nematode n : unsegmented worms with elongated rounded body pointed at both ends; mostly free-living but some are parasitic [syn: nematode worm, roundworm]

English

Pronunciation

(US) IPA: /ˈnɛməˌtoʊd/

Noun

1. a roundworm, a small invertebrate animal of the phylum Nematoda

The nematodes or roundworms (Phylum Nematoda from Greek (nema): "thread" + -ode "like") are one of the most common phyla of animals, with over 80,000 different described species (over 15,000 are parasitic). They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as Antarctica and oceanic trenches. Further, there are a great many parasitic forms, including pathogens in most plants, animals, and also in humans.

The nematodes were originally named nematoidea by Rudolphi (1808). They were renamed nematodes by Burmeister 1837 (as a family; Leuckart 1848 and von Siebold 1848 both promoted them to the rank of order), then nematoda (Diesing 1861), though Nathan Cobb (1919) argued that they should be called nemata or nemates (and in English 'nemas' rather than 'nematodes'). After some confusion which saw the nematodes placed (often together with the horsehair worms, nematomorpha) as a class or order in various groups such as Aschelminthes, Lankester (1877) definitively promoted them to the level of phylum.

Morphology

Roundworms are unsegmented, bilaterally symmetric and triploblastic protostomes with a complete digestive system. Roundworms have no circulatory or respiratory systems so they use diffusion to breathe. Although they lack a circulatory system, nutrients are transported throughout the body via fluid in the pseudocoelom. They are thin and are round in cross section. Nematodes are one of the simplest animal groups to have a complete digestive system, with a separate orifice for food intake and waste excretion, a pattern followed by all subsequent, more complex animals. The body cavity is a pseudocoelom (persistent blastula), which lacks the muscles of coelomate animals that protects the body from drying out, from digestive juices, or from other harsh environments. Although this cuticle allows movement and shape changes via a hydrostatic skeletal system, it is very inelastic so does not allow the volume of the worm to increase. Therefore, as the worm grows, it has to molt and form new cuticles. The cuticles don't allow volume to increase so as to keep hydrostatic pressure inside the organism very high. For this reason, the roundworms do not possess circular muscles (just longitudinal ones) as they're not required. This hydrostatic pressure is the reason the roundworms are round.

Nematodes have a simple nervous system, with a main ventral nerve cord and a smaller dorsal nerve cord. Sensory structures at the anterior end are called amphids, while sensory structures at the posterior end are called phasmids.

Most free-living nematodes are microscopic, though a few parasitic forms can grow to over a meter in length (typically as parasites of very large animals such as whales). There are no circular muscles, so the body can only undulate from side to side. Contact with solid objects is necessary for locomotion; its thrashing motions vary from mostly to completely ineffective at swimming.

Nematodes generally eat bacteria, fungi and protozoans, although some are filter feeders. Excretion is through a separate excretory pore. Nematodes also contract bacterial infections within excretion pores.

Reproduction

Reproduction is usually sexual. Males are usually smaller than females (often much smaller) and often have a characteristically bent tail for holding the female for copulation. During copulation, one or more chitinized spicules move out of the cloaca and are inserted into genital pore of the female. Amoeboid sperm crawl along the spicule into the female worm. Nematode sperm is thought to be the only eukaryotic cell without the globular protein G-actin.

Eggs may be embryonated or unembryonated when passed by the female, meaning that their fertilized eggs may not yet be developed. In free-living roundworms, the eggs hatch into larva, which eventually grow into adults; in parasitic roundworms, the life cycle is often much more complicated.

Some nematodes, specifically Heterorhabditis spp., undergo a process called endotokia matricida; intrauterine birth causing maternal death. The hermaphroditic nematode keeps its self-fertilized eggs inside its uterus until they hatch. The juvenile nematodes will then ingest the parent nematode. This process is significantly promoted in environments with a low or reducing food supply.. In certain fertile areas the topsoil is estimated to contain in the billions of nematodes per acre. In the 1914 edition of the Yearbook of the United States Department of Agriculture N.A.Cobb wrote on the abundance of nematodes

Nematodes in Agriculture

Depending on the species, a nematode may be beneficial or detrimental to plant health.

From an agricultural perspective, there are two categories of nematode: predatory ones, which will kill garden pests like cutworms, and pest nematodes, like the root-knot nematode, which attack plants.

Predatory nematodes can be bred by soaking a specific recipe of leaves and other detritus in water, in a dark, cool place, and can even be purchased as an organic form of pest control.

Rotations of plants with nematode resistant species or varieties is one means of managing parasitic nematode infestations.

Phylogeny

While it has recently been suggested that nematodes are related to the arthropods and priapulids and should be grouped with them in the Ecdysozoa (molting animals), there is substantial resistance within the nematology community. Grouping organisms based on behaviors is not generally accepted. While there seems to be some evolutionary connection between these phyla, the exact nature of their relationship is still being debated.

That the roundworms have a large number of peculiar apomorphies and in many cases a parasitic lifestyle confounds analyses; the DNA sequence data hitherto analyzed is equivocal on ecdysozoan monophyly. Genetic analyses of roundworms suggest that - as is also indicated by their unique morphological features - the group has been under intense selective pressure during its early radiation, resulting apparently in accelerated rates of both morphological and molecular evolution. Until a strong phylogenetic tree based on combined evidence is produced, most agree that the Nematoda should simply be referred to as part of the Metazoa.

Robustness

• Hundreds of nematode worms (C. elegans), featured in a research project on mission STS-107, survived the Space Shuttle Columbia Disaster.

See also

• Ascariasis: A human disease caused by the parasitic roundworm Ascaris lumbricoides
• Caenorhabditis elegans: An important model organism often used to study cellular differentiation, sometimes referred to as "worm" by scientists.
• List of parasites (human)
• Toxocariasis: A helminth infection of humans caused by the dog or cat roundworm, Toxocara canis or Toxocara cati
• Xiphinema: List of species

References

External links

• http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html
• Nematode.net: Repository of parasitic nematode sequences.
• http://www.nematodes.org/
• NeMys World free-living Marine Nematodes database
• Nematode Virtual Library
• International Federation of Nematology Societies
• Society of Nematologists
• Australasian Association of Nematologists
• Research on nematodes and longevity