Annelida

Abstract

Annelids are a taxon of protostomes comprising more than 17,000 worldwide-distributed species, which can be found in marine, limnic, and terrestrial habitats (Zhang 2011). Their phylogeny was under discussion for a long time, but recent phylogenomic analyses resulted in a solid backbone of this group (Struck et al. 2011; Weigert et al. 2014). According to these analyses, most of the annelid diversity is part of Errantia or Sedentaria, which both form reciprocally monophyletic sister groups (Fig. 9.1) and are now known as Pleistoannelida (Struck 2011). The Sedentaria also include the Clitellata, Echiura, and Pogonophora (Siboglinidae) as derived from the annelid taxa. Outside Sedentaria and Errantia, several groups can be found in the basal part of the annelid tree, namely, Sipuncula, Amphinomida, Chaetopteridae, Magelonidae, and Oweniidae. The latter two taxa together represent the sister taxon of all other annelids. Given this hypothesis, it has to be assumed that the early diversification of extant annelids took place at least in the Lower Cambrian (520 Ma ago) (Weigert et al. 2014). The phylogenetic position of Myzostomida, a group of commensals or parasites of echinoderms (and, rarely, cnidarians), remains still uncertain. Whereas there is strong support for an annelid ancestry, its exact position awaits to be determined (Bleidorn et al. 2014). Likewise, the phylogenetic position of several interstitial taxa is still under debate (Westheide 1987; Worsaae and Kristensen 2005; Worsaae et al. 2005; Struck 2006). A position of Diurodrilidae outside Annelida, as suggested by Worsaae and Rouse (2008), was rejected by molecular data (Golombek et al. 2013), and the position of the enigmatic Lobatocerebrum and Jennaria remains unresolved (Rieger 1980, 1991). Likewise, the position of Annelida within Protostomia is still uncertain. However, recent phylogenomic analyses recover a clade uniting annelids with Mollusca, Nemertea, Brachiopoda, and Phoronida, but without strong support for any sister group relationship (Edgecombe et al. 2011).

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