In July–August 2004, the Mid-Atlantic Ridge Ecosystems (MAR-Eco) expedition collected Demospongiae (Porifera) from the Northern Mid-Atlantic Ridge (MAR) north of the Azores, between 41°N and 61°N. Demosponges were found at 10 stations, at depths ranging from 753 to 3046 m. Twenty-two species were identified: 17 Tetractinellida, one Polymastiida, one Suberitida, two Poecilosclerida and one Dendroceratida. The study of this material is an opportunity to revise the taxonomy and the North Atlantic distribution of each of these deep-sea species. Some species are particularly rare and poorly known (Tetilla longipilis, Tetilla sandalina, Craniella azorica, Polymastia corticata) and two are new to science: Forcepia (Forcepia) toxafera sp. nov. and Iotroata paravaridens sp. nov. This study suggests that the MAR is not a longitudinal barrier for the dispersal of deep-sea demosponges while on the contrary, the Charlie-Gibbs Fracture Zone (CGFZ) may be a latitudinal border for the dispersal of deep-sea demosponges, due to great depths and currents.

The composition, taxonomic structure and distribution patterns of bathyal benthic fauna of the northern Mid-Atlantic Ridge between the Azores and the southern tip of the Reykjanes Ridge were studied from specimens collected on the ‘G.O. Sars MAR–ECO’ expedition in July 2004. Material was obtained from 16 Campelen 1800 trawls taken north of the Azores (42°N) and in two areas around the Charlie-Gibbs Fracture Zone (CGFZ) (51°N and 53°N) at depths 1237–3527 m. A total of 192 species were recorded. Overall the MAR–ECO benthic fauna was dominated by echinoderms (49.5% of the total species sampled), sponges (18.2%) and anthozoans (16.7%). Between-station species similarity was analysed using non-metric multidimensional scaling. Our analysis revealed differences between stations from the CGFZ and the Azorean regions. Stations from the Azores formed one cluster whereas stations from the CGFZ separated into two groups related to depth: stations from depths 1263 m to 1916 m and those from 2350 m to 3512 m. The taxonomic structure of the bathyal fauna was also different in the Azores than in the CGFZ areas. At the CGFZ, taxa with the highest percentage of species present were different above and below the 2000 m bathymetric contour, in contrast close to the Azores taxa with the highest percentage of species were the same shallower and deeper than 2000 m. Our results point at turnover in benthic bathyal fauna between the Charlie-Gibbs Fracture Zone and the Azores, which could be related to the presence of the Sub-Polar Front.