Bathymetric structuring of scavenging amphipods in two sub-Arctic fjords

The removal of carcasses and consequently recycling of organic matter from food falls depends on a variety of benthic scavenging species in marine ecosystems. Amphipods, more specifically lysianassids, are among the most important scavengers in the deep sea and coastal habitats. Arctic coastal waters were found to have a highly diverse amphipod community, while very little is known about the lysianassid distribution and species composition in sub-Arctic ecosystems. In this study, sampling with baited traps along a depth transect in two North Norwegian fjords revealed only four lysianassid species that were scarce or even absent in shallow waters (30–100 m depth) while highly abundant in deeper waters (200–500 m). This depth distribution pattern was found across seasons in spring and winter. Both fjords differed in their species composition with a single species, Orchomenella obtusa , dominating the innermost fjord from 200 m depth down to the deep basin. This depth restriction did not result in strong genetic population differentiation of this species (based on COI markers) between the fjords, which are separated by a shallow sill, indicating sufficient connectivity despite their lack of pelagic larvae. The reason for the observed depth distribution remains unclear, but the low diversity of amphipod scavengers and low abundances in shallow waters may have ramifications for organic matter cycling and system functioning in the different depth zones of sub-Arctic fjords.