Biodiversity patterns of megabenthic non-crustacean invertebrates from an exploited ecosystem of the Northwestern Mediterranean Sea

We evaluated seasonal patterns of biodiversity of the megabenthic non-crustacean invertebrates on soft bottoms of the continental shelf and slope of the Catalan Sea (Balearic Sea, Northwestern Mediterranean). Scientific demersal trawls were carried out between 30 and 400 m during winter and summer of 2013 in 37 and 45 stations respectively. Among a total of 188 species, a limited number of echinoderms, cephalopods and occasionally ascidians dominated the bulk of the community, covering up to 95% of the total wet weight. Seasonal and bathymetric variations were found in the different biodiversity patterns, with higher values in summer and in shallower depths, regarding overall wet weight (>14,000 kg/km2), density (>1 million ind/km2), richness and diversity (143 species; H’=1.74). The statistical differences in density and wet weight between bathymetric strata reflected in particular groups of species typifying each environment, but without any exclusiveness in depth preferences, since generalist species were the main components of the community. Depth was the main driver of statistical differences in the community structure, but a combined influence of geographic location was also identified from an interaction effect. Sediment type may be influencing habitat preferences, with echinoderms (especially crinoids Leptometra and Antedon), dominating on sandy substrates, and cephalopod mollusks (especially cephalopods Illex, Eledone, Octopus) dominating on muddy substrates. Trawling activity may also influence the dominance of some target and non-target species. Several infrequent bivalves, opistobranchs and species from phyla Porifera, Cnidaria, Bryozoa and Sipuncula constituted an important understudied component of the richness of this community. This is the first study dealing with bathymetric variation of the megabenthic non-crustacean community of the Catalan Sea within this gradient of depths above the middle slope and also considering seasonal variability.