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64-534: The Chromadorea are a class of the roundworm phylum, Nematoda . They contain a single subclass ( Chromadoria ) and several orders. With such a redundant arrangement, the Chromadoria are liable to be divided if the orders are found to form several clades , or abandoned if they are found to constitute a single radiation. Formerly, they were treated as a subclass in the paraphyletic " Adenophorea " assemblage, which has been mostly abandoned by modern authors. It

128-458: A characteristically bent or fan-shaped tail. During copulation , one or more chitinized spicules move out of the cloaca and are inserted into the genital pore of the female. Amoeboid sperm crawl along the spicule into the female worm. Nematode sperm is thought to be the only eukaryotic cell without the globular protein G-actin . Eggs may be embryonated or unembryonated when passed by

192-458: A collaborative wiki called 959 Nematode Genomes is underway to improve the systematics of this phylum. 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 following classes and subclasses are presented: Nematode eggs from

256-483: A genetically determined correlation between DNA repair capacity and lifespan. In female C. elegans , germline processes that control DNA repair and formation of chromosomal crossovers during meiosis were shown to progressively deteriorate with age. Different free-living species feed on materials as varied as bacteria , algae , fungi , small animals, fecal matter, dead organisms, and living tissues. Free-living marine nematodes are important and abundant members of

320-534: A glandular and muscular ejaculatory duct associated with the vas deferens and cloaca . In females, the ovaries each open into an oviduct (in hermaphrodites, the eggs enter a spermatheca first) and then a glandular uterus . The uteri both open into a common vulva/vagina, usually located in the middle of the morphologically ventral surface. 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

384-445: A large ventral, a smaller dorsal and two pairs of sublateral cords extend posteriorly. 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. The nervous system

448-431: A model for diversity that would allow him to compare species richness data among sets with different sample sizes; he developed rarefaction curves as a method to compare the shape of a curve rather than absolute numbers of species. Following initial development by Sanders, the technique of rarefaction has undergone a number of revisions. In a paper criticizing many methods of assaying biodiversity, Stuart Hurlbert refined

512-502: 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. At the anterior end of the animal a dense, circular nerve ring which serves as the brain surrounds the pharynx. From this ring six labial papillary nerve cords extend anteriorly, while six nerve cords;

576-410: A range of reproductive modes, including sexual reproduction , facultative sexuality (in which most, but not all, generations reproduce asexually), and both meiotic and mitotic parthenogenesis . 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

640-426: A synonym. 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

704-555: Is Bursaphelenchus xylophilus , the pine wood nematode, present in Asia and America and recently discovered in Europe. This nematode is transmitted from tree to tree by sawyer beetles ( Monochamus ). Rarefaction (ecology) The issue that occurs when sampling various species in a community is that the larger the number of individuals sampled, the more species that will be found. Rarefaction curves are created by randomly re-sampling

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768-511: Is a stub . You can help Misplaced Pages by expanding it . Nematode (see text ) The nematodes ( / ˈ n ɛ m ə t oʊ d z / NEM -ə-tohdz or NEEM - ; ‹See Tfd› Greek : Νηματώδη ; Latin : Nematoda ), roundworms or eelworms constitute the phylum Nematoda . They are a diverse animal phylum inhabiting a broad range of environments. Most species are free-living, feeding on microorganisms , but many species are parasitic . The parasitic worms ( helminths ) are

832-444: Is a lethal parasite of gastropods such as slugs and snails . Some members of the genus Steinernema such as Steinernema carpocapsae and Steinernema riobrave are generalist parasites of webworms , cutworms , armyworms , girdlers , some weevils , wood-borers and corn earworm moths . These organisms are grown commercially as biological pest control agents which can be used as an alternative to pesticides ; their use

896-615: Is also suspected that the Chromadorea may not be monophyletic as delimited here; at least the Monhysterida seem to be a distinct and far more ancient lineage than the rest. Members of this class' bodies usually have annules, their amphids elaborate and spiral, and they all have three esophageal glands . They usually live in marine sediments, although they can live elsewhere. They have a more sophisticated pharynx than most roundworms. Members of this class can be identified by

960-533: Is also the only place in the nematode body that contains cilia , which are all nonmotile and with a sensory function. 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

1024-560: Is considered very safe. Plant-parasitic nematodes include several groups causing severe crop losses, taking 10% of crops worldwide every year. The most common genera are Aphelenchoides ( foliar nematodes ), Ditylenchus , Globodera (potato cyst nematodes), Heterodera (soybean cyst nematodes), Longidorus , Meloidogyne ( root-knot nematodes ), Nacobbus , Pratylenchus (lesion nematodes), Trichodorus , and Xiphinema (dagger nematodes). Several phytoparasitic nematode species cause histological damages to roots, including

1088-415: Is extremely rare or common , or when beta diversity is very high. Rarefaction assumes that the number of occurrences of a species reflects the sampling intensity, but if one taxon is especially common or rare, the number of occurrences will be related to the extremity of the number of individuals of that species, not to the intensity of sampling. The technique does not account for specific taxa. It examines

1152-705: Is less than K whenever at least one group is missing from this subsample. Therefore the rarefaction curve , f n {\displaystyle f_{n}} is defined as: From this it follows that 0 ≤ f(n) ≤ K. Furthermore, f ( 0 ) = 0 , f ( 1 ) = 1 , f ( N ) = K {\displaystyle f(0)=0,f(1)=1,f(N)=K} . Despite being defined at discrete values of n, these curves are most frequently displayed as continuous functions. Rarefaction curves are necessary for estimating species richness. Raw species richness counts, which are used to create accumulation curves, can only be compared when

1216-460: 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

1280-447: Is often ornamented with ridges, rings, bristles, or other distinctive structures. 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'

1344-402: Is present, with the pharynx connecting directly to a muscleless intestine 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 a rectum , which expels waste through the anus just below and in front of the tip of the tail. The movement of food through

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1408-455: Is unclear. Most nematode species are dioecious , with separate male and female individuals, though some, such as Caenorhabditis elegans , are androdioecious , consisting of hermaphrodites and rare males. Both sexes possess one or two tubular gonads . 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 wide seminal vesicle and then during intercourse into

1472-546: The Moltrasio Formation , and the Rhynie chert , where the earliest known fossils are known from. Nematodes are very small, slender worms: typically about 5 to 100 μm thick, and 0.1 to 2.5 mm long. 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. The body

1536-807: 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 Later work has suggested the presence of 12 clades. The Secernentea—a group that includes virtually all major animal and plant 'nematode' parasites—apparently arose from within

1600-539: The Vermes . The name of the group Nematoda, informally called "nematodes", came from Nematoidea , originally defined by Karl Rudolphi (1808), from Ancient Greek νῆμα ( nêma, nêmatos , 'thread') and -ειδἠς ( -eidēs , 'species'). It was treated as family Nematodes by Burmeister (1837). At its origin, the "Nematoidea" erroneously included Nematodes and Nematomorpha , attributed by von Siebold (1843). Along with Acanthocephala , Trematoda , and Cestoidea , it formed

1664-459: 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 ,

1728-651: The meiobenthos . 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 a model organism . C. elegans has had its entire genome sequenced, the developmental fate of every cell determined, and every neuron mapped. Nematodes that commonly parasitise humans include ascarids ( Ascaris ), filarias , hookworms , pinworms ( Enterobius ), and whipworms ( Trichuris trichiura ). The species Trichinella spiralis , commonly known as

1792-413: The uterus until they hatch. The juvenile nematodes then ingest the parent nematode. This process is significantly promoted in environments with a low food supply. The nematode model species C. elegans , C. briggsae , and Pristionchus pacificus , among other species, exhibit androdioecy , which is otherwise very rare among animals. The single genus Meloidogyne (root-knot nematodes) exhibits

1856-565: 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 " Chromadoria " seems to be another paraphyletic assemblage, with

1920-459: The Adenophorea. In 2019, a study identified one conserved signature indel (CSI) 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. A major effort by

1984-605: The Introverta—depending on the validity of the former—are often ranked as a superphylum . For an up-to-date view (as of 2022), see Phylogenomic Analysis of the Phylum Nematoda: Conflicts and Congruences With Morphology, 18S rRNA, and Mitogenomes . 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

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2048-696: The Nematoda has always been considered to be well resolved. Morphological characters and molecular phylogenies agree with placement of the roundworms as a sister taxon to the parasitic Nematomorpha ; together, they make up the Nematoida . Along with the Scalidophora (formerly Cephalorhyncha), the Nematoida form the clade Cycloneuralia , but much disagreement occurs both between and among the available morphological and molecular data. The Cycloneuralia or

2112-630: 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 published in the mega journal Zootaxa puts this figure at over 25,000. 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. A subsequent publication challenged this claim, estimating

2176-408: The animal can thrust into its prey. In some species, the style is hollow and can be used to suck liquids from plants or animals. 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. No stomach

2240-465: The bee less active, thus less effective in pollen collection. Depending on its species, a nematode may be beneficial or detrimental to plant health. From agricultural and horticulture perspectives, the two categories of nematodes are the predatory ones, which kill garden pests; and the pest nematodes, which attack plants, or act as vectors spreading plant viruses between crop plants. Predatory nematodes include Phasmarhabditis hermaphrodita which

2304-402: The cause of soil-transmitted helminthiases . They are taxonomically classified along with arthropods , tardigrades and other moulting animals in the clade Ecdysozoa . Unlike the vaguely similar 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

2368-495: The digestive system is the result of the body movements of the worm. The intestine has valves or sphincters at either end to help control food movement through the body. Nitrogenous waste is excreted in the form of ammonia through the body wall, and is not associated with any specific organs. However, the structures for excreting salt to maintain osmoregulation are typically more complex. In many marine nematodes, one or two unicellular ' renette glands ' excrete salt through

2432-445: The eggshell is complicated and includes several layers; a detailed anatomical and terminological framework has been proposed for these layers in 2023. Nematodes as a whole possess a wide range of modes of reproduction. Some nematodes, such as Heterorhabditis spp., undergo a process called endotokia matricida : intrauterine birth causing maternal death. Some nematodes are hermaphroditic , and keep their self-fertilized eggs inside

2496-426: The female, meaning their fertilized eggs may not yet be developed. A few species are known to be ovoviviparous . The eggs are protected by an outer shell, secreted by the uterus. In free-living roundworms, the eggs hatch into larvae , which appear essentially identical to the adults, except for an underdeveloped reproductive system; in parasitic roundworms, the lifecycle is often much more complicated. The structure of

2560-479: The figure to be at least 40,000 species. Although the highest estimates (up to 100 million species) 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 1 million species. Nematodes have successfully adapted to nearly every ecosystem : from marine (salt) to fresh water, soils, from

2624-400: The first used term in case of synonyms. The phylogenetic 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, they were proposed to form the group Ecdysozoa together with moulting animals, such as arthropods . The identity of the closest living relatives of

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2688-490: The formation of visible galls (e.g. by root-knot nematodes), which are useful characters for their diagnostic in the field. Some nematode species transmit plant viruses through their feeding activity on roots. One of them is Xiphinema index , vector of grapevine fanleaf virus , an important disease of grapes, another one is Xiphinema diversicaudatum , vector of arabis mosaic virus . Other nematodes attack bark and forest trees. The most important representative of this group

2752-473: The genera occur as parasites of vertebrates ; about 35 nematode species occur in humans. The word nematode comes from the Modern Latin compound of nemat- 'thread' (from Greek nema , genitive nematos 'thread', from the stem nein 'to spin'; cf. needle ) + -odes 'like, of the nature of' (cf. -oid ). In 1758, Linnaeus described some nematode genera (e.g., Ascaris ), then included in

2816-450: The infected ants for berries, and eat them. Parasite eggs passed in the bird's feces are subsequently collected by foraging C. atratus and are fed to their larvae , thus completing the lifecycle of M. neotropicum . 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

2880-418: 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 . 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

2944-437: 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. 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, because of the zoological rule that gives priority to

3008-465: The nemas and gordiids was realized by Vejdovsky when he named a group to contain the horsehair worms the order 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 (later emended as Nemata, Latin plural of nema ), listing Nematoidea sensu restricto as

3072-431: The number of individuals present, which is not always true. Rarefaction analysis assumes that the individuals in an environment are randomly distributed, the sample size is sufficiently large, that the samples are taxonomically similar, and that all of the samples have been performed in the same manner. If these assumptions are not met, the resulting curves will be greatly skewed. Rarefaction only works well when no taxon

3136-446: The number of items in group i (i = 1, ..., K ). M j = number of groups consisting in j elements From these definitions, it therefore follows that: In a rarefied sample we have chosen a random subsample n from the total N items. The relevance of a rarefied sample is that some groups may now be necessarily absent from this subsample. We therefore let: It is true that X n {\displaystyle X_{n}}

3200-625: The number of species is sampled to predict the number of genera in a particular community; similar techniques had been used to determine this level of diversity in studies several years before Sanders quantified his individual to species determination of rarefaction. Rarefaction techniques are used to quantify species diversity of newly studied ecosystems, including human microbiomes, as well as in applied studies in community ecology, such as understanding pollution impacts on communities and other management applications. Deriving rarefaction: N = total number of items K = total number of groups N i =

3264-415: The number of species present in a given sample, but does not look at which species are represented across samples. Thus, two samples that each contain 20 species may have completely different compositions, leading to a skewed estimate of species richness. The technique does not recognize species abundance , only species richness. A true measure of diversity accounts for both the number of species present and

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3328-429: The number of taxa should be plotted as a function of the accumulated number of individuals, not accumulated number of samples, because datasets may differ systematically in the mean number of individuals per sample. One cannot simply divide the number of species found by the number of individuals sampled in order to correct for different sample sizes. Doing so would assume that the number of species increases linearly with

3392-411: The obsolete group Entozoa , created by Rudolphi (1808). They were also classed along with Acanthocephala in the obsolete phylum Nemathelminthes by Gegenbaur (1859). In 1861, K. M. Diesing treated the group as order Nematoda. In 1877, the taxon Nematoidea, including the family Gordiidae (horsehair worms), was promoted to the rank of phylum by Ray Lankester . The first clear distinction between

3456-491: 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. 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. These findings suggest

3520-422: The phylum into two classes— Aphasmidia and 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, though

3584-504: 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, 0.9–3.6 km (3,000–12,000 ft) below

3648-433: The pool of N samples multiple times and then plotting the average number of species found in each sample (1,2, ... N). "Thus rarefaction generates the expected number of species in a small collection of n individuals (or n samples) drawn at random from the large pool of N samples.". The technique of rarefaction was developed in 1968 by Howard Sanders in a biodiversity assay of marine benthic ecosystems, as he sought

3712-623: The presence of eight conserved signature indels (CSIs) exclusively shared by the class. These molecular markers are found in essential proteins such as tRNA (guanine-N(1))-methyltransferase and can serve as a reliable molecular method of distinguishing the Chromadorea from other classes within the phylum Nematoda. Provisionally, the following orders are placed here: The Benthimermithida are also occasionally placed here. The Ascaridida appear to be nested within Rhabditida. This Chromadorea nematode (or roundworm-) related article

3776-503: The problem that he saw with Sanders' rarefaction method, that it overestimated the number of species based on sample size, and attempted to refine his methods. The issue of overestimation was also dealt with by Daniel Simberloff , while other improvements in rarefaction as a statistical technique were made by Ken Heck in 1975. Today, rarefaction has grown as a technique not just for measuring species diversity, but of understanding diversity at higher taxonomic levels as well. Most commonly,

3840-524: The species richness has reached a clear asymptote . Rarefaction curves produce smoother lines that facilitate point-to-point or full dataset comparisons. One can plot the number of species as a function of either the number of individuals sampled or the number of samples taken. The sample-based approach accounts for patchiness in the data that results from natural levels of sample heterogeneity. However, when sample-based rarefaction curves are used to compare taxon richness at comparable levels of sampling effort,

3904-843: 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 × 10 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 have been shown to 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

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3968-414: The trichina worm, occurs in rats, pigs, bears, and humans, and is responsible for the disease trichinosis . 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

4032-452: The wasp, and their offspring await the birth of the next generation of wasps as the fig ripens. A parasitic tetradonematid nematode discovered in 2005, Myrmeconema neotropicum , induces fruit mimicry in the tropical ant Cephalotes atratus . Infected ants develop bright red gasters (abdomens), tend to be more sluggish, and walk with their gasters in a conspicuous elevated position. These changes likely cause frugivorous birds to confuse

4096-432: 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. 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

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