In the Polar Regions, sea ice habitats are undergoing rapid environmental change. Because sea ice constitutes an important substrate for numerous species, as well as an important carbon source during critical periods of the year, these changes have a significant impact on ecosystem functioning, biodiversity, species distribution, and population sizes of both commercially exploited species, and species valuable from a conservation perspective. Species dwelling at the ice-water interface (e.g. Antarctic krill Euphausia superba and Arctic cod Boreogadus saida) are assumed to play key roles in these ecosystems. As an important trophic carbon transmitter from the sea ice into pelagic food webs and ultimately to the deep sea benthos, under-ice fauna can contribute significantly to the carbon flux in polar ecosystems. Whether the function of Arctic and Antarctic under-ice communities as trophic carbon transmitters is comparable in spite of great differences in the environmental regimes of the two Polar Oceans, however, is an open question. Quantifying under-ice communities was hampered in the past by the inaccessibility of the ice underside to conventional sampling gear. Using a new under-ice trawl, it was demonstrated that Antarctic krill concentrates under sea ice almost year-round, and that krill dwelling under ice are often under-estimated by pelagic nets and sonars. A diverse suite of species, including copepods, amphipods and pteropods, shares this attraction to the Antarctic ice underside at least temporarily. An Arctic expedition in 2012 using an improved version of this sampling gear brought evidence of a similarly rich under-ice community even in the biologically poor-considered central Arctic Ocean. Using a bio-environmental sensor array during under-ice fishing enabled fine-scale characterization of sea ice habitat properties as a basis for statistical modeling of under-ice species distribution. In the Arctic sea ice system, the trophic role that in the Southern Ocean is attributed to Antarctic krill may be resembled by the ice amphipod Apherusa glacialis (as a consumer of ice algae), and Arctic cod (as a key prey of top predators). This talk will evaluate published and unpublished results from under-ice fishing in the Southern Ocean in comparison to recent findings from the Arctic Ocean. With the availability of two unique datasets from oceanic sea ice systems in both Polar Oceans, similarities and differences between their under-ice communities will be highlighted, and hypotheses on the future fate of sea ice ecosystems will be discussed.