Mertensia ovum, also known as the Arctic comb jelly or sea nut, is a cydippid comb jelly or ctenophore first described as Beroe ovum by Johan Christian Fabricius in 1780. Unusually among ctenophores, which normally prefer warmer waters, it is found in the Arctic and adjacent polar seas, mostly in surface waters down to 50 metres (160 ft). A study in the Barents Sea found that it ingests prey ranging from small copepods to amphipods and krill, but that its staple diet consists of large copepod species such as Calanus finmarchicus, Calanus glacialis, Calanus hyperboreus and Metridia longa.