Mollusc collected with bottom otter trawl in the northern Alboran Sea: main assemblages, spatial distribution and environmental linkage

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English. Molluscan assemblages of circalittoral and bathyal soft bottoms in the northern Alboran Sea were studied using an experimentalbottom otter trawl. Samples of fauna were collected from 190 hauls over four MEDITS surveys carried out in spring between 2012and 2015 at depths ranging from 30 to 800 m. Near-bottom measurements (temperature and salinity) and sediment samples weretaken in the same locations as the faunal sampling. A total of 101 species grouped into 55 families were recorded. Cephalopodswere the most abundant group, with Abralia veranyi dominating the abundance, Octopus vulgaris the biomass, and Illex coindetiiand Todarodes sagittatus being the most frequently collected species. Multivariate analyses carried out separately with abundancedata for demersal species, benthic species, bivalves, gastropods, and cephalopods, as well as total molluscs, generally resulted inthree main molluscan assemblages corresponding to a shelf assemblage (30-200 m depth), an upper slope assemblage (201-350 mdepth), and a middle slope assemblage (351-800 m depth). PERMANOVA analyses revealed that significant depth-related differences of distinct molluscs groups were more acute than for the geographical sectors of the Alboran Sea (western, central, eastern,and insular sectors). Abundance, biomass, and species richness all decreased with depth, with a clear dominance of cephalopodson the slope. Significant geographical differences were detected, principally for demersal and benthic species as well as gastropodsand cephalopods, between the insular sector (Alboran Island) and the western and eastern continental sectors. Depth and temperature were the most influential variables in the different CCA analyses using datasets of molluscs with different lifestyles and fromdistinct classes, and sedimentological variables displayed a much more significant relationship with benthic molluscs than withdemersal ones.