Nematomorpha (sometimes called Gordiacea, and commonly known as horsehair worms or Gordian worms) are a phylum of parasitoid animals superficially similar to nematode worms in morphology, hence the name. They range in size in most species from 50 to 100 millimetres (2.0 to 3.9 in) long and can reach in extreme cases up to 2 metres, 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 the larvae are parasitic on arthropods, such as beetles, cockroaches, mantids, orthopterans, and crustaceans. About 351 freshwater species are known and a conservative estimate suggests that there may be about 2000 freshwater species worldwide. The name "Gordian" stems from the legendary Gordian knot. This relates to the fact that nematomorpha often tie themselves in knots.
Description and biologyEdit
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.
Reproductively, they are dioecious, 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.
In Spinochordodes tellinii and Paragordius tricuspidatus which have orthopterans as their vector, the infection acts on the insect's brain and causes it to seek water and drown itself, thus returning the nematomorph to water. 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.
There are a few cases of accidental parasitism in vertebrate hosts including dogs and humans. Several cases involving Parachordodes, Paragordius, or Gordius have been recorded in human hosts in Japan and China.
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 a paler brown colour.
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. The earliest Nematomorph could be Maotianshiania, from the Lower Cambrian; this organism is, however, very different from extant species; fossilized worms resembling the modern forms have been reported from Early Cretaceous Burmese amber dated to 100–110 million years.
Relationships within the phylum are still somewhat unclear, but two classes are recognised. The five marine species of nematomorph are contained in Nectonematoida. 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 into two families and seven genera within order 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.
- Hanelt, B, F. Thomas, and A. Schmidt-Rhaesa (2005). "Biology of the phylum Nematomorpha". Advances in Parasitology. 59: 244–305. doi:10.1016/S0065-308X(05)59004-3.
- Zhang, Z.-Q. (2011). "Animal biodiversity: An introduction to higher-level classification and taxonomic richness" (PDF). Zootaxa. 3148: 7–12.
- Poinar Jr., G (January 2008). "Global diversity of hairworms (Nematomorpha: Gordiaceae) in freshwater". Hydrobiologia. 595 (1): 79–83. doi:10.1007/s10750-007-9112-3.
- Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
- Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 307–308. ISBN 0-03-056747-5.
- Thomas, F.; Schmidt-Rhaesa, A.; Martin, G.; Manu, C.; Durand, P.; Renaud, F. (May 2002). "Do hairworms (Nematomorpha) manipulate the water seeking behaviour of their terrestrial hosts?" (PDF). J. Evol. Biol. 15 (3): 356–361. doi:10.1046/j.1420-9101.2002.00410.x. — 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.
- Schmidt-Rhaesa, Andreas (2002). "Two Dimensions of Biodiversity Research Exemplified by Nematomorpha and Gastrotricha". Integrative and Comparative Biology. 42 (3): 633–640. doi:10.1093/icb/42.3.633. PMID 21708759.
- Ponton, Fleur; Camille Lebarbenchon; Thierry Lefèvre; David G. Biron; David Duneau; David P. Hughes; Frédéric Thomas (April 2006). "Parasitology: Parasite survives predation on its host". Nature. 440 (7085): 756. doi:10.1038/440756a. PMID 16598248.
- Hong, Eui-Ju; Sim, Cheolho; Chae, Joon-Seok; Kim, Hyeon-Cheol; Park, Jinho; Choi, Kyoung-Seong; Yu, Do-Hyeon; Yoo, Jae-Gyu; Park, Bae-Keun (2015). "A Horsehair Worm, Gordius sp. (Nematomorpha: Gordiida), Passed in a Canine Feces". The Korean Journal of Parasitology. 53 (6): 719. doi:10.3347/kjp.2015.53.6.719. PMID 26797439.
- Yamada, Minoru; Tegoshi, Tatsuya; Abe, Niichiro; Urabe, Misako (2012). "Two Human Cases Infected by the Horsehair Worm, Parachordodes sp. (Nematomorpha: Chordodidae), in Japan". The Korean Journal of Parasitology. 50 (3): 263. doi:10.3347/kjp.2012.50.3.263. PMID 22949758.
- Saito, Y; Inoue, I; Hayashi, F; Itagaki, H (1987). "A hairworm, Gordius sp., vomited by a domestic cat". Nihon juigaku zasshi. the Japanese journal of veterinary science. 49 (6): 1035–7. PMID 3430914.
- Malcolm S. Bryant, Robert D. Adlard & Lester R.G. Cannon 2006. Gordian Worms: Factsheet. Queensland Museum. 
- Nematomorpha – Bumblebees
- Sun, W.; Hou, X. (1987). "Early Cambrian worms from Chengjiang, Yunnan, China: Maotianshania gen. nov" (Paywall). Acta Palaeontologica Sinica. 26 (3): 299–305.
- Poinar George; Ron Buckley (September 2006). "Nematode (Nematoda: Mermithidae) and hairworm (Nematomorpha: Chordodidae) parasites in Early Cretaceous amber". Journal of Invertebrate Pathology. 93 (1): 36–41. doi:10.1016/j.jip.2006.04.006. PMID 16737709.
- Pechenik, 'Biology of the Invertebrates, 2010, pg 457.
- "Gordioidea". Integrated Taxonomic Information System.
- "Chordodidae". Integrated Taxonomic Information System.
- 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. ISBN 978-0-07-127041-0.
- 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.
- Hanelt B, Thomas F, Schmidt-Rhaesa A (2005). "Biology of the phylum Nematomorpha". Advances in Parasitology. 59: 244–305. doi:10.1016/S0065-308X(05)59004-3.
- Poinar GO Jr (1991). "Nematoda and Nematomorpha". In Thorp JH, Covich AP. 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 Science. 30: 219–231.
- Hairworm Biodiversity Survey.
- Capinera, J. L. Horsehair Worms, Hairworms, Gordian Worms, Nematomorphs, Gordius spp. (Nematomorpha: Gordioidea). University of Florida IFAS. Published 1999, revised 2005.
- Nematomorph worm – Behavior modification of cricket by nematomorph worm. YouTube.
- Gordian Worm discussed on RadioNZ Critter of the Week, 6 November 2015
- Minor, M.A., and A.W. Robertson. (2006). Nematoda. Soil Bugs – An Illustrated Guide to New Zealand Soil Invertebrates (updated 29-Apr-2015), http://soilbugs.massey.ac.nz (13 August 2016).