Nikol Kmentová, Armando J. Cruz-Laufer, Antoine Pariselle, Karen Smeets, Tom Artois, Maarten P. M. Vanhove

Dactylogyridae is one of the most studied families of parasitic flatworms with more than 1000 species and 166 genera described to date including ecto-, meso-, and endoparasites. Dactylogyrid monogeneans have been used as model organisms for host-parasite macroevolutionary and biogeographical studies due to the scientific and economic importance of some of their host lineages. Consequently, an array of phylogenetic research into different dactylogyrid lineages has been produced over the past years but the last family-wide study was published over a decade ago. Here, we provide a new phylogeny of Dactylogyridae including representatives of all the genera with available molecular data. First, we discuss morphological, host range, biogeographical, and freshwater-marine patterns. Second, we provide an overview of the current state of the systematics of the family, and its subfamilies and genera. Third, we elaborate on the implications of taxonomic, citation, and confirmation bias in past studies. We found two well-supported main lineages which we assigned to the subfamilies Dactylogyrinae and Ancyrocephalinae. The subfamilies further include 11 well-supported clades whose members share only few diagnostic morphological features. Our study highlights the discrepancy between morphological similarities and molecular phylogenetic relationships in some dactylogyrid lineages. Environmental changes might have induced morphological adaptation, e.g. changes in the attachment organ in response to marine-freshwater habitat switches or reduction of eyespots related to water turbidity. Moreover, synonymisation of some of the para- or polyphyletic genera is proposed. We conclude that a strong taxonomic bias further limits knowledge on biogeographical evolutionary patterns that can be inferred from these results. Finally, we propose addressing potential citation and confirmation biases through a ‘level playing field’ multiple sequence alignment as provided by this study.