The specific characteristics of seamounts frequently promote the occurrence of abundant megabenthic fauna as cold-water corals and sponges, which are considered as Vulnerable Marine Ecosystems (VMEs). The deep-sea megabenthic fauna of the Ormonde seamount (NE Atlantic, Portugal) has been described and quantitatively investigated for the first time. Four underwater video transects, performed by means of a Remotely Operated Vehicle (ROV), from depths between 660–1960 m were analysed. This study revealed the occurrence of several VMEs on the seamount, mainly characterized by dense sponge aggregations. The composition of megabenthic communities display a bathymetric gradient in its distribution, with four communities identified and influenced by different water masses and substrate type. The highest occurrence of sponges was found between 700-1200 m depth under the influence of the Mediterranean Outflow Water on rocky substrates. Cnidarians were mostly present on soft substrates within the same water mass, and also within the North Atlantic Deep Water. The replacement in species composition dominates along the depth gradient and flanks of the seamount. This study reveals the high biodiversity of Ormonde seamount, being a hotspot for deep-sea sponge aggregations. The results contribute to increase the current knowledge on the North Atlantic deep-sea ecosystems and to develop efficient conservation and management measures to protect these VMEs.
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