The present study investigated whether the cyclopoid copepod Oithona similis Claus 1866 is a cosmopolitan or a conglomerate of cryptic species. Adult and subadult females (C5 stages) of O. similis were closely examined morphologically and via DNA-barcoding from four study areas: the Arctic Ocean, the Southern Ocean, the North Sea and the Mediterranean Sea. Sampling was done during two expeditions with RV Polarstern in the Arctic Ocean (ARK XXIII-3, ARK XXV-1) and at one expedition in the Southern Ocean (ANT XXIV-2). Further samples from three stations in the North Sea and one station in the Mediterranean Sea were provided. Based on the shape of the rostrum, body size and the formula and structure of the outer setae of the exopodits of the swimming legs, five different morphotypes were identified: Oithona similis (Arctic Ocean, Mediterranean Sea, North Sea, Southern Ocean), O. atlantica (Arctic Ocean), O. frigida (Southern Ocean), O. nana (North Sea) and Oithona sp. (North Sea). Via CO1-sequencing in total eight different haplotypes of O. similis were found in this study: three in the Arctic Ocean, three in the Southern Ocean, one in the North Sea as well as in the Mediterranean Sea and one in the Mediterranean Sea. Only one haplotype was found in one than one sampling area. In addition to the number of haplotypes, this clearly indicates that O. similis is not a cosmopolitan but a conglomerate of cryptic species. Additionally to the Oithona similis groups, three other copepod species groups were identified morphologically as well as via sequencing: O. frigida in the Southern Ocean and in the North Sea O. nana (close to the island of Helgoland)and Oithona sp. (close to the island of Sylt). Oithona nana was chosen as the basis of a neighbor joining tree because it is not as closely related to O. similis as the other species are. Morphological differences regarding the appendages of the swimming legs of O. frigida and O. similis were obvious and were clearly reflected in the results of the CO1 sequences. The differences reflected in the appendage structures of the swimming legs were also obvious between O. similis and O. nana. Another haplotype named Oithona sp. shares the swimming leg appendage structure with O. nana, but has a bended rostrum like O. similis. The differentiation between these species is also clearly reflected in their position in the neighbour joining tree. Thus, O. similis and other Oithona species inhabiting the investigation areas can clearly be differentiated morphologically and genetically. The CO1- sequences of the Oithona similis haplotype containing individuals from two different places in the North Sea and the Mediterranean Sea differ from the sequences of the species sampled at the other regions. The fact that the same haplotype was found at different places in the North Sea as well as in the Mediterranean Sea shows that this species is widely distributed and might be quite flexible concerning environmental conditions. It is also possible that species of the genus Oithona are advected into the southern North Sea with Atlantic water Overall, almost no morphological differences were found within and between regions for individuals of the Oithona similis species groups from the Southern Ocean, the Arctic Ocean, the North Sea and the Mediterranean Sea. Exceptions are individuals from the Arctic Ocean that were described as Oithona atlantica. One aim of this study was to examine whether possibly existing cryptic species in the nominal O. similis either show no morphological differences or only very slight ones that make it impossible to differentiate between them morphologically. Since individuals that were described as Oithona atlantica prior to sequencing do not form an own haplotype, and as no other morphological differences within the O. similis individuals were found, this can be confirmed at least concerning the examined morphological characters
