Animal Diversity Web

Di­ver­sity

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)

• Biogeographic Regions
• nearctic
  • native
• palearctic
  • native
• oriental
  • native
• ethiopian
  • native
• neotropical
  • native
• australian
  • native
• oceanic islands
  • native
• arctic ocean
  • native
• atlantic ocean
  • native
• mediterranean sea
  • native

• Other Geographic Terms
• holarctic
• cosmopolitan

Habi­tat

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)

• Habitat Regions
• temperate
• tropical
• terrestrial
• saltwater or marine
• freshwater

• Aquatic Biomes
• pelagic
• benthic
• lakes and ponds
• rivers and streams
• temporary pools
• coastal

• Wetlands
• marsh
• swamp
• bog

• Other Habitat Features
• urban
• suburban
• agricultural
• riparian

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

  • Gordiacea (von Siebold, 1843)

• 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)

• Other Physical Features
• heterothermic
• bilateral symmetry

• Sexual Dimorphism
• sexes shaped differently

De­vel­op­ment

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)

• Development - Life Cycle
• metamorphosis

Re­pro­duc­tion

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)

• Mating System
• polygynandrous (promiscuous)

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)

• Key Reproductive Features
• iteroparous
• seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• parthenogenic
• sexual
• asexual
• fertilization
  • internal
• oviparous

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

• Parental Investment
• no parental involvement

Lifes­pan/Longevity

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)

Be­hav­ior

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)

• Key Behaviors
• natatorial
• diurnal
• nocturnal
• crepuscular
• parasite
• motile
• sedentary
• solitary

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)

• Communication Channels
• tactile
• chemical

• Perception Channels
• tactile
• chemical

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)

• Primary Diet
• carnivore
  • eats body fluids
  • insectivore
  • eats non-insect arthropods

Pre­da­tion

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)

• Known Predators

  • green frogs (Lithobates clamitans clamitans)
  • rock bass (Ambloplites rupestris)
  • green sunfish (Lepomis cyanellus)
  • longear sunfish (Lepomis megalotis)
  • brown trout (Satmo trutta)

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)

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

Con­trib­u­tors

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

Ref­er­ences

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