Nematodespecies 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 ofextant 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 byrarefaction curves,[8][9] together with the use ofDNA barcoding[10] and the increasing acknowledgment of widespreadcryptic species among nematodes,[11] have placed the figure closer to one million species.[12]
Nematodes have successfully adapted to nearly everyecosystem: 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 andspecies counts, and are found in locations as diverse as mountains, deserts, andoceanic trenches. They are found in every part of the Earth'slithosphere,[13] 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.[13] They represent 90% of all animals on theocean floor.[14] 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.[15] 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.[15][16] They play crucial roles in polar ecosystems.[17][18] The roughly 2,271genera are placed in 256families.[19] The many parasitic forms includepathogens in most plants and animals. A third of the genera occur asparasites ofvertebrates; about 35 nematode species arehuman parasites.[19]
The wordnematode comes from theModern Latin compound ofnema- 'thread' (from Greeknema, genitivenematos 'thread', from the stemnein 'to spin'; cf.needle) +-odes 'like, of the nature of' (cf.-oid). The addition firstly of '-oid' and then to '-ode' renders 'threadlike'.[20]
The name of the group Nematoda, informally called "nematodes", came fromNematoidea, originally defined byKarl Rudolphi in 1808,[22] fromAncient Greek νῆμα (nêma, nêmatos, 'thread') and -ειδἠς (-eidēs, 'species'). It was treated asfamily Nematodes byBurmeister in 1837.[22]
In 1861,Karl Moriz Diesing treated the group as order Nematoda.[22] In 1877, thetaxon Nematoidea, including the familyGordiidae (horsehair worms), was promoted to the rank of phylum byRay Lankester.[22] The first clear distinction between the nemas and gordiids was realized byFrantišek Vejdovsky when he named the group containing the horsehair worms the order Nematomorpha in 1886.[25]
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 ofnema), listing Nematoideasensu 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]
Thephylogenetic relationships of the nematodes and their close relatives among theprotostomes are unresolved. Traditionally, they were held to be a lineage of their own, but in the 1990s, they were proposed to form the groupEcdysozoa together withmoulting animals, such asarthropods. The identity of the closest living relatives of the Nematoda has always been considered to be well resolved. Morphological andmolecular phylogenetics agree with placing the roundworms as asister taxon to the parasiticNematomorpha; together, they make up theNematoida. Along with theScalidophora (formerly Cephalorhyncha), the Nematoida form the cladeCycloneuralia, 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 asuperphylum.[29][30]
Due to the lack of knowledge regarding many nematodes, their systematics is contentious. An early and influential classification was proposed by Chitwood and Chitwood[31]—later revised by Chitwood[32]—who divided the phylum into two classes—Aphasmidia andPhasmidia. These were later renamedAdenophorea (gland bearers) andSecernentea (secretors), respectively.[33] The Secernentea share several characteristics, including the presence ofphasmids, 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.
The Secernentea seem to be a natural group of close relatives, while the Adenophorea appear to be aparaphyletic assemblage of roundworms that retain a good number ofancestral traits. The oldEnoplia do not seem to be monophyletic, either, but do contain two distinct lineages. The old groupChromadorea seems to be another paraphyletic assemblage, with theMonhysterida representing a very ancient minor group of nematodes. Among the Secernentea, theDiplogasteria may need to be united with theRhabditia, while the Tylenchia might be paraphyletic with the Rhabditia.[36]
The understanding of roundworm systematics andphylogeny as of 2002 is summarised below:
Later work has suggested the presence of 12 clades.[37] In 2019, a study identified oneconserved 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]
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:
Nematode eggs from theclades Ascaridina, Spirurina, and Trichocephalida have been discovered incoprolites from theOligocene-aged Tremembé Formation, which represented apalaeolake in present-daySão Paulo with a diversefossil assemblage of birds, fish, and arthropods that lent itself to fostering high nematode diversity.[41] Nematodes have also been found in variouslagerstätten, such asBurmese amber, theMoltrasio Formation, and theRhynie chert, where the earliest known fossils are known from.
Internal anatomy of a maleC. elegans nematodeCross-section of femaleAscaris. The large circles filled with small green circles are the uterus and eggs. The long narrow feature is thedigestive tract. The smaller red and orange circles are theovaries andoviducts. The cluster of green and black blobs in the upper right and lower left are the nerve cords (ventral anddorsal). Surrounding the internal organs are the frilly green longitudinal muscles, the dark hypodermis, and the green outercuticle.
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] Theepidermis is either asyncytium or a single layer of cells, and is covered by a thickcollagenouscuticle. The cuticle is often of a complex structure and may have two or three distinct layers. Underneath the epidermis lies a layer of longitudinalmuscle 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 thenerve cords; this is a unique arrangement in the animal kingdom, in which nerve cells normally extend fibers into the muscles rather thanvice versa.[44]
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 sharpstylet, 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, suckingpharynx, also lined with cuticle. Digestive glands are found in this region of the gut, producingenzymes that start to break down the food. In stylet-bearing species, these may even be injected into the prey.[44]
Nostomach is present, with the pharynx connecting directly to a musclelessintestine that forms the main length of the gut. This produces further enzymes and also absorbs nutrients through its single-cell-thick lining. The last portion of the intestine is lined by a cuticle, forming arectum, which expels waste through theanus just below and in front of the tip of the tail. The movement of food through the digestive system is the result of the body movements of the worm. The intestine has valves orsphincters at either end to help control food movement through the body.[44]
Nitrogenous waste is excreted in the form ofammonia through the body wall, and is not associated with any specific organs. However, the structures for excreting salt to maintainosmoregulation are typically more complex.[44]
There is an excretory gland, also known as aventral cell, orrenette cell in all species of Adenophorea. In Secernentia there is an excretory canal system that may or may not use a gland cell.[42]
At the anterior end of the animal a dense, circularnerve ring which serves as thebrain 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. Theventral nerve is the largest, and has a double structure forward of theexcretorypore. 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 containscilia; these are all nonmotile and with a sensory function.[46][47]
The body is covered in numerous sensorybristles andpapillae 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 probablychemoreception organs. A few aquatic nematodes possess what appear to bepigmented eye-spots, but whether or not these are actually sensory in nature is unclear.[44]
Extremity of a male nematode showing thespicule, used for copulation, bar=100 μm[48]
Most nematode species aredioecious, with separate male and female individuals, though some, such asCaenorhabditis elegans, areandrodioecious, consisting ofhermaphrodites and rare males. Both sexes possess one or two tubulargonads. In males, the sperm are produced at the end of the gonad and migrate along its length as they mature. The testis opens into a relatively wideseminal vesicle and then during intercourse into a glandular and muscular ejaculatory duct associated with thevas deferens andcloaca. In females, the ovaries each open into anoviduct (in hermaphrodites, the eggs enter aspermatheca first) and then a glandularuterus. The uteri both open into a common vulva/vagina, usually located in the middle of the morphologically ventral surface.[44]
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. Duringcopulation, one or morechitinizedspicules move out of the cloaca and are inserted into the genital pore of the female.Amoeboidsperm crawl along the spicule into the female worm. Nematode sperm is thought to be the onlyeukaryotic cell without the globular proteinG-actin.[49]
Eggs may beembryonated or unembryonated when passed by the female, meaning their fertilized eggs may not yet be developed. A few species are known to beovoviviparous. The eggs are protected by an outer shell, secreted by the uterus. In free-living roundworms, the eggs hatch intolarvae, which appear essentially identical to the adults, except for an underdeveloped reproductive system; in parasitic roundworms, the lifecycle is often much more complicated.[44] The structure of the eggshell is complicated and includes several layers; a detailed anatomical and terminological framework has been proposed for these layers in 2023.[50]
Nematodes as a whole possess a wide range of modes of reproduction.[51] Some nematodes, such asHeterorhabditis spp., undergo a process calledendotokia matricida: intrauterine birth causing maternal death.[52] Some nematodes arehermaphroditic, and keep their self-fertilized eggs inside theuterus until they hatch. The juvenile nematodes then ingest the parent nematode. This process is significantly promoted in environments with a low food supply.[52]
The genusMesorhabditis 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 essentiallyclones of the female.[44]
The nematodeCaenorhabditis elegans is often used as a model organism for studyingaging at the molecular level. For example, inC. elegans aging negatively impactsDNA repair, and mutants ofC. 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 femaleC. elegans,germline processes that control DNA repair and formation ofchromosomal crossovers duringmeiosis were shown to progressively deteriorate with age.[55]
Different free-living species feed on materials as varied asbacteria,algae,fungi, small animals, fecal matter, dead organisms, and living tissues. Free-living marine nematodes are important and abundant members of themeiobenthos. They play an important role in the decomposition process, aid in recycling of nutrients in marine environments, and are sensitive to changes in the environment caused by pollution. One roundworm of note,C. elegans, lives in the soil and has found much use as amodel organism.C. elegans has had its entire genome sequenced,[56] the developmental fate of every cell determined, and every neuron mapped.[57]
Nematodes that commonly parasitise humans includeascarids (Ascaris),filarias,hookworms,pinworms (Enterobius), andwhipworms (Trichuris trichiura). The speciesTrichinella spiralis, commonly known as the trichina worm, occurs in rats, pigs, bears, and humans, and is responsible for the diseasetrichinosis.Baylisascaris usually infests wild animals, but can be deadly to humans, as well.Dirofilaria immitis is known for causing heartworm disease by inhabiting the hearts, arteries, and lungs of dogs and some cats.Haemonchus contortus is one of the most abundant infectious agents in sheep around the world, causing great economic damage to sheep. In contrast,entomopathogenic nematodes parasitize insects and are mostly considered beneficial by humans, but some attack beneficial insects.[citation needed]
One form of nematode is entirely dependent uponfig wasps, which are the sole source offig 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 parasitictetradonematid nematode discovered in 2005,Myrmeconema neotropicum, induces fruit mimicry in the tropical antCephalotes atratus. Infected ants develop bright redgasters (abdomens), tend to be more sluggish, and walk with their gasters in a conspicuous elevated position. These changes likely causefrugivorous birds to confuse the infected ants for berries, and eat them. Parasite eggs passed in the bird'sfeces are subsequently collected by foragingC. atratus and are fed to theirlarvae, thus completing the lifecycle ofM. neotropicum.[58]
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]
Rotations of plants with nematode-resistant species or varieties is one means of managing parasitic nematode infestations. For example, plantingTagetes marigolds as a cover crop just prior to planting a nematode-susceptible plant, has been shown to suppress nematodes.[67] Another is treatment with natural antagonists such as the fungusGliocladium roseum.Chitosan, a naturalbiocontrol, elicits plant defense responses to destroy parasiticcyst nematodes on roots ofsoybean,corn,sugar beet,potato, andtomato crops without harming beneficial nematodes in the soil.[68]Soil steaming is an efficient method to kill nematodes before planting a crop, but indiscriminately eliminates both harmful and beneficial soil fauna.
The golden nematodeGlobodera rostochiensis is a particularly harmful pest that has resulted in quarantines and crop failures worldwide. It can be controlled, however.CSIRO, the scientific research body of the Australian government, found a 13- to 14-fold reduction of nematode population densities in plots havingChinese mustardBrassica juncea green manure or seed meal in the soil.[69]
A number of pathogenic intestinal nematodes cause diseases in humans, includingascariasis,trichuriasis, andhookworm disease.Anisakis species parasitise fish, andmarine mammals and when consumed by humans can causeanisakiasis agastric or gastroallergic disease.[70] 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.[71]
Studies have shown that parasitic nematodes infectAmerican eels causing damage to the eel's swim bladder,[72] dairy animals like cattle and buffalo,[73] and all species of sheep.[74]
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 thenitrogen cycle by way of nitrogen mineralization.[75] But plant parasitic nematodes cause billions of dollars in annual crop damage worldwide.[76]
One group ofcarnivorous fungi, thenematophagous fungi, are predators of soil nematodes.[77] They can set enticements for the nematodes in the form of lassos or adhesive structures.[78][79][80] They can also release powerful toxins when in contact with nematodes.[81]
The nematodeCaenorhabditis elegans an importantmodel organism, was used as part of an ongoing research project conducted on the 2003Space ShuttleColumbia missionSTS-107, and survived there-entry breakup. It is believed to be the first known species to survive a virtually unprotected atmospheric descent to Earth's surface.[82][83] The Antarctic nematodePanagrolaimus davidi was able to withstand intracellular freezing depending on how well it had been fed.[84] In 2023 an individual ofPanagrolaimus kolymaensis was revived after 46,000 years in Siberian permafrost.[85]
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