Mouthpart-atlas of Arctic sympagic amphipods-trophic niche separation based on mouthpart morphology and feeding ecology

The four amphipod species Onisimus nanseni, O. glacialis (Lysianassidae), Gammarus wilkitzkii (Gammaridae), and Apherusa glacialis (Caltiopiidae) have been identified earlier as autochthonous sympagic organisms that spend the whole life cycle in close association with the Arctic ice pack. Even one and a half centuries after the discovery of the ice-bound ecosystem, the ecology of these amphipod species, and in particular of Onisimus spp. and Apherusa glacialis, is only poorly known. In this study we evaluate and separate the trophic niches of the four amphipod species by analysing the morphology of mouthparts and accessory feeding appendages and compiling all information available on species distribution and feeding ecology. Morphological features and ecological observations imply that interspecific trophic niche overlap among these four amphipod species is reduced; each species has evolved a specific trophic strategy in selecting a different set of primary and supplemental food sources: O. nanseni is detritivorous (necrophageous), O. glacialis is detritivorous (herbivorous), G. wilkitzkii is carnivorous (detritivorous), and A. glacialis is herbivorous (detritivorous). Food choice plasticity in general, and facultative detritivory in particular, can be interpreted as an adaptation to the highly variable and dynamic character of the ice ecosystem. There is evidence that in both Onisimus species, as well as in A. glacialis, niche separation is furthermore provided by their temporary absence from the sympagic environment, which questions the affiliation of these species to the group of autochthonous sympagic organisms.