Nematomorphahorsehair worms(Also: Gordian worms)


The phylum Nematomorpha (also known as horsehair worms) is comprised of two orders: Nectonematoidea (monogeneric (Nectonema), with four known species) and Gordioidea, in which the remainder of the over 300 described nematomorphan species are placed. There are 19 currently recognized genera within Nematomorpha, and estimates of global species diversity for this phylum are as high as 2,000. While Nectonema species are marine, planktonic worms, gordioids are found in freshwater, most commonly along the banks of ponds and streams, and some are semi-aquatic and live in damp soil. Nematomorphans are parasitic as larvae (Nectonema species parasitize marine invertebrates, while gordioids utilize terrestrial arthropods) and are free-living and aquatic as adults. (Brusca and Brusca, 2003; Hanelt, et al., 2013; Poinar Jr., 2008; Schmidt-Rhaesa, 2012; Shapiro, 2012)

Geographic Range

Nectonema species are found in coastal, marine, and pelagic environments as adults, and are found as parasites in decapod crustaceans as larvae. They are known from the waters of Indonesia, Japan, New Zealand, the Northern Atlantic, and the Mediterranean. Gordioids are found in freshwater streams and ponds as adults (a few are found in damp soil) and most typically in terrestrial insects, as parasitic larvae. They are known from every continent, with the exception of Antarctica. (Brusca and Brusca, 2003; Shapiro, 2012)


Generally speaking, nematomorphans are found in aquatic or occasionally terrestrial (either as semi-aquatic individuals themselves or within terrestrial hosts) environments throughout the world. Adult Nectonema species are free-swimming and pelagic, and are sometimes found near the coast during high tides. They are most often collected as larvae from their decapod crustacean hosts. Gordioids may be found in nearly any freshwater environment, including not only rivers, lakes, and streams, but even puddles or grasses after a heavy rain. They are also found as parasitic larvae in their hosts, which are typically terrestrial arthropods and insects. (Brusca and Brusca, 2003; Hanelt, et al., 2013; Shapiro, 2012)

Systematic and Taxonomic History

The first formal description of a nematomorph species was provided by Linnaeus in 1758, who erected the genus Gordius for the species Gordius aquaticus. These organisms were long included in taxonomic classifications with members of phylum Nematoda, which were themselves subjected to a number of changes in name and taxonomic rank throughout the 19th and early 20th century. The name Nematomorpha was not applied to organisms in this group until they were elevated to the level of phylum by the Czech zoologist František Vejdovský in 1886, replacing the name Gordiacea, which had earlier been applied to nematomorphans as an order within the now obsolete groups Entozoa (unranked), Nemathelminthes (Phylum), and Nematoidea (Phylum). (Bleidorn, et al., 2002; Miralles and de Villalobos, 1995; Vejdovsky, 1886; von Siebold, 1843)

Morphological and molecular data support the monophyly of the Nematomorpha. These data also indicate a sister group relationship between the marine genus Nectonema, and the remaining freshwater species in the phylum, as well as the validity of the families Gordiidae, Chordodidae, and the subfamily Chordodinae. Two previously recognized families Lanochordodidae and Spinochordodidae, have been found to be either paraphyletic, or synonyms of previously described taxa. (Bleidorn, et al., 2002; Nielsen, 2001; Schmidt-Rhaesa, 1996)

The phylum Nematomorpha is included in the clade Nematoida, where it forms the sister group to the phylum Nematoda. This clade is nested within the superphylum Ecdysozoa, a group of protostome animals possessing multilayered cuticles, which are periodically shed as the animals develop and grow. The placement of Nematoida within Ecdysozoa is still a matter of debate, as morphological and molecular analyses have alternatively placed them as the sister group of the phylum Tardigrada, or as a sister group to Panarthropoda, a clade consisting of tardigrades, arthropods, and onychophorans. (Adouette, et al., 2000; Blaxter, et al., 1998; Bleidorn, et al., 2002; Dunn, et al., 2008; Giribet, et al., 2007; Nielson, 2003; Schmidt-Rhaesa, 1998; Zrzavý, 2001)

  • Synonyms
  • Synapomorphies
    • Nematomorphans have mostly subpharyngeal brains.
    • Nematomorphans have a reduction of the pharyngeal musculature.
    • These animals have a distinctive larval form.

Physical Description

Nematomorphans can be up to 1 meter long (10 to 20 cm on average) and 1 to 3 mm in diameter. They are typically tan to black in color. Adult nematomorphans are covered in a very thick cuticle that is secreted by the epidermis and is comprised of two layers. These include an inner, lamellate, fibrous layer (the number of sheets in this layer varies somewhat from species to species and are in different areas of an organism’s body) and an outer, homogeneous layer. The outer layer often bears areoles (groups of bumps, warts or papillae). Some areoles have an apical spine (likely touch-sensitive) or pore (potentially lubricant producing), and the spatial patterning of these areoles is often used as a diagnostic characteristic at the species level. The epidermis is unciliated and very thin, covering a thin basal lamina and produced into either one (dorsal) or two (dorsal and ventral) cords containing nerve tracks. Under these layers is a thick sheet of longitudinal muscles, which gives rise to the rete system (hollow tubular extensions); these muscles also play a large role in providing body support. Depending on the species, nematomorphans may have a spacious blastocoelom (e.g. Nectonema sp.) or one filled with mesenchyme. Although they are unciliated, nematomorphans possess natatory bristles that aid in swimming and floating. Some species have two or three caudal lobes at their posterior ends. Nematomorphans are somewhat sexually dimorphic, as a male's cloaca may be swollen and serve as seminal vesicles. (Brusca and Brusca, 2003; Hanelt, et al., 2005; Poinar Jr., 2008)

  • Sexual Dimorphism
  • sexes shaped differently


Development of Nectonema species has not been studied in great detail, in contrast to that of gordioids. In these latter species, fertilized eggs are released by females and laid in gelatinous strings. Cleavage is holoblastic, but not clearly spiral or radial, and leads to a coeloblastula stage. Larvae develop inside egg cases and are very small (around 100 µm in length) upon hatching. They have 2 to 3 rings of cuticular hooks and stylets, which they use to penetrate their hosts. Gordioid eggs develop into semi-sessile larvae over 7 to 14 days and can survive for up to two weeks before finding a host. They cannot swim and are found at the bottom of the water column. Larvae may be ingested directly by a host or, more often, they are ingested by paratenic hosts (definitive hosts are not typically aquatic). Once in a paratenic host, a larva will encyst, remaining there for up to a year. The encysted larvae are ingested by the definitive host when it feeds on the paratenic host. Larvae grow into juveniles within their hosts, which may take anywhere from 4 to 20 weeks. They molt once before leaving their hosts, at which point they have usually reached their full adult sizes, often filling the entire body cavities of their hosts. Juveniles must be released into water and current research indicates that nematomorphans have the ability to influence their hosts' behavior in order to insure this; infected cricket hosts such as Nemobius sylvestris are known to behave erratically, to the point of suicidally jumping into water when the juveniles are ready to be released. (Brusca and Brusca, 2003; Hanelt and Janovy Jr, 1999; Hanelt and Janovy Jr, 2004; Hanelt, et al., 2013; Hanelt, et al., 2005; Sanchez-Moreno, et al., 2008; Shapiro, 2012)


Observations of mating indicate male nematomorphans become highly active during breeding in response to the presence of potential mates. Upon locating a receptive female, a male will wrap his body around her, dropping sperm near her cloacal pore. From there, it is assumed that sperm enter the cloaca, fertilizing eggs in the seminal receptacle. Nematomorphans are sometimes found in large breeding knots. A female may lay millions of eggs during her lifetime. (Brusca and Brusca, 2003; Hanelt, et al., 2013)

Nematomorphans are dioecious and reproduce sexually. Males have one or two testes, which open to a cloaca via a sperm duct. The cloaca may become swollen, acting as a seminal vesicle. Females may have a pair of elongate ovaries, which open to the cloaca via a seminal receptacle, or no ovaries at all, with oocytes scattered throughout the body cavity. A female may lay millions of eggs during a breeding season. Nematomorphans are known to breed during the late spring, summer, and early fall, and are capable of overwintering. A newly identified species of gordioid, Paragordius obami, is parthenogenetic, with no males; this is the only species of nematomorphan not known to reproduce sexually. ("Horsehair Worms: Integrated pest management around the home", 2013; "Horsehair or Gordian Worm", 2012; Brusca and Brusca, 2003; Hanelt, et al., 2012; Hanelt, et al., 2013)

Nematomorphans exhibit no parental investment beyond the production of gametes. (Brusca and Brusca, 2003)

  • Parental Investment
  • no parental involvement


Although a specific lifespan has not been reported for nematomorphan species, they are known to survive for multiple years. ("Horsehair Worms: Integrated pest management around the home", 2013)


Although they are unciliated, nematomorphans possess natatory bristles which, when moved by the body wall muscles, aid in swimming and floating. They are solitary outside of breeding. (Brusca and Brusca, 2003)

Nematomorphan larvae parasitize and can influence their hosts' behaviors. (Hanelt and Janovy Jr, 1999; Hanelt and Janovy Jr, 2004; Hanelt, et al., 2013; Sanchez-Moreno, et al., 2008)

Communication and Perception

Nematomorphans have a circumpharyngeal cerebral ganglion located in the region of the head known as the calotte, and single or paired nerve cords that run in the epidermal nerve tracks. Some species also have modified, pigmented cells located on their calottes, which may be photosensitive. Nematomorphans are highly tactically sensitive; some of their cuticular areoles may be touch receptors. Areoles may also be chemosensitive. Some species have four “giant cells” that are connected to the central nervous system and have many microvilli, and are assumed to be involved in additional sensory functions. (Brusca and Brusca, 2003; Schmidt-Rhaesa, 2012)

Food Habits

Nematomorphan larvae are parasitic, eating and absorbing their hosts' body tissues in early stages and feeding on nutrients from bodily fluids later. They do not feed as adults, but they may be able to absorb nutrients from the water through their body walls. (Brusca and Brusca, 2003; Shapiro, 2012)

Known food sources (hosts) of gordioid species include crickets, beetles, grasshoppers, cockroaches and mantids. Known hosts of Nectonema species are most often decapod crustaceans, such as crabs and shrimps. (Hanelt, et al., 2013)


Predators of nematomorphan larvae are those species that serve as paratenic and definitive hosts. Known predators of adult nematomorphans include fishes and frogs. (Cochran, 2002; Cochran, et al., 1999)

Ecosystem Roles

Nematomorphans are parasitic as larvae. They may infect paratenic hosts. When in these intermediary hosts, the larvae encyst. Paratenic hosts often include trematode flatworms, insect larvae (particularly flying insects), small crustaceans, snails, and fishes. Definitive hosts of gordioids are typically terrestrial insects and arthropods, while definitive hosts of Nectonema species are marine decapod crustaceans. (Baker, 1985; De Villalobos, et al., 1999; Hanelt and Janovy Jr, 2003; Hanelt and Janovy Jr, 2004; Poinar Jr. and Brockerhoff, 2001; Poinar Jr. and Weissman, 2004; Sanchez-Moreno, et al., 2008; Schmidt-Rhaesa, 2012; Schmidt-Rhaesa, et al., 2009; Shapiro, 2012)

Species Used as Host

Economic Importance for Humans: Positive

Nematomorphan infestations have been used at times as a means of pest control, but it has not been widely successful. Additionally, their life cycle and, in particular, their parasitic behavior and control of their hosts has been a source of scientific research. ("Horsehair Worms: Integrated pest management around the home", 2013; Baker, 1985; Hanelt, et al., 2013; Sanchez-Moreno, et al., 2008)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

Nematomorphans do not parasitize humans, livestock, or other domestic animals. There are no known adverse effects of nematomorphans on humans. ("Horsehair Worms: Integrated pest management around the home", 2013)

Conservation Status

Species in this phylum are not considered endangered or threatened in any way. (Hanelt, et al., 2013)

  • IUCN Red List [Link]
    Not Evaluated


Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Animal Diversity Web Staff.


Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

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Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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living in the southern part of the New World. In other words, Central and South America.

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living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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living in landscapes dominated by human agriculture.


reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents


Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.


an animal that mainly eats meat


uses smells or other chemicals to communicate


the nearshore aquatic habitats near a coast, or shoreline.


having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.


active at dawn and dusk

  1. active during the day, 2. lasting for one day.

union of egg and spermatozoan


mainly lives in water that is not salty.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.


a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

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Found in northern North America and northern Europe or Asia.


An animal that eats mainly insects or spiders.

internal fertilization

fertilization takes place within the female's body


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


marshes are wetland areas often dominated by grasses and reeds.


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


having the capacity to move from one place to another.


specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.


found in the oriental region of the world. In other words, India and southeast Asia.

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reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death


development takes place in an unfertilized egg


An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.


Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female


lives alone


living in residential areas on the outskirts of large cities or towns.


a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


Living on the ground.


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.


living in cities and large towns, landscapes dominated by human structures and activity.


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