Distribution and diversity of deep-sea sponge grounds on the Rosemary Bank Seamount, NE Atlantic

A towed camera survey revealed extensive sponge grounds on the deep slopes of the Rosemary Bank Seamount in the NE Atlantic. An Agassiz trawl deployed in the same area yielded samples for taxonomic validation and comparison to the visual survey. The sponge grounds were observed between 1200 and 1440 m depth. Eight species were identified in the trawl samples from five genera: Geodia, Thenea, Pheronema, Aphrocallistes and Craniella. In addition, there were 2 unresolved species of Craniella, one of Hexadella and 2 other unidentifiable Porifera species. Seven taxa were consistently identified from the video footage. Craniella longipilis was numerically dominant across all depths, but other species showed significant change in abundance with depth. At shallower depths Pheronema carpenteri was more prevalent, whereas the encrusting species Hexadella sp. increased in frequency with depth. Although all species have been previously reported from the North Atlantic, community composition at Rosemary Bank appears to be more diverse than other regions. Estimates of the number of sponges within the surveyed area suggest around 88 million sponges may be present. The community appears to be largely in pristine condition and the recent designation of Rosemary Bank as a nature conservation MPA should help ensure it remains so.