Nematode

Nematode
Temporal range: Early Devonian–Recent[1] Possible Cambrian occurrence [2]
Caenorhabditis elegans,
a model species of roundworm
Scientific classification Edit this 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
  • Nematodes Burmeister, 1837
  • Nematoidea sensu stricto Cobb, 1919
  • Nemates Cobb, 1919
  • Nemata Cobb, 1919 emend.

The nematodes (/ˈnɛmətoʊdz/ NEM-ə-tohdz or NEEM-; ‹See Tfd›Greek: Νηματώδη; Latin: Nematoda), roundworms or eelworms constitute the phylum Nematoda.[3][4] 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.[3] The parasitic worms (helminths) are 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 the ancestral protostome Hox genotype, which shows that the reduction has occurred within the nematode phylum.[5]

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][15] They represent 90% of all animals on the ocean floor.[20] 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.[21] 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.[21][22] They have been shown to play crucial roles in polar ecosystems.[23][24] The roughly 2,271 genera are placed in 256 families.[25] 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 occur in humans.[25]

  1. ^ Poinar, George; Kerp, Hans; Hass, Hagen (January 2008). "Palaeonema phyticum gen. n., sp. n. (Nematoda: Palaeonematidae fam. n.), a Devonian nematode associated with early land plants". Nematology. 10 (1): 9–14. doi:10.1163/156854108783360159.
  2. ^ Maas, Andreas; Waloszek, Dieter; Haug, Joachim; Müller, Klaus (January 2007). "A possible larval roundworm from the Cambrian 'Orsten' and its bearing on the phylogeny of Cycloneuralia". Memoirs of the Association of Australasian Palaeontologists. 34: 499–519.
  3. ^ a b "Nematodes". Retrieved 11 October 2023.
  4. ^ Hay, Frank. "Nematodes - the good, the bad and the ugly". APS News & Views. apsnet.org. American Phytopathological Society. Retrieved 28 November 2020.
  5. ^ Baker, Emily A.; Woollard, Alison (2019). "How weird is the worm? Evolution of the developmental gene toolkit in Caenorhabditis elegans". Journal of Developmental Biology. 7 (4): 19. doi:10.3390/jdb7040019. PMC 6956190. PMID 31569401.
  6. ^ Hodda, M (2011). Zhang, Z.-Q. (ed.). "Phylum Nematoda (Cobb, 1932)". Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness. Zootaxa. 3148: 63–95. doi:10.11646/zootaxa.3148.1.11.
  7. ^ Zhang, Z. (2013). Zhang, Z.-Q. (ed.). "Animal biodiversity: An update of classification and diversity in 2013". Animal biodiversity: An update of classification and diversity (Addenda 2013). Zootaxa. 3703 (1): 5–11. doi:10.11646/zootaxa.3703.1.3.
  8. ^ Cite error: The named reference Lambshead-1993 was invoked but never defined (see the help page).
  9. ^ Cite error: The named reference RAnderson was invoked but never defined (see the help page).
  10. ^ Cite error: The named reference Lambshead_Boucher was invoked but never defined (see the help page).
  11. ^ Cite error: The named reference Qing_Bert was invoked but never defined (see the help page).
  12. ^ Cite error: The named reference Floyd2002 was invoked but never defined (see the help page).
  13. ^ Cite error: The named reference Derycke_et_al was invoked but never defined (see the help page).
  14. ^ Cite error: The named reference Blaxter2016 was invoked but never defined (see the help page).
  15. ^ a b Cite error: The named reference Borgonie 2011 was invoked but never defined (see the help page).
  16. ^ Cite error: The named reference Lemonick 2011 was invoked but never defined (see the help page).
  17. ^ Cite error: The named reference Bhanoo 2011 was invoked but never defined (see the help page).
  18. ^ Cite error: The named reference Nature-2011-06-02 was invoked but never defined (see the help page).
  19. ^ Cite error: The named reference Drake 2011-06-01 was invoked but never defined (see the help page).
  20. ^ Danovaro R, Gambi C, Dell'Anno A, Corinaldesi C, Fraschetti S, Vanreusel A, et al. (January 2008). "Exponential decline of deep-sea ecosystem functioning linked to benthic biodiversity loss". Curr. Biol. 18 (1): 1–8. Bibcode:2008CBio...18....1D. doi:10.1016/j.cub.2007.11.056. PMID 18164201. S2CID 15272791.
  21. ^ a b van den Hoogen, Johan; Geisen, Stefan; Routh, Devin; Ferris, Howard; Traunspurger, Walter; Wardle, David A.; et al. (24 July 2019). "Soil nematode abundance and functional group composition at a global scale". Nature. 572 (7768): 194–8. Bibcode:2019Natur.572..194V. doi:10.1038/s41586-019-1418-6. hdl:20.500.11755/c8c7bc6a-585c-4a13-9e36-4851939c1b10. ISSN 0028-0836. PMID 31341281. S2CID 198492891. Archived from the original on 2 March 2020. Retrieved 10 December 2019.
  22. ^ Platt HM (1994). "foreword". In Lorenzen S, Lorenzen SA (eds.). The phylogenetic systematics of freeliving nematodes. Ray Society (Series). Vol. 162. London, UK: Ray Society. ISBN 978-0-903874-22-9. OCLC 1440106662.
  23. ^ Cary, S. Craig; Green, T.G. Allan; Storey, Bryan C.; Sparrow, Ashley D.; Hogg, Ian D.; Katurji, Marwan; et al. (15 February 2019). "Biotic interactions are an unexpected yet critical control on the complexity of an abiotically driven polar ecosystem". Communications Biology. 2 (1): 62. doi:10.1038/s42003-018-0274-5. PMC 6377621. PMID 30793041.
  24. ^ Adams, Byron J.; Wall, Diana H.; Storey, Bryan C.; Green, T.G. Allan; Barrett, John E.; Cary, S. Craig; et al. (15 February 2019). "Nematodes in a polar desert reveal the relative role of biotic interactions in the coexistence of soil animals". Communications Biology. 2 (1): 63. doi:10.1038/s42003-018-0260-y. PMC 6377602. PMID 30793042.
  25. ^ a b Anderson, Roy C. (8 February 2000). Nematode Parasites of Vertebrates: Their development and transmission. CABI. p. 1. ISBN 978-0-85199-786-5.