Eurythenes gryllus (Lichtenstein, 1822) is a presumed cosmopolitan eurybathic benthopelagic giant deep-sea amphipod. However, previous studies already highlighted genetically divergent lineages in the Atlantic and Pacific Oceans, which appeared to be vertically stratified, and minor morphological differences between populations were also recorded. With an aim to quantify the geographic and bathymetric patterns of genetic variations, the genetic diversity in Eurythenes gryllus was investigated at the global scale (Arctic, Atlantic, Pacific and Southern Oceans) using three different genes (COI, 16S rRNA, 285 rRNA). This genetic analysis was accompanied by a thorough morphological study. Phylogenetic and phylogeographic analyses revealed the existence of at least eight well-supported clades, two bathyal and six abyssal, which were separated by genetic distances at the level of usual interspecific divergences. A subsequent morphological analysis confirmed the genetic findings and revealed small but consistent differences between the different clades, which will be described as separate species. Furthermore, a clear genetic break was observed between specimens sampled above and below 3000 m. This bathymetric break below 3000 m has already been reported for several organisms and regions, suggesting its role as a ubiquitous phylogeographic barrier for barophysical tolerance. The Eurythenes clade, comprising specimens sampled at bathyal sites in the Arctic and Southern Oceans, is presumably the true E. gryllus. This represents, to our knowledge, the first molecular evidence for a bipolar distribution in a macro-benthic deep-sea organism. The present results clearly highlight the difficult nature of research on the systematics of deep-sea crustaceans and shows that the abyss is a more complex environment than previously assumed, likely to harbour an important hidden diversity.

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