Nematode
Nematode Temporal range: [2]
Possible Cambrian occurrence | |
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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 |
Clade: | Protostomia |
Superphylum: | Ecdysozoa |
Clade: | Nematoida |
Phylum: | Nematoda Diesing, 1861 |
Classes | |
(see text) | |
Synonyms | |
|
The nematodes (
They are classified along with
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.[4][5] 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.[6] A subsequent publication challenged this claim, estimating the figure to be at least 40,000 species.[7] Although the highest estimates (up to 100 million species) have since been deprecated, estimates supported by rarefaction curves,[8][9] together with the use of DNA barcoding[10] and the increasing acknowledgment of widespread cryptic species among nematodes,[11] have placed the figure closer to one million species.[12]
Nematodes have successfully adapted to nearly every
Etymology
The word nematode comes from the Modern Latin compound of nema- 'thread' (from Greek nema, genitive nematos 'thread', from the stem nein 'to spin'; cf. needle) + -odes 'like, of the nature of' (cf. -oid). The addition firstly of '-oid' and then to '-ode' renders 'threadlike'.[20]
Taxonomy and systematics
-
Eophasma jurasicum, a fossilized nematode
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Oxyuridae Pinworm
-
Steinernema feltiae
-
Steinernema feltiae
History

In 1758, Carl Linnaeus described nematodes of a few genera including Ascaris and Dracunculus, then included in the Vermes.[21]
The name of the group Nematoda, informally called "nematodes", came from
At its origin, the "Nematoidea" erroneously included Nematodes and
In 1861,
In 1910, Grobben proposed the phylum Aschelminthes, and the nematodes were included as class Nematoda alongside the classes Rotifera, Gastrotricha, Kinorhyncha, Priapulida, and Nematomorpha.[26]
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 (later emended as Nemata, Latin plural of nema), listing Nematoidea sensu restricto as a synonym.[27]
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.[28]
Phylogeny
The
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[34] suggested the existence of five clades:[35]
- 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.[37] In 2019, a study identified one conserved signature indel (CSI) found exclusively in members of the phylum Nematoda through comparative genetic analyses.[38] 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.[38] An analysis of the mitochondrial DNA suggests that the following groupings are valid[39]
- 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.[40] 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
- Subclass
Fossil record
Nematode eggs from the
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.[42]
The body is often ornamented with ridges, rings, bristles, or other distinctive structures.[43]
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.[44] 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.[44]
Digestive system
The oral cavity is lined with cuticles, which are often strengthened with structures, such as ridges, especially in carnivorous species, which may bear several 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.[44] 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.[44]
No
Excretory system
There is an excretory gland, also known as a ventral cell, or renette cell in all species of Adenophorea. In Secernentia there is an excretory canal system that may or may not use a gland cell.[42]
Nervous system
At the anterior end of the animal a dense, circular nerve ring which serves as the brain surrounds the pharynx.[44] 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.[45] 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.[44]
The nervous system is the only place in the body that contains
The body is 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.[44]
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.[44]
Aging
The nematode Caenorhabditis elegans is often used as a model organism for studying aging at the molecular level. For example, in C. elegans aging negatively impacts DNA repair, and mutants of C. elegans that are long-lived were shown to have increased DNA repair capability.[54] These findings suggest a genetically determined correlation between DNA repair capacity and lifespan.[54] In female C. elegans, germline processes that control DNA repair and formation of chromosomal crossovers during meiosis were shown to progressively deteriorate with age.[55]
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.[citation needed]
A parasitic
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 entomopathogenic nematodes as beneficial agents to control fungus gnats. The nematodes enter the larvae of the gnats by way of their anus, mouth, and spiracles (breathing pores) and then release bacteria which kills the gnat larvae. Commonly used nematode species to control pests on greenhouse crops include Steinernema feltiae for fungus gnats and western flower thrips, Steinernema carpocapsae used to control shore flies, Steinernema kraussei for control of black vine weevils, and Heterorhabditis bacteriophora to control beetle larvae.[66]
Rotations of plants with nematode-resistant species or varieties is one means of managing parasitic nematode infestations. For example, planting
The golden nematode
Disease in humans

A number of pathogenic intestinal nematodes cause diseases in humans, including
Trichinosis starts in the intestines but larvae can migrate to muscle. Filarial nematodes cause filariases.
Studies have shown that parasitic nematodes infect American eels causing damage to the eel's swim bladder,[73] dairy animals like cattle and buffalo,[74] and all species of sheep.[75]
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.[76] But plant parasitic nematodes cause billions of dollars in annual crop damage worldwide.[77]
One group of carnivorous fungi, the nematophagous fungi, are predators of soil nematodes.[78] They can set enticements for the nematodes in the form of lassos or adhesive structures.[79][80][81] They can also release powerful toxins when in contact with nematodes.[82]
Survivability
The nematode Caenorhabditis elegans an important model organism, was used as part of an ongoing research project conducted on the 2003 Space Shuttle Columbia mission STS-107, and survived the re-entry breakup. It is believed to be the first known species to survive a virtually unprotected atmospheric descent to Earth's surface.[83][84] The Antarctic nematode Panagrolaimus davidi was able to withstand intracellular freezing depending on how well it had been fed.[85] In 2023 an individual of Panagrolaimus kolymaensis was revived after 46,000 years in Siberian permafrost.[86]
See also
- Biological pest control – Controlling pests using other organisms
- List of organic gardening and farming topics– Overview of and topical guide to organic gardening and farming
- List of parasites of humans
- Nematode.net
- Soil food web
- Worm bagging – Form of vivipary observed in nematodes
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External links
- Harper Adams University College Nematology Research
- Nematodes/roundworms of man
- Introduction to the Nematoda
- European Society of Nematologists
- http://webarchive.loc.gov/all/20020914155908/http://www.nematodes.org/
- NeMys World free-living Marine Nematodes database
- Nematode Virtual Library
- Society of Nematologists
- Australasian Association of Nematologists Archived 26 February 2015 at the Wayback Machine
- Phylum Nematoda – nematodes on the UF
- Featured Creatures Web site—University of Florida Institute of Food and Agricultural Sciences (IFAS)