The amphipod Themisto gaudichaudii is a so far neglected but key pelagic player in the Southern Ocean. This voracious predator is found along a wide latitudinal gradient with temperatures differing by almost 20°C between its distributional limits. Due to its high abundances, swarm-forming behaviour and importance for higher trophic levels, it is often referred to as krill from the north. Whilst Antarctic krill (Euphausia superba) is more restricted to colder waters further south, T. gaudichaudii is more abundant in northern ice-free zones and is prone to extend its range southwards with the rising water temperatures observed in the Southwest Atlantic sector. In this region, density changes of salps and Antarctic krill have already been noted over the last decades, with krill decreasing whilst salps are on the increase. However, the interactions between the three key players – Themisto, krill and salps – are too little understood to evaluate cascading top-down effects and potential ecological shifts in the Southern Ocean pelagic realm. Knowledge gaps in the biology and ecology of Themisto gaudichaudii are being explored with a focus on its genetic and trophic connectivity. Phylogeographic analyses confirm its spreading potential, showing several well-supported genetic lineages occurring in sympatry and across a wide geographical range. Its feeding ecology is documented by reviewing literature data and applying molecular diet analyses, to verify whether Themisto predates on krill and salps or competes with them for the same resource, i.e. phytoplankton. Finally, consequences of these possible zooplankton range shifts on higher trophic levels are discussed.