Role of Astrophorina sponges (Demospongiae) in food-web interactions at the Flemish Cap (NW Atlantic)

Deep-sea sponges are important contributors to carbon and nitrogen cycling due to their large filtration capacity. Species of the suborder Astrophorina form dense sponge grounds in the North Atlantic, where they serve as prey for spongivores, but also have non-trophic interactions with commensal epi- and endobionts. At the Flemish Cap (NW Atlantic) Astrophorina sponges are present in four previously described deep-sea epifaunal assemblages: the deep-sea coral assemblage, the lower slope assemblages 1 and 2, and the deep-sea sponge assemblage. To investigate their role in trophic and non-trophic interactions at the Flemish Cap, we developed trophic-/ non-trophic interaction web models for each of the four faunal assemblages using the published literature. By excluding the sponges from the models, we estimated how many trophic, facultative and obligatory non-trophic links would be lost, and how this removal affected food-web properties (number of compartments, links, link density, and connectance). Astrophorina sponges were mostly linked via facultative non-trophic links to 60, 59, 86, and 92 compartments in the deep-sea coral, the lower slope 1 and 2, and the deep-sea sponge assemblages, respectively. Direct trophic links only existed to Echinasteridae and Pterasteridae. As removing Astrophorina sponges from the interaction webs of the different assemblages had the highest impact on food-web properties compared to removing any other fauna present, these sponges were considered "highest impact taxa". They were also identified, along with sea pens, as "structural species"/ "habitat formers" and "foundation species" based on non-trophic interactions in the deep-sea coral and deep-sea sponge assemblages.