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The Phylum Nematoda, the round worms, is among the least understood taxon of freshwater invertebrates. Based upon species that have been named and described, there are probably somewhere more than 27,000 species. Most of these are parasites. However, nematologists seem to agree that the actual species richness of the nematodes surpasses that of the arthropods. So, there are more than a million more nematodes to describe!

According to Abebe et al. (2008), there are c. 360 species of North American freshwater nematodes in more than 160 genera (53 families, 11 orders). However, because of the difficulty of identifying species and the complicated nature of higher nematode classification, it is difficult to find much agreement among different sources.

Traditionally, the Phylum Nematoda had been grouped with the other “pseudocoelomate” phyla, but today the prevailing hypothesis is that the round worms are members of the Ecdysozoa with the arthropods and other molting invertebrates. Most free-living nematodes are small (μm-mm) and ubiquitous. Sexes are typically separate, with a high degree of sexual dimorphism.

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Bibliography

• Zhang, Z.-Q. 2011. Animal biodiversity: An introduction to higher-level classification and taxonomic richness. Zootaxa 3148: 7-12. [Available online]
• Hodda, M. 2011. Phylum Nematoda Cobb 1932. In: Zhang, Z.-Q. (Ed.) Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness. Zootaxa 3148: 63-95. [Available online]
• Andrássy, I. 2009. Aporcelinus, a new genus of aporcelaimoid nematodes (Dorylaimida), and its species. International Journal of Nematology 19(2): 121-136. [Available online]
• Abebe, E., W. Decraemer & P. De Ley. 2008. Global diversity of nematodes (Nematoda) in freshwater. Hydrobiologia 595: 67-78. [Available online]
• De Ley, P., W. Decraemer & Eualem-Abebe. 2006. Introduction: Summary of present knowledge and research addressing the ecology and taxonomy of freshwater nematodes. [in] Eualem-Abebe et al. (eds.). Freshwater Nematodes: Ecology and Taxonomy: 3-30.
• Abebe, E., W. Traunspurger & I. Andrassy (eds.). 2006. Freshwater Nematodes. 752 pp. [Available online]
• Brusca, R.C. & G.J. Brusca. 2003. Invertebrates, 2nd edition. Sinauer Associates, Inc., Sunderland, Massachusetts. 936 pp.
• Smith, D.G. 2001. Nematoda (Roundworms). Pennak's Freshwater Invertebrates of the United States, 4th edition: 191-215.
• Andrássy, I. 1992. A census of genera and subgenera of free-living nematodes described ensuing Andrássy's book, 1976. Opusc. Zool. Budapest 25: 137-148. [Available online]
• Ferris, V.R., L.M. Ferris, & J.P. Tjepkema. 1973. Genera of freshwater nematodes (Nematoda) of eastern North America. U.S. Environmental Protection Agency, Biota of Freshwater Ecosystems Identification Manual 10. 38 pp. [Available online]
• Chitwood, B.G. & M.W. Allen. 1959. Nemata. [in] W.T. Edmondson (ed.). Ward & Whipple Fresh-Water Biology, 2nd edition: 368-401.
• Pennak, R.W. 1940. Ecology of the microscopic Metazoa inhabiting the sandy beaches of some Wisconsin lakes. Ecological Monographs 10(4): 537-615. [Available online]

Web Pages

• Animal Diversity Web
• Integrated Taxonomic Information System
• Micrographia Gallery
• NeMys
• Synopsis of the Described Nematoda of the World
• Systema Naturae 2000
• Tree of Life
• WikiSpecies
• Wikipedia
• World Register of Marine Species