Nematomorpha

(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

List of Species

No common name

Gordius aquaticus

order

Gordioidea

family

Gordiidae

taxonomy

Gordius aquaticus Linnaeus, 1758, Europe.

other common names

None known.

physical characteristics

Body ranges from light to dark brown. Body length ranges from 4.7–19.3 in (120–490 mm) with a maximum diameter of 0.02 in (0.6 mm). Male posterior is bifurcating, female posterior entire. Male has postcloacal crescent, which is a parabolic fold of the cuticle just past the cloaca on the posterior end. Areoles are completely lacking.

distribution

Found throughout Europe. In the south, they have been reported from southern France to Turkey. In the north, they have been reported from Belgium to Finland.

habitat

Adults are free-living in freshwater environments. This species is often collected from ponds and from slower streams and temporary waters such as wheel ruts or puddles filled with rainwater. Juveniles develop within coleopteran hosts such as ground beetles.

behavior

The behavior of this group has not been intensively studied.

feeding ecology and diet

Parasitic during larval stage, non-feeding as adult.

reproductive biology

These worms reproduce once per year. Worms either over winter within coleopteran hosts or as free-living adults in sediments or leaf litter. Adult worms emerge from hosts during late summer or early fall, and begin mating. Egg production begins about a month after copulation, and can last as long as four weeks. Larvae hatch from eggs and enter aquatic insect larvae such as midges or mayflies. Within these insects, parasites form cysts able to survive insect metamorphosis to adult flies. Flying insects, carrying cysts, are eaten by beetle hosts, completing the life cycle. Development to adult worms in beetle hosts may take up to three months.

conservation status

Not threatened.

significance to humans

As with all gordiids, this species does not infect humans. However, the accidental expulsion of this worm from its insect host in domestic water supplies, toilets, pools, and livestock watering tanks has caused much unwarranted distress.

No common name

Paragordius varius

order

Gordioidea

family

Gordiidae

taxonomy

Paragordius varius Leidy, 1851, type locality unknown, although it was likely on the East Coast of the United States.

other common names

None known.

physical characteristics

Body ranges from light yellow to nearly black. Body length ranges from 3.9–13.8 in (100–350 mm) with a maximum diameter of 0.03 in (0.7 mm). Male posterior is bifurcating, female posterior trifurcating. Male lacks postcloacal crescent. Only one kind of areoles are present, which are small and flat.

distribution

Found throughout the Americas. They have been reported in North America from Canada and the United States; in Central America, they have been reported from Costa Rica and Guatemala; and in South America from countries including Columbia, Brazil, and Argentina. Within the United States, they have been reported from 26 states, including Hawaii. They have also been reported from Cuba.

habitat

Adults are free-living in freshwater environments. This species is often collected in slower streams and has often been encountered in temporary waters such as rain puddles and many places where rainwater collects. Juveniles develop within orthopteran hosts such as crickets and grasshoppers.

behavior

The behavior of this group has not been intensively studied.

feeding ecology and diet

Parasitic during larval stage, non-feeding as adult.

reproductive biology

These worms produce several generations annually. Worms most likely overwinter as cysts in aquatic insects. During the spring, aquatic insects, carrying cysts, metamorphose into flying adults. Crickets and grasshoppers are infected when they scavenge dead insects harboring cysts. Development to adult worms takes up to one month. The fast development of this species allows up to three generations to be produced during a single year in temperate climates. The reproductive biology of this group in more tropical regions is unknown.

conservation status

Not threatened.

significance to humans

As with all gordiids, this species does not infect humans. However, this species is often encountered by humans in the United States. Such findings of adult worms in a pet's water dish, puddles, toilets, hot tubs, or even near childrens' playgrounds result in numerous calls to local health authorities annually.

No common name

Nectonema agile

order

Nectonematoidea

family

Nectonematidae

taxonomy

Nectonema agile Verill, 1873, Woods Hole, Massachusetts, United States.

other common names

None known.

physical characteristics

Body ranges from grayish white to pale cream. Semi-transparent anterior tips. Body length ranges from 1.2–7.9 in (32–200 mm) with a maximum diameter of 0.03 in (0.75 mm). Both male and females ends entire. Sensory bristles in two rows cover the length of the adult body.

distribution

Found sporadically throughout the world. The locations include the eastern seaboard of North America, Brazil, the Mediterranean, and the Black Sea.

habitat

Adults are free-living in marine environments. This species is often collected in intertidal areas, especially at night in the glare of artificial light. Juveniles are parasites of decapod crustaceans such as crabs and shrimp.

behavior

Nothing is known.

feeding ecology and diet

Parasitic during larval stage, non-feeding as adult.

reproductive biology

Nothing is known.

conservation status

Not threatened.

significance to humans

None known.

Resources

Books

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.

Periodicals

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.

Ben Hanelt, PhD