Nematomorpha (sometimes calledGordiacea, and commonly known ashorsehair worms,hairsnakes,[1][2][3] orGordian worms) are aphylum ofparasitoidanimals superficially similar tonematodeworms inmorphology, hence the name. Most species range in size from 5 to 10 centimetres (2 to 4 in), reaching 2 metres (6 ft 7 in) in extreme cases, and 1 to 3 millimetres (0.039 to 0.118 in) in diameter. Horsehair worms can be discovered in damp areas, such as watering troughs, swimming pools, streams, puddles, and cisterns. The adult worms are free-living, but thelarvae areparasitic onarthropods, such asbeetles,cockroaches,mantises,orthopterans, andcrustaceans.[4] About 351 freshwater species are known[5] and a conservative estimate suggests that there may be about 2000 freshwater species worldwide.[6] The name "Gordian" stems from the legendaryGordian knot. This relates to the fact that nematomorphs often coil themselves in tight balls that resemble knots.[7]
Reproductively, they havetwo distinct sexes, with the internal fertilization of eggs that are then laid in gelatinous strings. Adults have cylindricalgonads, opening into thecloaca. 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 thehaemocoel and absorb nutrients directly through their skin. Development into the adult form takes weeks or months, and the larvamoults several times as it grows in size.[8]
The adults are mostly free-living in freshwater or marine environments, and males and females aggregate into tight balls (Gordian knots) during mating.[9][10]
InSpinochordodes tellinii andParagordius tricuspidatus, which havegrasshoppers andcrickets as their hosts, the infection acts on the infected host's brain.[11] This causes the host insect to seek water and drown itself, thus returning the nematomorph to water.[9]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.[12] The nematomorpha parasite affects hostHierodula patellifera's light-interpreting organs so the host is attracted to horizontallypolarized light. Thus the host goes into water and the parasite's lifecycle completes.[13] Many of the genes the parasites use for manipulating their host have been acquired throughhorizontal gene transfer from the host genome.[14]
There are a few cases of accidental parasitism in vertebrate hosts, including dogs,[15] cats,[16] and humans. Several cases involvingParachordodes,Paragordius, orGordius have been recorded in human hosts in Japan and China.[17]
Mermithid worm (Phylum Nematoda), often confused with horsehair worms (Phylum Nematomorpha). Dark structures are eggs, not seen in horsehair worms. Germany 2021.
Owing to their use oforthopterans as hosts, nematomorphs can be significant factors in shaping community ecology. One study conducted in a Japaneseriparian 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 theKirikuchi 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.[18]
Nematomorphs can be confused with nematodes, particularlymermithid 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.[19]
The phylum is placed along with theEcdysozoa clade of moulting organisms that include theArthropoda. Their closest relatives are the nematodes. The two phyla make up the groupNematoida in the cladeCycloneuralia. During the larval stage, the animals show a resemblance to adultkinorhyncha and some species ofLoricifera andPriapulida, all members of the groupScalidophora.[20] The earliest Nematomorph could beMaotianshania, from the Lower Cambrian; this organism is, however, very different from extant species;[21] fossilized worms resembling the modern forms have been reported from midCretaceousBurmese amber dated to 100 million years ago.[22]
Relationships within the phylum are still somewhat unclear, but two classes are recognised. The five marine species of nematomorph are contained inNectonematoida.[23] This order is monotypic containing the genusNectonemaVerrill, 1879: adults are planktonic and the larvae parasitisedecapod crustaceans, especially crabs.[23] They are characterized by a double row of natotorysetae along each side of the body, dorsal and ventral longitudinal epidermal cords, a spacious and fluid-filledblastocoelom and singulargonads.
The approximately 320 remaining species are distributed between two families,[24] within the monotypic classGordioida. Gordioidean adults are free-living in freshwater or semiterrestrial habitats and larvae parasitise insects, primarilyorthopterans.[23] Unlike nectonematiodeans, gordioideans lack lateral rows of setae, have a single, ventral epidermal cord and their blastocoels are filled withmesenchyme in young animals but become spacious in older individuals.
^Heston, Joshua."Horse Hair Snake".State of the Ozarks. Retrieved1 January 2021.
^Troxell, W.H. (30 January 1903)."Erroneous Beliefs"(PDF).Emmitsburg Chronicle. No. 37.Archived(PDF) from the original on 2021-05-09. Retrieved1 January 2021.
Pechenik, Jan A. (2010). "Four Phyla of Likely Nematode Relatives".Biology of the Invertebrates (6th International ed.). Singapore: Mc-Graw Hill Education (Asia). pp. 452–457.ISBN978-0-07-127041-0.
Baker GL, Capinera JL (1997). "Nematodes and nematomorphs as control agents of grasshoppers and locusts".Memoirs of the Entomological Society of Canada.171:157–211.doi:10.4039/entm129171157-1.
Poinar GO Jr (1991). "Nematoda and Nematomorpha". In Thorp JH, Covich AP (eds.).Ecology and Classification of North American Freshwater Invertebrates. San Diego, CA: Academic Press. pp. 249–283.
Thorne G (1940). "The hairworm,Gordius robustus Leidy, as a parasite of the Mormon cricket,Anabrus simplex Haldeman".Journal of the Washington Academy of Sciences.30:219–231.
