| Summary: | [eng] Hydrozoans are one of the most widespread components of the marine gelatinous zooplankton and benthos and play an essential ecological role, affecting the food web and biogeochemical cycles. Despite their importance, hydrozoans have generally received little attention in ecological studies and almost every group, including members of the genus Euphysa, is in need of a thorough taxonomic revision. Three species of Euphysa have been recorded in Norway based on morphological identification, although these records should be assessed with caution since morphology alone does not always reflect taxonomic diversity. In order to determine which Euphysa species are actually present in Northeast Atlantic waters, an integrative approach for species delimitation combining both morphological and genetic information was implemented. The genetic approach included phylogenetic analyses (Bayesian inference and Maximum Likelihood), genetic distances and statistical parsimony haplotype network, as well as molecular species delimitation tools (ABGD and PTP). Two mitochondrial genes (cytochrome oxidase I (COI) and 16S ribosomal RNA subunit) and a nuclear marker (internal transcribed spacer (ITS)) of 65 specimens of Euphysa and relevant outgroups from Norwegian and adjacent waters were sequenced, obtaining a total of 133 new DNA barcodes. The systematic resolution of the three employed molecular markers was tested and discussed. An undescribed species was discovered and the morphological diagnostic features of each of the other three Euphysa species occurring in Norway was revised. This is the first molecular assessment of the diversity in genus Euphysa. This work provides an updated inventory of the species of Euphysa occurring in the Northeastern Atlantic as well as powerful complementary data for advancing our understanding of hydrozoan systematics. It also demonstrates the presence of cryptic diversity within this genus, which shows that morphological classifications are not always congruent with molecular data and gives a taste of what can be expected for the rest of the genera within Corymorphidae.
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