Nematomorpha

Nematomorphs possess an external cuticle without cilia. Internally, they have only longitudinal muscle and a non-functional gut, with no excretory, respiratory or circulatory systems. The nervous system consists of a nerve ring near the anterior end of the animal and a ventral nerve cord running along the body.{{Cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages= 307–308|isbn= 978-0-03-056747-6}}

Reproductively, they have two distinct sexes, with the internal fertilization of eggs that are then laid in gelatinous strings. Adults have cylindrical gonads, opening into the cloaca. The larvae have rings of cuticular hooks and terminal stylets that are believed to be used to enter the hosts. Once inside the host, the larvae live inside the haemocoel and absorb nutrients directly through their skin. Development into the adult form takes weeks or months, and the larva moults several times as it grows in size.

The adults are mostly free-living in freshwater or marine environments, and males and females aggregate into tight balls (Gordian knots) during mating.{{Cite journal|first=Andreas|last=Schmidt-Rhaesa|year=2002|title=Two Dimensions of Biodiversity Research Exemplified by Nematomorpha and Gastrotricha|journal=Integrative and Comparative Biology|volume=42|issue=3|pages=633–640|doi=10.1093/icb/42.3.633|pmid=21708759 |doi-access=free}}

Image:Spinochordodes in Meconema.jpg and its Meconema host]]

In Spinochordodes tellinii and Paragordius tricuspidatus, which have grasshoppers and crickets as their hosts, the infection acts on the infected host's brain.{{cite journal |doi = 10.1016/S0020-7519(03)00014-6 |author = Thomas, F.|year=2003 |title=Biochemical and histological changes in the brain of the cricket Nemobius sylvestris infected by a manipulative parasite Paragordius tricuspudatus (Nematomorpha) |journal=International Journal for Parasitology |volume=33 |pages=435–443 |issue = 4| pmid=12705936 | url=https://hal.science/hal-00307180 |display-authors=etal}} This causes the host insect to seek water and drown itself, thus returning the nematomorph to water.{{Cite journal |last = Thomas |first = F. |author2 = Schmidt-Rhaesa, A. |author3 = Martin, G. |author4 = Manu, C. |author5 = Durand, P. |author6 = Renaud, F. |date = May 2002 |title = Do hairworms (Nematomorpha) manipulate the water seeking behaviour of their terrestrial hosts? |journal = Journal of Evolutionary Biology |volume = 15 |issue = 3 |pages = 356–361 |url = http://www.erin.utoronto.ca/~w3gwynne/BIO418/Nemato.pdf |doi = 10.1046/j.1420-9101.2002.00410.x |url-status = dead |archive-url = https://web.archive.org/web/20150924002153/http://www.erin.utoronto.ca/~w3gwynne/BIO418/Nemato.pdf |archive-date = 2015-09-24 |citeseerx = 10.1.1.485.9002 |s2cid = 86278524 }} — according to Thomas et al., the "infected insects may first display an erratic behaviour which brings them sooner or later close to a stream and then a behavioural change that makes them enter the water", rather than seeking out water over long distances. P. tricuspidatus is also remarkably able to survive the predation of their host, being able to wiggle out of the predator that has eaten the host.{{Cite journal |last = Ponton |first = Fleur |author2=Camille Lebarbenchon |author3=Thierry Lefèvre |author4=David G. Biron |author5=David Duneau |author6=David P. Hughes |author7=Frédéric Thomas |date=April 2006 |title = Parasitology: Parasite survives predation on its host |journal = Nature |pmid = 16598248 |volume = 440 |issue = 7085 |doi = 10.1038/440756a |page = 756 |bibcode = 2006Natur.440..756P |s2cid = 7777607 |url = https://hal.umontpellier.fr/hal-02992927/file/Ponton%20et%20al%202006%20nature.pdf }} The nematomorpha parasite affects host Hierodula patellifera{{'}}s light-interpreting organs so the host is attracted to horizontally polarized light. Thus the host goes into water and the parasite's lifecycle completes.{{Cite web|title=Parasites manipulate praying mantis's polarized-light perception, causing it to jump into water|url=https://phys.org/news/2021-06-parasites-mantis-polarized-light-perception.html|access-date=2021-07-04|website=phys.org|language=en}} Many of the genes the parasites use for manipulating their host have been acquired through horizontal gene transfer from the host genome.[https://www.popularmechanics.com/science/animals/a45575538/horsehair-worm-horizontal-gene-transfer/ This Parasitic Worm 'Steals' Genes From Its Unsuspecting Host]

There are a few cases of accidental parasitism in vertebrate hosts, including dogs,{{Cite journal |doi=10.3347/kjp.2015.53.6.719 |pmid=26797439|title=A Horsehair Worm, Gordius sp. (Nematomorpha: Gordiida), Passed in a Canine Feces |journal=The Korean Journal of Parasitology |volume=53 |issue=6 |pages=719–24 |year=2015 |last1=Hong |first1=Eui-Ju |last2=Sim |first2=Cheolho |last3=Chae |first3=Joon-Seok |last4=Kim |first4=Hyeon-Cheol |last5=Park |first5=Jinho |last6=Choi |first6=Kyoung-Seong |last7=Yu |first7=Do-Hyeon |last8=Yoo |first8=Jae-Gyu |last9=Park |first9=Bae-Keun |pmc=4725239}} cats,{{Cite journal |last1=Saito |first1=Y |last2=Inoue |first2=I |last3=Hayashi |first3=F |last4=Itagaki |first4=H |year=1987 |title=A hairworm, Gordius sp., vomited by a domestic cat |journal=Nihon Juigaku Zasshi. The Japanese Journal of Veterinary Science |volume=49 |issue=6 |pages=1035–7 |doi=10.1292/jvms1939.49.1035 |pmid=3430914 |doi-access=free}} and humans. Several cases involving Parachordodes, Paragordius, or Gordius have been recorded in human hosts in Japan and China.{{Cite journal|doi=10.3347/kjp.2012.50.3.263|pmid=22949758|title=Two Human Cases Infected by the Horsehair Worm, Parachordodes sp. (Nematomorpha: Chordodidae), in Japan and America |journal=The Korean Journal of Parasitology |volume=50 |issue=3 |pages=263–7 |year=2012 |last1=Yamada |first1=Minoru |last2=Tegoshi |first2=Tatsuya |last3=Abe |first3=Niichiro |last4=Urabe |first4=Misako|pmc=3428576}}

File:Habitus saitenwurm zarbach benecke.jpgOwing to their use of orthopterans as hosts, nematomorphs can be significant factors in shaping community ecology. One study conducted in a Japanese riparian ecosystem showed that nematomorphs could cause orthopterans to become 20 times more likely to enter water than uninfected orthopterans; these orthopterans constituted up to 60% of the annual energy intake for the Kirikuchi char. The absence of nematomorphs from riparian communities can thus lead to char preying more heavily on other aquatic invertebrates, potentially causing more widespread physiological effects.{{Cite journal|last1=Sato|first1=Takuya|last2=Watanabe|first2=Katsutoshi|last3=Kanaiwa|first3=Minoru|last4=Niizuma|first4=Yasuaki|last5=Harada|first5=Yasushi|last6=Lafferty|first6=Kevin D.|date=2011|title=Nematomorph parasites drive energy flow through a riparian ecosystem|journal=Ecology|language=en|volume=92|issue=1|pages=201–207|doi=10.1890/09-1565.1|pmid=21560690|bibcode=2011Ecol...92..201S |issn=1939-9170|hdl=2433/139443|s2cid=20274754 |hdl-access=free}}

{{See also|List of bilaterial animal orders}}

Nematomorphs can be confused with nematodes, particularly mermithid worms. Unlike nematomorphs, mermithids do not have a terminal cloaca. Male mermithids have one or two spicules just before the end, apart from having a thinner, smoother cuticle without areoles and paler brown colour.{{Cite web |first1=Malcolm S. |last1=Bryant |first2=Robert D. |last2=Adlard |first3=Lester R.G. |last3=Cannon |year=2006 |title=Gordian Worms: Factsheet |website=Queensland Museum |url=http://www.qm.qld.gov.au/inquiry/factsheets/Gordian_Worms_20070605.pdf |access-date=2008-03-10 |url-status=dead |archive-url=https://web.archive.org/web/20080722175823/http://www.qm.qld.gov.au/inquiry/factsheets/Gordian_Worms_20070605.pdf |archive-date=2008-07-22 }}

The phylum is placed along with the Ecdysozoa clade of moulting organisms that include the Arthropoda. Their closest relatives are the nematodes. The two phyla make up the group Nematoida in the clade Cycloneuralia. During the larval stage, the animals show a resemblance to adult kinorhyncha and some species of Loricifera and Priapulida, all members of the group Scalidophora.[http://www.bumblebee.org/invertebrates/NEMATOMORPHA.htm Nematomorpha – Bumblebees] The earliest Nematomorph could be Maotianshania, from the Lower Cambrian; this organism is, however, very different from extant species;{{Cite journal|last1= Sun|first1= W.|last2= Hou|first2= X.|year= 1987|title= Early Cambrian worms from Chengjiang, Yunnan, China: Maotianshania gen. nov.|journal= Acta Palaeontologica Sinica|volume=26|issue=3|pages=299–305|url= http://en.cnki.com.cn/Article_en/CJFDTOTAL-GSWX198703007.htm|format=Paywall}} fossilized worms resembling the modern forms have been reported from mid Cretaceous Burmese amber dated to 100 million years ago.{{Cite journal |author1=Poinar George |author2=Ron Buckley | date = September 2006 | title = Nematode (Nematoda: Mermithidae) and hairworm (Nematomorpha: Chordodidae) parasites in Early Cretaceous amber | journal = Journal of Invertebrate Pathology | volume = 93 | issue = 1| pages = 36–41 | doi = 10.1016/j.jip.2006.04.006 | pmid = 16737709 |bibcode=2006JInvP..93...36P }}

Relationships within the phylum are still somewhat unclear, but two classes are recognised. The five marine species of nematomorph are contained in Nectonematoida.Pechenik, 'Biology of the Invertebrates, 2010, pg 457. This order is monotypic containing the genus Nectonema Verrill, 1879: adults are planktonic and the larvae parasitise decapod crustaceans, especially crabs. They are characterized by a double row of natotory setae along each side of the body, dorsal and ventral longitudinal epidermal cords, a spacious and fluid-filled blastocoelom and singular gonads.

The approximately 320 remaining species are distributed between two families,{{ITIS|id=699878 |taxon=Gordioidea}} within the monotypic class Gordioida. Gordioidean adults are free-living in freshwater or semiterrestrial habitats and larvae parasitise insects, primarily orthopterans. Unlike nectonematiodeans, gordioideans lack lateral rows of setae, have a single, ventral epidermal cord and their blastocoels are filled with mesenchyme in young animals but become spacious in older individuals.

{{Reflist}}

• {{Cite book|last=Pechenik|first=Jan A.|title=Biology of the Invertebrates|edition=6th International|year=2010|publisher=Mc-Graw Hill Education (Asia)|location=Singapore|isbn=978-0-07-127041-0|pages=452–457|chapter=Four Phyla of Likely Nematode Relatives}}

• {{Cite journal |vauthors=Baker GL, Capinera JL |year=1997 |title=Nematodes and nematomorphs as control agents of grasshoppers and locusts |journal=Memoirs of the Entomological Society of Canada |volume=171 |pages=157–211 |doi=10.4039/entm129171157-1}}
• {{Cite book |chapter=Biology of the phylum Nematomorpha |last1=Hanelt |first1=B. |first2=F. |last2=Thomas |first3=A. |last3=Schmidt-Rhaesa |title=Advances in Parasitology |volume=59 |year=2005 |pages=243–305 |doi=10.1016/S0065-308X(05)59004-3 |pmid=16182867 |isbn=9780120317592 }}
• {{Cite book |author=Poinar GO Jr |year=1991 |chapter=Nematoda and Nematomorpha |pages=249–283 |veditors=Thorp JH, Covich AP |title=Ecology and Classification of North American Freshwater Invertebrates |publisher=Academic Press |location=San Diego, CA}}
• {{Cite journal |author=Thorne G |year=1940 |title=The hairworm, Gordius robustus Leidy, as a parasite of the Mormon cricket, Anabrus simplex Haldeman |journal=Journal of the Washington Academy of Sciences |volume=30 |pages=219–231}}

{{EB1911 poster|Nematomorpha}}

• Capinera, J. L. [http://edis.ifas.ufl.edu/in274 Horsehair Worms, Hairworms, Gordian Worms, Nematomorphs, Gordius spp. (Nematomorpha: Gordioidea).] University of Florida IFAS. Published 1999, revised 2005.
• [https://www.youtube.com/watch?v=76BnVsaLiR0 Nematomorph worm – Behavior modification of cricket by nematomorph worm.] YouTube.
• Gordian worms discussed on RNZ Critter of the Week, [http://www.radionz.co.nz/national/programmes/afternoons/audio/201777689/critter-of-the-week 6 November 2015]

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Category:Ecdysozoa phyla

Category:Nematoida

Category:Parasitic protostomes

Category:Suicide-inducing parasitism