Both the Arctic and the Southern Ocean, in particular the southwest Atlantic sector, are experiencing rapid environmental changes. In the southwest Atlantic sector of the Southern Ocean, a long-term trend of density changes of key pelagic species has been noted over the last decades. A decline in Antarctic krill is hypothesized, whilst salps are on the rise and shifting their distribution poleward. A similar poleward expansion is anticipated for a third key player, the amphipod crustacean Themisto gaudichaudii, leading to an increasing overlap of the distributions of these three species. In Arctic waters, two congeneric amphipod species co-exist: T. libellula, a genuine Arctic species and T. abyssorum, considered sub-Arctic boreal. Despite their overlapping distributions, the two species seem to occupy distinct ecological niches and are preyed upon by different predators. Due to the ongoing Atlantification of the Arctic, a range expansion of T. abyssorum concomitant with a retraction of T. libellula’s range are very likely to occur. Due to major knowledge gaps in the ecology, biology and genetic connectivity of Themisto species, the consequences of these shifts for food web structures and biogeochemical cycles remain largely unexplored. In this context, we carried out a comparative study of Themisto populations at both poles. Their distribution, genetic and trophic connectivity were investigated with population genetics, molecular diet analyses and feeding experiments.