Integrative taxonomy of the Eurytemora velox (Copepoda, Calanoida) species complex reveals a new cryptic species

Abstract This study investigated the variability within populations of the copepod Eurytemora velox . Integrative taxonomy confirmed significant genetic heterogeneity among the studied populations supported the hypothesis of E. velox being a species complex. To assess morphological variability, morphometric traits and ordination analyses were utilized. For molecular assessment, the mitochondrial cytochrome c oxidase subunit 1 (COI) and cytochrome b (cyt b) and the nuclear 18S ribosomal DNA (18S) and the internal transcribed spacer 2 (ITS2) were employed. Both morphological and molecular data facilitated the identification of at least two species: E. velox s. str. and a new cryptic species, E. nilsseni sp. nov. The former one, E. velox s. str. is an expansive, widely distributed species that inhabits diverse water bodies, including transformed river systems, particularly those with altered water parameters such as high salinity. Within this species, no population-specific morphological differences were observed; however, COI data revealed clear genetic structuring in E. velox s. str., with distinct mitochondrial haplotype clades associated with different drainage basins, whereas nuclear markers showed only limited differentiation among populations. In contrast, E. nilsseni sp. nov. is an endemic and likely relict species recorded exclusively in ponds on the Skagerrak coast islands.