| Summary: | Caudofoveata is by far the least known group of molluscs. The shell-less, wormshaped caudofoveate molluscs are characterized by a mantle covered in calcareous sclerites, giving them a distinctive, shiny appearance. Caudofoveata consists of 141 recognized species found from intertidal habitats to the deep-sea, where they live burrowing in sediments. Three families are recognized within Caudofoveata; Prochaetodermatidae SalviniPlawen, 1975, Chaetodermatidae Ihering, 1876, and Limifossoridae Salvini-Plawen, 1970. Evolutionary relationships of the group have been debated, yet studies investigating the phylogeny of the group have been lacking. In this thesis, the phylogenetic relationships within and among the families of Caudofoveata are investigated using molecular phylogenetics and analyses of whole mitochondrial genomes. Special emphasis was put on the fauna of the northern Atlantic Ocean, including the description of a new species, and a thorough investigation of one of the most common species in the area. In order to investigate relationships among and within families of Caudofoveata, phylogenetic analyses were performed using six sequenced fragments of selected mitochondrial and nuclear genes from 38 species representing all three recognized families of Caudofoveata with species from worldwide sampling locations. In resulting trees, contrary to traditional views, Prochaetodermatidae is sister to a clade containing the other two currently recognized families, Chaetodermatidae and Limifossoridae. The monophyly of Prochaetodermatidae is highly supported, but Limifossoridae and Chaetodermatidae are not recovered as monophyletic. Most of the caudofoveate genera are also not recovered as monophyletic in our analyses. Thus, results from our molecular data suggest that current classification of the group is in need of revision and indicate different evolutionary scenarios than previous hypotheses based on morphology. Within the family Chaetodermatidae, Chaetoderma nitidulum Lovén, 1844 is a common species of ...
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