Biodiversity of Spongosorites coralliophaga (Stephens, 1915) on coral rubble at two contrasting cold-water coral reef settings

Cold-water coral reefs (CWRs) in the northeastAtlantic harbor diverse sponge communities. Knowledge ofdeep-sea sponge ecology is limited and this leaves us with afragmented understanding of the ecological roles that spongesplay inCWR ecosystems. We present the first study of faunalbiodiversity associated with the massive demosponge Spongosorites coralliophaga (Stephens, 1915) that typically colonizes coral debris fields of CWRs. Our study focused on thesessile fauna inhabiting sponges mixed with coral rubble attwo contrasting settings in the northeast Atlantic: the shallowinshore (120–190 m water depth) Mingulay Reef Complex(MRC) and the deep offshore (500–1200 m) LogachevMound (LM) coral province. MRC is dominated by thescleractinian Lophelia pertusa, while LM is dominated by L.pertusa and Madrepora oculata. Nine sponge–coral rubbleassociations were collected from MRC and four from LM.Measurements of abundance, species richness, diversity,evenness, dry biomass, and composition of sessile fauna onsponge and coral rubble microhabitats were undertaken.Differences in community composition between the two regions were mainly a response to changes in fauna with depth. Fauna composition was also different between sponge and coral rubble within each region. Infauna constituted a minor component of the sponge-associated fauna inMRC but had a higher contribution in LM. Sponge and coral rubble sessile fauna in both regions was mainly composed of cnidarians and molluscs, similarly to some previous studies. Sponges’ outer surfaces atMRC were colonized by a speciesrich community with high abundance and biomass suggesting that S. coralliophaga at MRC acts as a settlement surface for various organisms but such a role is not the case at LM. This difference in the role of S. coralliophaga as a biologicalstructure is probably related to differences in fauna compositionwith depth, bottom current speed, and the quantity/quality of food supplied to the benthos.