The predatory amphipod Themisto, is a so far neglected key pelagic player covering a distribution from temperate to polar regions where it regionally represents the dominant food source for higher trophic levels. Poleward range expansions of several species have already been documented or predicted. In the Atlantic sector of the Southern Ocean, Themisto gaudichaudii, is anticipated to extend its range southward, where it may impact other key players such as krill and salps. In the Arctic, a range expansion of the sub-Arctic T. abyssorum and a corresponding range contraction of the Arctic T. libellula is very likely a manifestation of the ongoing ‘Atlantification’ and corresponding reduced sea-ice levels. Due to earlier knowledge gaps in the ecology, biology and genetic connectivity of Themisto species, the likelihood of range shifts and their consequences for the food web remained hitherto unexplored. Using morphological and molecular tools, we investigated the distribution, connectivity and feeding ecology of Arctic and Antarctic Themisto species. Distinct genetic structures pinpoint climate change winners versus losers within Themisto. Metabarcoding analyses of Themisto populations in the Southern Ocean show a diet predominantly composed of krill, which may further impact krill-consuming top predators, but unexpectedly also of ctenophores, so far often considered as a trophic ‘dead end’.