Distributional patterns of isopods (Crustacea) in Icelandic and adjacent waters

Since the 1990s, the marine benthos of the Greenland-Iceland-Faeroe (GIF) Ridge has been sampled through the BIOICE (Benthic Invertebrates of Icelandic waters) and subsequent IceAGE (Icelandic Marine Animals: Genetics and Ecology) projects. Isopod crustaceans formed one of the prominent macrofaunal groups. Most isopod families occurred on both sides of the ridge, but showed side-specific abundance patterns reflecting known distribution from the literature. Our results from 35,536 isopod specimens from 55 epibenthic sled stations show a depth pattern at the family level indicating typical shallow water families like, for example, Paramunnidae or typical deep-water families like, for example, Haploniscidae and Ischnomesidae, while other families did not show a clear depth trend. We hypothesize that the ridge influences the distribution of the families through its effects on the hydrography and sediment characteristics. Total organic carbon (TOC) and mud content significantly explained isopod family distributions, possibly reflecting different habitat use and lifestyle (e.g., infaunal, epifaunal). Our analysis of a subset of 27 selected BIOICE and IceAGE stations and 100 isopod species (22,574 specimens), mostly covering the upper 1000 m depths between Iceland and Norway along the Iceland-Faeroe Ridge (IFR), resulted in four main species assemblages differing in species diversity. Whereas north of the IFR, we found 76 species; south of the IFR, we found 52 species; 40 species occurred both north and south of the IFR. Although the subset of selected stations is limited to a comparably small portion of the IceAGE sampling grid and to a comparably restricted depth range of the upper 1000 m, our result shows the slope area of Northeast Iceland and around the Faeroes and the Norwegian Channel with the highest number of species, especially stations in the thermocline between 400 and 800 m depth, where we observed the turnover from shallow to deep-water faunal elements.