Nematomorpha

(invertebrate zoology) A group of the Aschelminthes or a separate phylum that includes the horsehair worms.

(Hair worms)

Phylum: Nematomorpha

Number of families: 2

Thumbnail description
Parasitic worms as juveniles in marine or terrestrial arthropods, free-living as adults

Evolution and systematics

The Nematomorpha consists of two clades: the class Nectonematoida and the class Gordiida. The nectonematids are parasites of marine crustaceans such as crabs and shrimp. The gordiids are usually parasites of terrestrial arthropods such as crickets, grasshoppers, beetles, mantids, and cockroaches. Based on molecular evidence, the phylum Nematomorpha has been shown to be the sister group to nematodes. The phylum contains two orders, two families, and two genera—all corresponding to the two clades. Overall, the phylum contains approximately 230 species.

Fossil gordiids have been found emerging from a cockroach trapped in fossilized amber dated at 15–45 million years ago. However, it has been suggested that this group might date back to the Carboniferous.

Physical characteristics

Nematomorphs are long, thin, cylindrical worms. Their shape leads many to refer to these worms as hair worms. Gordiid adults can be from 2–118 in (5–300 cm) long and 0.02–0.40 in (0.5–10 mm) thick. The color of gordiids ranges from black and brown to yellow and white depending on sex, and species. Most have a white anterior tip immediately followed by a thin dark band or collar. Nectonematid adults can be up to 11.8 in (300 mm) long and 0.07 in (1.7 mm) in diameter. Most gordiids contain surface ornamentations, areoles, which are made up of raised bumps. The color of nectonematids ranges from grayish white to yellow. Most nematomorph males can be distinguished from females by having a slight inward curving posture of their posterior ends.

Distribution

Gordiids have been recorded from every continent except Antarctica. Nectonematids have been reported from oceans around the globe, including the shores of North America, South America, Europe, Japan, New Zealand, and Indonesia.

Habitat

Nematomorphs are parasites as juveniles but free-living as adults. Adult gordiids are usually found in slow-moving freshwater streams or ponds. In streams, worms are either attached to vegetation hanging over the banks or in between rocks on the bottom. Worms attach by winding their muscular bodies into tight coils. Gordiids have also been recorded from larger rivers such as the Mississippi River and lakes such as the Great Lakes. The habitat of adult nectonematids is largely unknown. Adults have often been found at the surface in the littoral zone, but have also been dredged from the seafloor several hundred feet (meters) deep.

Behavior

Gordiids tend to entangle in large knots during mating. Often, hundreds of individuals can be found in a seemingly undoable tangle. This behavior has led the gordiids to be called Gordian worms from the Greek myth of Gordius. The behavior of nectonematids has not been studied.

Feeding ecology and diet

Adult nematomorphs do not feed. The gut is greatly reduced and in adults is non-functional. In some adults, the mouth may be closed by skin, while in others, parts of the gut are missing. Worms get all of their energy from their hosts. Although the exact mechanism has not yet been resolved, juvenile worms are able to move host nutrients through their cuticle and into their gut.

Reproductive biology

Adult gordiids found in temperate climates appear in late spring or summer. In some species, mating is immediate, while in others mating can be delayed for as long as several months after emergence. Males wrap around the female with their posterior end and glides along the female surface until he reaches her cloaca. The male then deposits a drop of sperm onto the posterior end of the female, which usually covers the tip of the female. Within a month, females lay up to 6 million eggs, soon after which they die. Larvae hatch from eggs within 20 days, and penetrate and encyst in aquatic organisms. Cysts within aquatic insects are carried to land when the insect metamorphoses into a fly. Arthropod hosts are infected upon eating a fly containing cysts. The reproductive biology of tropical gordiids or nectonematids is unknown.

Conservation status

No species of nematomorphs are listed by the IUCN. Some evidence indicates that gordiid populations are not affected by human modifications of the landscape.

Significance to humans

Although several hundred reports exist of humans supposedly infected with gordiids, humans do not serve as hosts for these worms. Most, if not all, of these cases are due to incidental associations. Worms have been noted from the human digestive tract by being spit up or passed through the intestine. These worms are likely to have been swallowed as adults. No evidence exists that these worms are able to live within a human for an extended period. It is also likely that worms discovered in toilets or chamber pots were present in these vessels before use, carried in by insect hosts. Nectonematids do not appear to have immediate importance to humans.

Species accounts

Resources

Poinar, George O. "Nematoda and Nematomorpha." In Ecology and Classification of North American Freshwater Invertebrates, edited by James H. Thorp and Alan P. Covich. San Diego: Academic Press, 2001.

Schmidt-Rhaesa, Andreas. Süsswasserfauna von Mitteleuropa: Nematomorpha. Stuttgart, Germany: Gustav Fisher Verlag, 1997.

Hanelt, B., and J. Janovy Jr. "Untying a Gordian Knot: The Domestication and Laboratory Maintenance of a Gordian Worm. Paragordius varius (Nematomorpha: Gordiida)." Journal of Natural History In press, 2003.

Poinar, G., Jr., and A. M. Brockerhoff. "Nectonema zealandica n. sp. (Nematomorpha: Nectonematoidea) Parasitising the Purple Rock Crab Hemigrapsus edwardsi (Brachyura: Decapoda) in New Zealand, with Notes on the Prevalence of Infection and Host Defense Reactions." Systematic Parasitology 50, no. 2 (2001): 149–157.

Schmidt-Rhaesa, A. "Phylogenetic Relationships of the Nematomorpha: A discussion of Current Hypothesis." Zoologischer Anzeiger 236 (1998): 203–216.

Schmidt-Rhaesa, A. "The Life Cycle of Horsehair Worms (Nematomorpha)." Acta Parasitologica 46, no. 3 (2001): 151–158.

[Article by: Ben Hanelt, PhD]

A phylum of worms that was formerly considered to be a class of the phylum Aschelminthes; commonly called the hairworms, and closely allied to the nematodes. The adults are free-living in aquatic habitats, while the juveniles are parasitic in arthropods. The nematomorphs are found all over the world. They are divided into two classes, the Nectonematoidea and Gordioidea, with a total of 225 species. See also Nemata.

The body is long and slender with a maximum length of 5 ft (1.5 m) and a diameter of 0.02–0.12 in. (0.5–3 mm). The females are longer than the males. The posterior end may be rounded with a terminal cloaca, or it may form two or three lobes in a forklike structure. The body color is yellowish, brown, or almost black. The body wall consists of three layers: an outer, rather thick fibrous cuticle; an epidermis consisting of a single layer of cells; and innermost, a muscle layer with longitudinal fibers only.

The body cavity extends the length of the body. It may be filled with tissue so that only minor spaces are left around the digestive system and the gonads.

The sexes are always separate, and the gonads are paired and stringlike extending the length of the body. During copulation the male coils itself around the female and places a drop of sperm near the cloacal opening of the female. The sperm cells actively enter the seminal receptacle. The eggs are laid in water in strings, and the adults die after egg laying. When hatched, the larvae swim to an aquatic arthropod. They penetrate the body wall of the host by means of their characteristic proboscis, which is armed with hooks and three long stylets. The gradual development in the host lasts some months without any metamorphosis. When they are mature, the worms leave the host.

Nematomorpha
Scientific classification
Domain: Eukaryota Kingdom: Animalia Superphylum: Ecdysozoa Phylum: Nematomorpha Vejdovsky, 1886
Classes
Nectonematoida Gordioidea

Nematomorpha (sometimes called Gordiacea, and commonly known as Horsehair worms or Gordian worms) are a phylum of parasitic animals which are morphologically and ecologically similar to nematode worms, hence the name. They range in size from 1cm to 1 meter long, and 1 to 3 millimetres in diameter. Horsehair worms can be discovered in damp areas such as watering troughs, streams, puddles, and cisterns. The adult worms are free living, but the larvae are parasitic on beetles, cockroaches, Orthoptera and crustaceans ^[1]. About 326 species are known and a conservative estimate suggests that there may be about 2000 species worldwide.^[2] The name "Gordian" stems from the legendary Gordian knot. This relates to the fact that nematomorpha often tie themselves in knots.^[3]

Contents 1 Description and biology 2 Taxonomy 3 References 3.1 Further reading 4 External links 4.1 Videos

Description and biology

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. Reproductively, they are dioecious, with the internal fertilization of eggs that are then laid in gelatinous strings. The larvae that hatch have rings of cuticular hooks and terminal stylets that are believed to be used to enter the hosts. They are mostly free living but males and females aggregate into tight balls (Gordian knots) during mating.^[4][5]

Spinochordodes tellinii and its katydid host

In Spinochordodes tellinii, which has orthopterans as its vector, the infection acts on the insect's brain and causes it to seek water and drown itself, thus returning the nematomorph to water.^[4] They are also remarkably able to survive the predation of their host, being able to wriggle out of the predator which has eaten the host.^[6]

Taxonomy

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.^[7]

The phylum is placed along with the Ecdysozoa clade of moulting organisms that include the Arthropoda. Fossilized worms have been reported from Early Cretaceous Burmese amber dated to 100-110 million years apart from a fossil from the Mesozoic.^[8]

Relationships within the phylum are still somewhat unclear, but two classes are recognised:

• Class Nectonematoida: Marine, planktonic, with a double row of natotory setae along each side of the body; with dorsal and ventral longitudinal epidermal cords, blastocoelom spacious and fluid filled; gonads single; larvae parasitise decapod crustaceans
• Class Gordioidea: Freshwater and semiterrestrial; lack lateral rows of setae; with a single, ventral epidermal cord; blastocoelom filled with mesenchyme in young animals but becomes spacious in older individuals; larvae primarily parasitise Orthoptera

References

1. ^ 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. 
2. ^ Poinar Jr., G (2008). "Global diversity of hairworms (Nematomorpha: Gordiaceae) in freshwater". Hydrobiologia 595 (1): 79–83. doi:10.1007/s10750-007-9112-3. 
3. ^ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
4. ^ ^{a b} Thomas, F.; Schmidt-Rhaesa, A., Martin, G., Manu, C., Durand, P., and Renaud, F. (May 2002). "Do hairworms (Nematomorpha) manipulate the water seeking behaviour of their terrestrial hosts?". J. Evol. Biol. 15 (3): 356–361. doi:10.1046/j.1420-9101.2002.00410.x. ISSN 1010-061X. http://www.erin.utoronto.ca/~w3gwynne/BIO418/Nemato.pdf.  — 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.
5. ^ 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. 
6. ^ Ponton, Fleur; Camille Lebarbenchon, Thierry Lefèvre, David G. Biron, David Duneau, David P. Hughes, and Frédéric Thomas (April 2006). "Parasitology: Parasite survives predation on its host". Nature 440 (7085): 756. doi:10.1038/440756a. 
7. ^ Malcolm S. Bryant, Robert D. Adlard & Lester R.G. Cannon 2006. Gordian Worms: Factsheet. Queensland Museum. [1]
8. ^ Poinar, George; Buckley Ron (2006) Nematode (Nematoda: Mermithidae) and hairworm (Nematomorpha: Chordodidae) parasites in Early Cretaceous amber. Journal of invertebrate pathology 93(1):36-41

Further reading

• 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. 
• 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. p. 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. 

External links

• Nematomorpha Biodiversity Project website. Provides pictures, life cycle information and searchable databases.
• horsehair worms on the UF / IFAS Featured Creatures Web site

Videos

• Videos of a cricket infected with a Gordian worm with the worm emerging to mate, as well as emerging from predators which have eaten the infected cricket. From Nature, April 2006.
• Nematomorph worm at YouTube — explains the proteome research done on this worm
• The cricket suicide (video)

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