Nematoda

Abstract

Nematoda (roundworms) are mostly small animals in the range of only millimeters. While they are hard to see without a microscope, nematodes represent the largest animal phylum with an estimated number in the range of one to ten million species (Lambshead 1993). Nematodes are characterized by three general features. Besides species richness, these are numerical abundance and ecological omnipresence because they usually occur in high numbers and they are found in most ecosystems. For example, in some soil samples, nematodes can occur in excess of one million individuals per square meter (Floyd et al. 2002). The highest diversity of nematodes is found in marine environments and in terrestrial settings, often in association with arthropods or other invertebrates. Some nematodes are important parasites of plants, livestock, and humans. In the last 15 years, molecular phylogenetics has resulted in a comprehensive understanding of the relationships among nematodes that can serve as the basis for evolutionary considerations (van Megen et al. 2009). For example, molecular phylogenetics convincingly showed that parasitism has evolved at least seven times independently in nematodes, involving both plant and animal parasitism (Fig. 2.1; Blaxter et al. 1998). By now, many parasitic nematodes have their genome sequenced (Fig. 2.1), representing a promising starting point to understand associated biological processes (for a review see Sommer and Streit 2011).

Although commonly considered a subtaxon of Cycloneuralia, the Nematoda are covered separately in this chapter.

Chapter vignette artwork by Brigitte Baldrian.© Brigitte Baldrian and Andreas Wanninger.