Tratkiewicz, K.; Baczynski, J.; Gasiorowski, L.

Catenulida is a clade of free-living flatworms found abundantly in freshwater habitats across the globe. Despite their ubiquitous distribution, catenulids remain poorly studied; most of the species are known only from the asexual forms that lack distinct, taxonomically useful characters. Accordingly, the studies of catenulid diversity require application of integrative methods that combine morphological and molecular data. Here, we report the survey of catenulid diversity in Central and Western Europe, with a special focus on the species found in Polish freshwaters. We collected and identified 13 distinct morphotypes that were subsequently sequenced for four molecular markers - 18S, 28S, COI and ITS-5.8S. The obtained sequences, together with reference data from other catenulid species, were used to infer the comprehensive phylogeny of the clade. The analysis revealed several well-supported clades within the largest catenulid family, Stenostomidae, highlighting the major challenges in catenulid taxonomy, such as unresolved species complexes of Stenostomum leucops and Stenostomum simplex. By tracing evolution of morphological, developmental and ecological characters on the phylogeny our study provides insight into major character transitions in the key lineages of catenulids.