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)

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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