Water diviners multiplied: cryptic diversity in the Niphargus aquilex species complex in Northern Europe

As for many other groups, patterns of biodiversity for subter-ranean crustaceans in Europe indicate larger morphospecies ranges at higher latitudes (the so-called Rapoport effect). However, this observed correlation may be artefactual if many of those high-latitude, widely distributed morphospecies are actually species complexes. To test this hypothesis, we looked for previously undetected species boundaries within Niphargus aquilex and Niphargus schellenbergi, two closely related morphospecies of groundwater amphipods widely distributed in northern Europe, by sequencing fragments of the mitochondrial cytochrome c oxidase subunit I gene (COI) and of the nuclear 28S ribosomal RNA gene of 198 individuals collected across their area of distribution. Distance-based and allele sharing-based species delimitation approaches were congruent in revealing the existence of at least eight species within N. aquilex and at least two species with N. schellenbergi. Our data demonstrate that these two common morphospecies with large ranges are actually complexes of species with narrower distributions, suggesting that the Rapoport effect might be the result of increased morphological stasis at high latitudes rather than actual differences in sizes of distribution ranges.