Exploring the species boundaries in terrestrial clitellates (Annelida: Clitellata)

The biological diversity in the soil is much higher than most people can imagine, and a very important group of animals living there are earthworms and other clitellate worms. It has been found that several clitellate morphospecies are in fact complexes of so called cryptic species, and the number of species in this group may be much higher than previously believed. In this thesis, I explore the species boundaries of terrestrial clitellates by combining various data sources and methods as a basis for taxonomical decisions. A widely used approach for studying cryptic species is DNA-barcoding, where a single standardised marker is used for the identification of organisms and discovering new species. For animals, the marker generally used is the mitochondrial COI gene. In the thesis, I present four different cases, from three different clitellate families: 1) A variety of Rhyacodrilus falciformis was shown to be a distinct species, whereas other COI clusters were found to be part of the same species. 2) Extensive cryptic diversity was found in the genus previously known as Cognettia, a group that includes C. sphagnetorum, a well-studied model in soil biology. This taxon and C. glandulosus were both found to be complexes of cryptic species. The generic taxonomy of Cognettia was revised and the genus split into its two senior synonyms, Euenchytraeus and Chamaedrilus, and the species in the C. sphagnetorum complex were revised and described. 3) In the earthworm Aporrectodea longa, two well separated COI lineages were found to be part of the same species. 4) In the earthworm genus Lumbricus, the previously noted split between L. terrestris and L. herculeus was verified using nuclear data, and in the morphospecies L. rubellus seven cryptic species were found. The first evidence of limited hybridisation within the genus was also found, between L. terrestris and L. herculeus, as well as and between species within L. rubellus s.lat. To conclude, I have shown that the species diversity among terrestrial clitellates is larger than previously known based on morphology, and that species can be robustly delimited using a combination of mitochondrial and nuclear markers, supported with morphology. I have also found that using DNA-barcoding alone will often overestimate the number of clitellate species, and caution is required when using it.