Deep-sea dives reveal an unexpected hexactinellid sponge garden on the Rio Grande Rise (SW Atlantic). A mimicking habitat?

Long overlooked, sponge grounds are now recognised as important habitats which support high biodiversity. Some of these deep-sea sponge aggregations are formed by hexactinellids, in varying densities, which can occur on both soft and hard substrates. Despite their ecological importance as habitat framework building taxa, hexactinellids have been little studied and only 33 species in this Class were reported from the entire South Atlantic until now. Most of this knowledge was gained from studies conducted on the eastern South America continental slope. Here we report a new hexactinellid sponge garden found by the Shinkai 6500 manned submersible on the Rio Grande Rise, a prominent topographic feature of the deep SW Atlantic. This is the first sponge garden worldwide found to be dominated by the stiff, often dichotomously branching, Sarostegia oculata which may occur in densities of over 5 individuals per square meter. This sponge, of uncertain familial allocation, seems to always carry epibiotic zoanthids, thus mimicking the 3D skeletal framework of actual coral gardens