. Along with the Scalidophora, the Nematoida form the clade Cycloneuralia, but much disagreement occurs 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.

Systematics

Due to the lack of knowledge regarding many nematodes, their systematics is contentious. An early and influential classification was proposed by Chitwood and Chitwood—later revised by Chitwood—who divided the phylum into two classes—Aphasmidia and Phasmidia. These were later renamed Adenophorea and Secernentea, 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, though the Adenophorea were not in a uniform group.
Initial studies of incomplete DNA sequences suggested the existence of five clades:
The Secernentea seem to be a natural group of close relatives, while the Adenophorea appear to be a paraphyletic assemblage of roundworms that retain a good number of ancestral traits. The old Enoplia do not seem to be monophyletic, either, but do contain two distinct lineages. The old group Chromadorea seems 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 Dorylaimida
  • Class Enoplea
  • Class Secernentea
  • * Subclass Diplogasteria
  • * Subclass Rhabditia
  • * Subclass Spiruria
  • * Subclass Tylenchia
  • "Chromadorea" assemblage
Later work has suggested the presence of 12 clades. In 2019, a study identified one conserved signature indel found exclusively in members of the phylum Nematoda through comparative genetic analyses. The CSI consists of a single amino acid insertion within a conserved region of a Na/H exchange regulatory factor protein NRFL-1 and is a molecular marker that distinguishes the phylum from other species. An analysis of the mitochondrial DNA suggests that the following groupings are valid
In 2022 a new classification of the entire phylum Nematoda was presented by M. Hodda. It was based on current molecular, developmental and morphological evidence. Under this classification, the classes and subclasses are:
  • Class Enoplea
  • * Subclass Enoplia
  • * Subclass Oncholaimia
  • * Subclass Triplonchia
  • Class Dorylaimida
  • * Subclass Dorylaimia
  • * Subclass Bathyodontia
  • * Subclass Trichocephalia
  • Class Chromadorea
  • * Subclass Chromadoria
  • * Subclass Plectia

    Fossil record

Nematode eggs from the clades Ascaridina, Spirurina, and Trichocephalida have been discovered in coprolites from the Oligocene-aged Tremembé Formation, which represented a palaeolake in present-day São Paulo with a diverse fossil assemblage of birds, fish, and arthropods that lent itself to fostering high nematode diversity. Nematodes have also been found in various lagerstätten, such as Burmese amber, the Moltrasio Formation, and the Rhynie chert, where the earliest known fossils are known from.

Anatomy

Nematodes are very small, slender worms. Most are free-living, often less than 2.5 mm long and some only about 1 mm. Many nematodes are microscopic. Some soil nematodes can reach up to 7 mm in length, and some marine species can reach up to 5 cm. Some are parasitic and can reach lengths of 50 cm or more.
The body is often ornamented with ridges, rings, bristles, or other distinctive structures.
The head is relatively distinct. 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 edges. An adhesive 'caudal gland' is often found at the tip of the tail. The epidermis is either a syncytium or a single layer of cells, and is covered by a thick collagenous cuticle. The cuticle is often of a complex structure and may have two or three distinct layers. Underneath the epidermis lies a layer of longitudinal muscle cells. The relatively rigid cuticle works with the muscles to create a hydroskeleton, as nematodes lack circumferential muscles. Projections run from the inner surface of muscle cells towards the nerve cords; this is a unique arrangement in the animal kingdom, in which nerve cells normally extend fibers into the muscles rather than vice versa.