Molecular phylogeny of Catenulida (Platyhelminthes) with special focus on their diversity in Poland

Abstract 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 a 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. This analysis confirms the previous results on deep nodes of catenulid phylogeny, and also revealed several well-supported clades within the largest catenulid family Stenostomidae, each distinguished by unique combinations of morphological features. Moreover, based on our molecular and morphological data, we recognize Stenostomum heebuktense, as a younger synonym of Stenostomum gigerium and Catenula confusa, as a younger synonym of Catenula lemnae. We also highlight the major challenges in catenulid taxonomy, particularly the unresolved species complexes of Stenostomum leucops and Stenostomum simplex.