Acanthocephala

The Acanthocephala, thorny headed worms are a small monophyletic taxon of endoparasites with about 1,200 species presently known. Together with the Nematomorpha, it represents one of the two known exclusively parasitic phyla. A cosmopolitan taxon, acanthocephalans have a 2-host life cycle; all classes of vertebrates may serve as definitive hosts, arthropods as intermediate hosts and, in some instances, paratenic hosts are utilized. Early workers, such as Rudolpbi, Luhe, and Meyer, focusing on taxonomy and morphology, were mainly from Germany, but significant contributions were also made from the USA by, for example, Van Cleave and from Travassosin Brazil. More recently, a physiological approach was undertaken by workers such as Edmonds & Dixon (1966), Crompton (1970), and Starling (1985), and aspects of fine structure were explored and ecological studies were undertaken. A comprehensive review of the biology of the group, including aspects of life history and population dynamics,was undertaken by Crompton and Nickol ln 1985. Since then, work has continued on taxonomy and systematics, with increasing use of molecular tools, and further ultra structural studies have been carried out (Taraschewski2000). A major emphasis, however, has been to take an ecological approach to the acanthocephalans, and a comprehensive review of acanthocephalan ecology was presented by Kennedy (2006). In this chapter, we will look at cunent understandingof aspects of the phylogeny, systematics, and taxonomy of a group that, while lacking anatomical diversity and exhibiting the same fundamental life cycle and developmental stages, nevertheless can be considered highly successful. Acarithocephalans are as widely distributed among vertebrate hosts and biomes as the larger and more diverse parasite groups (Kennedy 2006).