PhD thesis - Biologisk Institut - Københavns Universitet

13 General introductionprocesses (e.g., Hessling and Westheide 2002, Denes et al. 2007, Hejnol andMartindale 2008, Maxmen 2008, Wanninger 2009, Boyle and Seaver 2010,Brinkmann and Wanninger 2010a, b).Sipuncula – a neglected taxon with annelid affinitiesSipuncula is a small, worm-shaped, exclusively marine lophotrochozoan taxon thatuniformly exhibits an unsegmented adult body, which is subdivided into a posteriortrunk and a retractable anterior introvert. Sipunculans are filter or deposit feeders thatlive in soft sediments, rock crevices, dead corals, or vacant mollusc shells, and arewidespread throughout the oceans (Rice 1975a). Their fossil record is generally sparsebut a description of six well preserved sipunculan specimens found in southwestChina dates their origin to more than 520 million years ago (Huang et al. 2004).Furthermore, the fossilised outer and inner morphology resembles extant sipunculanspecies, suggesting only limited changes since the Early Cambrium (Huang et al.2004). Cladograms based on morphological characters divide sipunculans into twoclasses, Sipunculidae and Phascolosomatidea, comprising four orders and six families(Aspidosiphonidae, Phascolosomatidae, Golfingiidae, Phascolionidae, Themistidae,and Sipunculidae) (Cutler and Gibbs 1985, Cutler 1994). Today, the use of DNAsequence data has changed this morphology-based view of the relationships amongthe currently recognised 147 species of Sipuncula (Maxmen et al. 2003, Schulze et al.2005, 2007). The two classes have not been recognised by Maxmen et al. (2003) andSchulze et al. (2005), whereas the class Phascolosomatidae was recovered in the byfar most comprehensive analysis on sipunculan phylogeny of Schulze et al. (2005).Congruently, all analyses strongly support Sipunculus nudus as the sister group to theremaining Sipuncula, while most of the previously recognised families and orderswere not recovered as monophyletic (Maxmen et al. 2003, Schulze et al. 2005, 2007).However, based on morphological characters and DNA sequence data, the monophylyof Sipuncula is strongly supported (Maxmen et al. 2003, Schulze et al. 2005, 2007).In general, sipunculans are dioecious (separate sexes), but there are a fewknown hermaphroditic and parthenogenetic species (asexual reproduction) (Åkesson1958, Rice 1970, Rajalu and Krishnan 1969, Pilger 1978, Cutler 1994). Four different,one direct, and three indirect developmental pathways have been described forSipuncula (Fig. 1; Rice 1975b, c). Since sipunculans share certain developmentalfeatures such as spiral cleavage, that gives rise to a trochophore larva with an apical