Species Identification and Delimitation in Nemerteans

Abstract

The accurate identification and delimitation of species constitute the basis for assessing the biodiversity and phylogenetic relationships within the taxon Nemertea. However, working on nemertean taxonomy is challenging for many reasons. The external and internal morphology are relatively simple with only a few diagnostic characters. This makes it very difficult to identify species based solely on visual inspection. Additionally, the muddled taxonomic history of many genera and species hampers assigning identified species to the current systematic background.<br /> As observed in many other taxa, the identification and delimitation of nemertean species has shifted from traditional morphological-based to molecular-based taxonomy. The usage of either methodology bears its own pitfalls. Most of our taxonomic knowledge is based on morphological data and relies on histological sections, which are difficult to compile and analyze. Additionally, the overall interpretation of morphological characters lacks a widely applied standard and is therefore subjective. DNA taxonomy holds great promise for faster species identification. This methodology is based on the comparison of sequence data, relying on the presence of a barcoding gap and high coverage of the target taxa within sequence databases. The latter is, however, not given in a majority of nemertean species, rendering a DNA-based approach for species identification problematic.<br /> Within this thesis, I present three representative examples encountered in nemertean taxonomy. Chapters 2 and 3 concern the description of previously unknown species, one of which represents a cryptic species. Chapter 4 deals with the re-description of a nemertean species with a confusing nomenclatural history. With this thesis I give suggestions as to how species can be delimited, identified, and described in the future. I conclude that in most cases, an integrative taxonomic approach combining molecular data, external characters, and histological-based morphology constitute a stable background to safely delimit and identify nemertean species. The species descriptions presented herein show that if one method fails or is of limited conclusiveness, the application of the other approaches can assist to succeed in delimiting species boundaries. Molecular sequence data are of major importance especially in terms of identifying and delimiting cryptic nemertean species. In these cases, molecular sequence data combined with external characters present a solid basis for species descriptions. Furthermore, molecular sequence data should always be included in species descriptions even if its relevance is not immediately apparent. As more data is assembled, it should and will provide a solid backbone for future barcoding identification and phylogenetic reconstructions.<br /> I suggest basing species re-descriptions on more than just one methodology in an integrative taxonomic approach. Including data from different methodological or biological backgrounds allows assessing species boundaries from multiple perspectives and additionally provides a link to the traditional knowledge of nemertean taxonomy.