Exploring species diversity within the amphipod family Iphimediidae using DNA-based delimitation, traditional and 3D-Geometric Morphometrics

As an isolated ocean at the end of the thermal continuum, the Southern Ocean hosts a highly endemic cold-adapted marine fauna. However, this unique biota is now challenged by climate-related changes, as some parts of Antarctica are the fastest warming areas on Earth. In order to monitor and mitigate climate-induced changes, a baseline knowledge of species biodiversity and distributions is mandatory. However, this knowledge is currently insufficient for most Southern Ocean taxa, as the actual number of species is still likely to be greatly underestimated. For instance, most of the geographically widespread to circum-Antarctic amphipod species that have been subjected to molecular scrutiny were revealed to be complexes of similar species with a more restricted distribution range. In this study, we reconstructed a novel molecular phylogeny of Antarctic Iphimediidae and used a range of DNA-based species delimitation methods (bPTP, GMYC, ABGD) to explore species diversity. Furthermore, traditional and 3D-geometric morphometric methods were applied on micro-CT scans to characterize patterns of morphological variation within and among clades and DNA-based putative species. For two of the species complexes, namely Gnathiphimedia sexdentata and Echiniphimedia hodgsoni, the shapes of selected anatomical traits of potential taxonomic value at the species level were quantified: (i) a spine-like tooth, (ii) the head, (iii) the coxa 4 and (iv) the first article of Antenna I peduncle. Our results revealed that ten nominal iphimediid species are complexes of multiple species, most of which can be distinguished by small but consistent morphological differences. Ten of these new species were formally described: Stegopanoploea brevidentata sp. nov., Maxilliphimedia acutilobata sp. nov., Maxilliphimedia oliveri sp. nov., Echiniphimedia spinosior sp. nov., Echiniphimedia maxima sp. nov., Iphimediella longidentata sp. nov., Iphimediella brachyodonta sp. nov., Iphimediella longilobata sp. nov., Labriphimedia adeliae sp. nov., and Labriphimedia anneninae sp. nov. Altogether, molecular and morphometric methods were combined to revise species diversity within the iconic Antarctic Iphimediidae, increasing the number of species from 39 to 49, with 14 additional putative species requiring further study for formal description.

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