Taxonomy and systematics in Gyrodactylusvon Nordmann, 1832 (Monogenea) : studies on a problematic species complex parasitizing salmonids

The two monogenean species G. salaris and G. thymalli, are almost identical at the nuclear molecular level. There is also no support from mtDNA (cox1) sequences for monophyly of all G. salaris or G. thymalli haplotypes. It has therefore been suggested that these taxa represent a case of incipient speciation. Based on this, whether ultimately considered one or more species, these taxa together are referred to as the G. salaris / G. thymalli species complex. It is argued in the present thesis that a total-evidence approach to the taxonomy of the G. salaris / G. thymalli species complex is at lower risk of oversimplifying the taxonomic conclusions than is a single-criterion approach for species delimitation. One aim is therefore to apply a comprehensive all-inclusive approach to the taxonomy of the G. salaris / G. thymalli species complex. <br> The morphology of the opisthaptoral hard-parts, is considered taxonomically informative, and therefore representing a non-linear approach to taxonomy in Gyrodactylus. However, traditional linear measurements are not necessarily the optimal approach in studies of morphology. Furthermore, environmental factors may influence on the size and possibly also the shape of the opisthaptoral hard-parts. Accordingly, one aim in the present thesis is to improve the methodology in studies of systematics in Gyrodactylus based on morphology. In the analytical approach to study morphology, shape descriptors from studies of geometric morphometrics proved to work well. It is therefore reasonable to assume that in future studies the application of geometric shape descriptors is justified.<br> The presented results indicated that taxonomic revisions in the G. salaris / G. thymalli species complex based on a single species criterion, despite advantages of comparability, are unlikely to lead to an overall satisfactory delimitation of species. All methods applied so far for assessing the taxonomy and systematics of the G. salaris / G. thymalli species complex document the importance of evolutionary relationship as the absolute basic grouping criterion. However, for example species definition exclusively based on mtDNA sequence data fails to reflect the significant differences in host preferences and pathogenicity. It is therefore concluded that the knowledge of phylogenetic relationships in the species complex should be supplemented with information on morphology and even more important, on ecology, when it comes to defining the boundaries between the taxa, whether on a species level or below.