Diversity, biology and evolutionary relationships

Nematodes are among the most abundant organisms on Earth, as they exist in almost every possible habitat and ecosystem (Bernard, 1992; De Ley, 2006; Ettema, 1998; Powers et al. 2009). Indeed, these organisms can be found in aquatic (marine and fresh water) and terrestrial ecosystems ranging from the tropics to the poles and from the highest to the lowest of elevations. Furthermore, nematodes have exploited a wide range of ecological niches encompassing free–living and parasitic species. Parasites have received the most attention and have been the subject of extensive research because of the damage they cause to crops, livestock, and humans (Anderson, 2000; Norton, 1978; Poinar, 1983; Stirling, Poinar, Jansson, 1988; Wallace, 1963; Zuckerman Rhode, 1981). However, several parasitic species are considered beneficial organisms to humans as they can be used as control agents of pests that are of agriculture, forestry or health importance (Bedding, Akhurst, Kaya, 1993; Gaugler Kaya, 1990; Grewal, Grewal, Adams, 2003; Petersen, 1985; Poinar; Stock Hunt, 2005; Wilson Gaugler, 2000; Wilson, Glen, & George, 1993).