Nematode

Definition

Any member of the phylum Nematoda; a cylindrical, unsegmented pseudocoelomate worm with a complete digestive tract and a flexible, collagen-rich cuticle. Nematodes are extraordinarily abundant in marine, freshwater, and terrestrial ecosystems—often exceeding all other metazoans in individual count—and occupy trophic roles from microbial grazers to apex predators of meiofauna. Parasitic nematodes include major plant pathogens (e.g., root-knot nematodes Meloidogyne spp.) and animal parasites (e.g., filarial worms, hookworms), while free-living species drive nutrient mineralization in soils and sediments. In entomological contexts, nematodes serve as critical food resources for predatory arthropods, as biocontrol agents (e.g., entomopathogenic Steinernema and Heterorhabditis that vector symbiotic bacteria into insect hosts), and as competitors or commensals within the microarthropod-dominated soil pore space.

Etymology

From Greek nēma, nēmatos 'thread' + -ode 'like', referring to the slender, filamentous body form.

Example

Entomopathogenic nematodes in the family Steinernematidae are commercially reared for biological control of soil-dwelling insect pests such as black cutworm (Agrotis ipsilon) and Japanese beetle (Popillia japonica); the nematode penetrates the host cuticle, releases its bacterial symbiont (Xenorhabdus), and kills the insect within 24–48 hours.

Synonyms

• roundworm
• Eelworm

Related Terms

• helminth
• entomopathogenic nematode
• meiofauna
• soil food web
• Steinernematidae
• Heterorhabditidae
• Meloidogyne
• pseudocoelom

Usage Notes

In strict zoological usage, 'nematode' refers specifically to phylum Nematoda, whereas 'roundworm' and 'eelworm' are broader vernacular terms sometimes applied to other vermiform taxa (e.g., nematomorphs, some annelids). In plant pathology, 'eelworm' traditionally denotes plant-parasitic nematodes (e.g., Ditylenchus, Anguina). Nematodes are not closely related to annelids or platyhelminths despite superficial resemblance; molecular phylogenetics places Nematoda within Ecdysozoa, the molting clade that includes arthropods. In soil ecology, nematode community composition is used as a bioindicator of ecosystem disturbance and decomposition pathways.