A species history is characterized by fluctuations of its geographic limits, however, climate change is currently redistributing life on Earth. Particularly in the Arctic, waters are warming at faster rates and sea ice is thinning and contracting. Amphipod crustaceans of the genus Themisto are highly abundant in Arctic waters where they play a major role as food for higher trophic levels such as seabirds, fish and seals. Throughout the Arctic region, two congeneric 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. T. libellula is bigger in size and feeds on copepods with a high lipid content. It is the main prey of some specialist feeders such as little auks that only feed on the largest size class, in which case T. abyssorum would not be a proper substitute as it is much smaller. Range expansion of T. abyssorum and retraction of T. libellula’s range is very likely to occur considering the ongoing Atlantification of the Arctic. However, many aspects of the biology, ecology and genetic connectivity of Arctic Themisto populations are still unstudied, despite their importance for a better understanding of the consequences of their potential distributional changes on the food web and biogeochemical cycles. In this context, we investigated the abundance, geographic and bathymetric distribution as well as the genetic connectivity of the two species T. abyssorum and T. libellula during two cruises with R/V Polarstern to Fram Strait and East Greenland.