| Summary: | © Inter-Research 2018. Gonatus fabricii is the most abundant cephalopod species in Arctic waters, and the only squid that completes its entire life cycle there. In order to understand its ecological role in the Arctic, we conducted stable isotope analyses of beaks from all ontogenetic groups from west and east Greenland waters and the Barents Sea, complemented with morphological data. The values of both δ13C and δ15N of G. fabricii were not related to sex. Values of δ13C showed a small ontoge-netic increase, and these values were geographically distinct, with highest levels found in the western part of the study area. Values of δ15N showed a dramatic ontogenetic increase (i.e. 10.0‰ δ15N; 2.6 trophic levels), from epipelagic juvenile forms to large bathypelagic adults, without significant geographical variation. The observed maximum value of δ15N (14.9‰) is the highest ever recorded in cephalopod beaks. The estimated trophic level (up to 5.1) compares only to top vertebrate predators in the Arctic: large piscivorous fishes, seals and toothed whales or large benthic scavenging fishes. Thus, G. fabricii is a top invertebrate predator in the Arctic, with the widest iso-topic niche observed to date for any species there. Among cephalopods its trophic level is only exceeded by its Antarctic congener, G. antarcticus, and by the Antarctic colossal squid Mesony-choteuthis hamiltoni. Thus, polar squids occupy higher trophic positions than do squids living in warmer regions. Finally, our study shows that G. fabricii descends to bathypelagic layers during ontogenesis, continuously increasing its trophic level by changing prey types and sizes, and avoiding predation pressure.
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