<p>This thesis focuses on phylogenetic and taxonomic studies of Catenulida (Platyhelminthes). Catenulida is a group of microscopic free-living worms mainly found in freshwater habitats. The Swedish catenulid fauna was previously virtually unknown. The taxonomy of Catenulida is difficult because of the paucity of good morphological characters, which makes species identification extremely difficult. </p><p>Molecular phylogenies are inferred from DNA sequences. Based on two molecular markers, 18S rDNA and 28S rDNA, the phylogenetic position of Catenulida has now been well established as the sister group to the rest of the flatworms, Rhabditophora. Within Catenulida there is a basal split between two major clades: Retronectidae + Catenulidae and Stenostomidae. The hypothesis of the marine Retronectidae as the sister group of the limnic Catenulida is rejected. </p><p>Four molecular markers, 18S rDNA, 28S rDNA, ITS-5.8S and CO1, are used as a backbone to infer phylogeny and to generate hypotheses about species delimitation in Catenulida using parsimony jackknifing and Bayesian analysis. <i>Anokkostenostomum</i> comes out non-monophyletic, and <i>Suomina</i> nested within <i>Catenula</i>, so two new synonymies are proposed: <i>Stenostomum</i> Schmidt, 1848 (<i>Anokkostenostomum</i> Noreña et al. 2005) and <i>Catenula</i> Duges, 1832 (<i>Suomina</i> Marcus, 1945) are proposed. </p><p>A first report on Swedish freshwater Catenulida are given. A total of 13 species are reported from Sweden. Four of them, all belonging to the taxon <i>Stenostomum</i> are new to science:<i> S. gotlandense</i> n.sp.; <i>S. handoelense</i> n.sp.; <i>S. heebuktense</i> n.sp. and <i>S. steveoi</i> n.sp.</p>

This thesis focuses on phylogenetic and taxonomic studies of Catenulida (Platyhelminthes). Catenulida is a group of microscopic free-living worms mainly found in freshwater habitats. The Swedish catenulid fauna was previously virtually unknown. The taxonomy of Catenulida is difficult because of the paucity of good morphological characters, which makes species identification extremely difficult. Molecular phylogenies are inferred from DNA sequences. Based on two molecular markers, 18S rDNA and 28S rDNA, the phylogenetic position of Catenulida has now been well established as the sister group to the rest of the flatworms, Rhabditophora. Within Catenulida there is a basal split between two major clades: Retronectidae + Catenulidae and Stenostomidae. The hypothesis of the marine Retronectidae as the sister group of the limnic Catenulida is rejected. Four molecular markers, 18S rDNA, 28S rDNA, ITS-5.8S and CO1, are used as a backbone to infer phylogeny and to generate hypotheses about species delimitation in Catenulida using parsimony jackknifing and Bayesian analysis. Anokkostenostomum comes out non-monophyletic, and Suomina nested within Catenula, so two new synonymies are proposed: Stenostomum Schmidt, 1848 (Anokkostenostomum Noreña et al. 2005) and Catenula Duges, 1832 (Suomina Marcus, 1945) are proposed. A first report on Swedish freshwater Catenulida are given. A total of 13 species are reported from Sweden. Four of them, all belonging to the taxon Stenostomum are new to science: S. gotlandense n.sp.; S. handoelense n.sp.; S. heebuktense n.sp. and S. steveoi n.sp.