The predatory amphipod Themisto, is a so far neglected key 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 been already been documented or predicted. We conducted a comparative study of Arctic and Antarctic Themisto species to investigate their distribution, genetic connectivity and feeding ecology in order to predict the likelihood of their range shifts and the consequences for polar food webs. Recently, increases in gelatinous zooplankton biomass, referred to as “jellification”, have been observed in several marine ecosystems. Abundance data on jellies are scarce for the Arctic, impeding our ability to detect changes of a similar magnitude in this rapidly changing region. We will use a combination of novel sampling techniques, molecular tools and modelling to investigate the current and future role of jellies in the Arctic food web, potential range expansions, and their impact of the ecosystem and fish stocks.