Roundworms may outnumber almost every animal
Nematodes—roundworms or eelworms—form phylum Nematoda across nearly every ecosystem, from polar soils to ocean trenches. Most freeload on microbes; many parasitise plants and animals. A 2013 biodiversity survey tallied over 25,000 described species, with total living diversity still wildly uncertain.
Ecdysozoan cousins of arthropods and tardigrades, they moult and carry reduced Hox gene sets like tardigrades. Unlike flatworms, they run a tube gut open at both ends. Species boundaries blur, so headcounts swing and ecological dominance is easier to feel than to census.
In soils and sediments they often swamp other animals in sheer abundance, quietly recycling nutrients while plant-parasitic lineages bankrupt harvests and animal parasites fill medical textbooks.
The name means threadlike, from Greek nema, thread. Linnaeus described a few genera, including Ascaris and Dracunculus, in 1758, lumping them with other worms, and Karl Rudolphi coined Nematoidea in 1808. Grobben placed them in a broader phylum, Aschelminthes, in 1910, but in 1919 Nathan Cobb argued they deserved a phylum of their own. In the 1990s they were grouped with arthropods and other moulting animals as Ecdysozoa, and a 2019 study found a single-amino-acid insertion in one protein that occurs only in nematodes.
Most free-living species are under 2.5 millimetres long and many are microscopic, though some soil species reach 7 millimetres, some marine ones 5 centimetres, and some parasites 50 centimetres or more. The head, unlike the two-sided body, is arranged in radial symmetry, carrying sensory bristles and a mouth of three or six lips, often toothed. Eggs from several parasitic clades have been found in fossil dung from the Oligocene Tremembé Formation near São Paulo, an ancient lake rich in birds, fish and arthropods. In 2022 M. Hodda proposed a new classification of the whole phylum.
Source: Nematode
