A morphologically generalized Eurasian Pleuroxus aduncus species group (Cladocera: Chydoridae) is paraphyletic and contains denticulated P. truncatus , with

Most publications on the water fleas (Crustacea: Cladocera) have focused, and continue to focus, on the genus Daphnia O.F. Müller, 1785 which represents a model for several branches of recent biological sciences. In contrast, the number of publications on other numerous genera remains minimal. We have revised the Pleuroxus aduncus species group (Anomopoda: Chydoridae), very common in the littoral zone of different continental water bodies of Northern Eurasia. Our study is based on both morphological analysis and molecular phylogenetic data (sequences of the mitochondrial COI and 16S , and nuclear 18S and 28S fragments). P. aduncus (Jurine, 1820) was redescribed based on European material; a new taxon, P. korovchinskyi sp. nov., was described from Eastern Siberia. Our results support the paraphyly of the aduncus group and led to a surprising conclusion: P. truncatus (O.F. Müller, 1785), a very characteristic taxon having a continuous row of denticles along its posterior valve margin, appeared within the morphologically generalized P. aduncus group. This situation has an analog in North America, where the denticulated taxon P. procurvus Birge, 1879 also was differentiated within a group of species lacking such denticles at the posterior valve margin. Our study has demonstrated that we need to continue a program of chydorid revisions, as well as of other “widely distributed” cladocerans.