Nematode
Nematode Temporal range:
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Caenorhabditis elegans, a model species of roundworm | |
Scientific classification ![]() | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Subkingdom: | Eumetazoa |
Clade: | ParaHoxozoa |
Clade: | Bilateria |
Clade: | Nephrozoa |
(unranked): | Protostomia |
Superphylum: | Ecdysozoa |
Clade: | Nematoida |
Phylum: | Nematoda Diesing, 1861 |
Classes | |
(see text) | |
Synonyms | |
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The nematodes (
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 published in the mega journal Zootaxa puts this figure at over 25,000.[6][7] Estimates of the total number of extant species are subject to even greater variation. A widely referenced article published in 1993 estimated there may be over 1 million species of nematode.[8] A subsequent publication challenged this claim, estimating the figure to be at least 40,000 species.[9] Although the highest estimates (up to 100 million species) have since been deprecated, estimates supported by rarefaction curves,[10][11] together with the use of DNA barcoding[12] and the increasing acknowledgment of widespread cryptic species among nematodes,[13] have placed the figure closer to 1 million species.[14]
Nematodes have successfully adapted to nearly every ecosystem: from marine (salt) 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,[15] even at great depths, 0.9–3.6 km (3,000–12,000 ft) below the surface of the Earth in gold mines in South Africa.[16][17][18][19][20] They represent 90% of all animals on the ocean floor.[21] 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.[22] 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.[22][23] They have been shown to play crucial roles in polar ecosystems.[24][25] The roughly 2,271
The many parasitic forms includeNathan Cobb, a nematologist, described the ubiquity of nematodes on Earth thus:
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.[27](p 472)
Etymology
The word nematode comes from the Modern Latin compound of nemat- "thread" (from Greek nema, genitive nematos "thread," from stem of nein "to spin"; see needle) + -odes "like, of the nature of" (see -oid).
Taxonomy and systematics





History
In 1758, Linnaeus described some nematode genera (e.g., Ascaris), then included in the Vermes.
The name of the group Nematoda, informally called "nematodes", came from
At its origin, the "Nematoidea" erroneously included Nematodes and
In 1861,
However, in 1910, Grobben proposed the phylum Aschelminthes and the nematodes were included as class Nematoda along with class Rotifera, class Gastrotricha, class Kinorhyncha, class Priapulida, and class Nematomorpha (The phylum was later revived and modified by Libbie Henrietta Hyman in 1951 as Pseudoceolomata, but remained similar). In 1932, Potts elevated the class Nematoda to the level of phylum, leaving the name the same. Despite Potts' classification being equivalent to Cobbs', both names have been used (and are still used today) and Nematode became a popular term in zoological science.[32]
Since Cobb was the first to include nematodes in a particular phylum separated from Nematomorpha, some researchers consider the valid taxon name to be Nemates or Nemata, rather than Nematoda,[33] because of the zoological rule that gives priority to the first used term in case of synonyms.
Phylogeny
The
For an up-to-date view (as of 2022), see https://www.frontiersin.org/articles/10.3389/fevo.2021.769565/full.
Nematode 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
Initial studies of incomplete DNA sequences[38] suggested the existence of five clades:[39]
- Dorylaimida
- Enoplia
- Spirurina
- Tylenchina
- Rhabditina
The Secernentea seem to be a natural group of close relatives, while the "Adenophorea" appear to be a
The understanding of roundworm systematics and
Phylum Nematoda
- Basal order Monhysterida
- Class Dorylaimida
- Class Enoplea
- Class Secernentea
- "Chromadorea" assemblage
Later work has suggested the presence of 12 clades.[41] The Secernentea—a group that includes virtually all major animal and plant 'nematode' parasites—apparently arose from within the Adenophorea.
In 2019, a study identified one conserved signature indel (CSI) found exclusively in members of the phylum Nematoda through comparative genetic analyses.[42] 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.[42]
A major effort by a collaborative wiki called 959 Nematode Genomes is underway to improve the systematics of this phylum.[43]
An analysis of the mitochondrial DNA suggests that the following groupings are valid[44]
- subclass Dorylaimia
- orders Trichinellida and Mermithida
- suborder Rhabditina
- infraorders Spiruromorpha and Oxyuridomorpha
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.[45]
Anatomy

Nematodes are very small, slender worms: typically about 5 to 100 µm thick, and 0.1 to 2.5 mm long.[46] The smallest nematodes are microscopic, while free-living species can reach as much as 5 cm (2 in), and some parasitic species are larger still, reaching over 1 m (3 ft) in length.[47]: 271 The body is often ornamented with ridges, rings, bristles, or other distinctive structures.[48]
The head of a nematode 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.[49]
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.[49]
Digestive system
The oral cavity is lined with cuticle, which is often strengthened with structures, such as ridges, especially in carnivorous species, which may bear a number of teeth. The mouth often includes a sharp stylet, which the animal can thrust into its prey. In some species, the stylet is hollow and can be used to suck liquids from plants or animals.[49]
The oral cavity opens into a muscular, sucking pharynx, also lined with cuticle. Digestive glands are found in this region of the gut, producing enzymes that start to break down the food. In stylet-bearing species, these may even be injected into the prey.[49]
No
Excretory system
In many marine nematodes, one or two unicellular 'renette glands' excrete salt through a pore on the underside of the animal, close to the pharynx. In most other nematodes, these specialized cells have been replaced by an organ consisting of two parallel ducts connected by a single transverse duct. This transverse duct opens into a common canal that runs to the excretory pore.[49]
Nervous system
At the anterior end of the animal a dense, circular nerve ring which serves as the brain surrounds the pharynx.[49] From this ring six labial papillary nerve cords extend anteriorly, while six nerve cords; a large ventral, a smaller dorsal and two pairs of sublateral cords extend posteriorly.[50] Each nerve lies within a cord of connective tissue lying beneath the cuticle and between the muscle cells. The ventral nerve is the largest, and has a double structure forward of the excretory pore. The dorsal nerve is responsible for motor control, while the lateral nerves are sensory, and the ventral combines both functions.[49]
The nervous system is also the only place in the nematode body that contains
The bodies of nematodes are covered in numerous sensory bristles and papillae that together provide a sense of touch. Behind the sensory bristles on the head lie two small pits, or 'amphids'. These are well supplied with nerve cells and are probably chemoreception organs. A few aquatic nematodes possess what appear to be pigmented eye-spots, but whether or not these are actually sensory in nature is unclear.[49]
Reproduction
Most nematode species are
Reproduction is usually sexual, though hermaphrodites are capable of self-fertilization. Males are usually smaller than females or hermaphrodites (often much smaller) and often have a characteristically bent or fan-shaped tail. During
Eggs may be
Nematodes as a whole possess a wide range of modes of reproduction.
The nematode model species C. elegans,
The genus Mesorhabditis exhibits an unusual form of parthenogenesis, in which sperm-producing males copulate with females, but the sperm do not fuse with the ovum. Contact with the sperm is essential for the ovum to begin dividing, but because no fusion of the cells occurs, the male contributes no genetic material to the offspring, which are essentially clones of the female.[49]
Free-living species
Different free-living species feed on materials as varied as
Parasitic species
Nematodes that commonly parasitise humans include
One form of nematode is entirely dependent upon fig wasps, which are the sole source of fig fertilization. They prey upon the wasps, riding them from the ripe fig of the wasp's birth to the fig flower of its death, where they kill the wasp, and their offspring await the birth of the next generation of wasps as the fig ripens.
A newly discovered parasitic tetradonematid nematode,
Similarly, multiple varieties of nematodes have been found in the abdominal cavities of the primitively social sweat bee, Lasioglossum zephyrus. Inside the female body, the nematode hinders ovarian development and renders the bee less active, thus less effective in pollen collection.[59]
Agriculture and horticulture
Depending on its species, a nematode may be beneficial or detrimental to plant health. From agricultural and
Greenhouse growers use
Rotations of plants with nematode-resistant species or varieties is one means of managing parasitic nematode infestations. For example,
The golden nematode
Epidemiology

A number of intestinal nematodes cause diseases affecting human beings, including
Gastrointestinal nematode infections in humans are common, with approximately 50% of the global population being affected. Developing countries are most heavily impacted, in part due to lack of access to medical care.[73]
Soil ecosystems
About 90% of nematodes reside in the top 15 cm (6") of soil. Nematodes do not decompose organic matter, but, instead, are parasitic and free-living organisms that feed on living material. Nematodes can effectively regulate bacterial population and community composition—they may eat up to 5,000 bacteria per minute. Also, nematodes can play an important role in the nitrogen cycle by way of nitrogen mineralization.[46]
One group of carnivorous fungi, the nematophagous fungi, are predators of soil nematodes.[74] They set enticements for the nematodes in the form of lassos or adhesive structures.[75][76][77]
Survivability
Nematode worms (C. elegans), part of an ongoing research project conducted on the 2003 Space Shuttle Columbia mission STS-107, survived the re-entry breakup. It is believed to be the first known life form to survive a virtually unprotected atmospheric descent to Earth's surface.[78][79] In a research project published in 2012, it was found that the Antarctic Nematodes (P. davidi) was able to withstand intracellular freezing depending on how well it was fed.[80] In 2023 it was reported that nematodes belonging to a species named Panagrolaimus kolymaensis had been revived after 46,000 years in Siberian permafrost.[81]
See also
- Biological pest control – Controlling pests using other organisms
- Capillaria – Genus of roundworms
- List of organic gardening and farming topics– Overview of and topical guide to organic gardening and farming
- List of parasites of humans
- Soil food web
- Worm bagging – Form of vivipary observed in nematodes.
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Further reading
- Atkinson, H.J. (1973). "The respiratory physiology of the marine nematodes Enoplus brevis (Bastian) and E. communis (Bastian): I. The influence of oxygen tension and body size" (PDF). .
- "Worms survived Columbia disaster". news.bbc.co.uk. BBC News. 1 May 2003. Retrieved 4 Nov 2008.
- Gubanov, N.M. (1951). "Giant nematoda from the placenta of Cetacea; Placentonema gigantissima nov. gen., nov. sp". Proceedings of the USSR Academy of Sciences (in Russian). 77 (6): 1123–1125.
- Kaya, Harry K.; Bedding, Robin A.; Akhurst, Raymond J. (1993). "An overview of insect-parasitic and entomopathogenic nematodes". In Kaya, Harry K.; Bedding, Robin A.; Akhurst, Raymond J. (eds.). Nematodes and the Biological Control of Insect Pests. Csiro Publishing. pp. 8–23. ISBN 9780643105911.
- "Giant kidney worm infection in mink and dogs". Merck Veterinary Manual (MVM) (online ed.). 2006. Archived from the original on 3 March 2016. Retrieved 10 February 2007 – via merckvetmanual.com.
- White JG, Southgate E, Thomson JN, Brenner S (August 1976). "The structure of the ventral nerve cord of Caenorhabditis elegans". PMID 8806.
- Lee, Donald L., ed. (2010). The Biology of Nematodes. London, UK: Taylor & Francis. ISBN 978-0415272117. Retrieved 16 December 2014 – via Google Books.
- de Ley P, Blaxter M (2004). "A new system for Nematoda: combining morphological characters with molecular trees, and translating clades into ranks and taxa". In Cook R, Hunt DJ (eds.). Nematology Monographs and Perspectives. Vol. 2. Leiden, NL: E.J. Brill. pp. 633–653.
External links


- Harper Adams University College Nematology Research
- Nematodes/roundworms of man
- http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html
- European Society of Nematologists
- Nematode.net: Repository of parasitic nematode sequences. Archived 2015-02-14 at the Wayback Machine
- http://webarchive.loc.gov/all/20020914155908/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 on BBC
- Nematode worms in an aquarium
- Phylum Nematoda – nematodes on the UF / *IFAS Featured Creatures Web site