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

Abstract

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 4 previously described deep-sea epifaunal assemblages: the deep-sea coral assemblage, 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 and non-trophic interaction web models for each of the 4 faunal assemblages using the published literature. By excluding the sponges from the models, we estimated how many trophic, and facultative and obligatory non-trophic, interactions 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 interactions to 59, 58, 84, and 90 compartments in the deep-sea coral, the lower slope 1 and 2, and the deep-sea sponge assemblages, respectively. Direct trophic interactions only existed with Syllidae, Echinasteridae, and Pterasteridae. Astrophorina sponges were considered highest impact taxa in all faunal assemblages and, together with sea pens, they were identified as structural species/habitat formers and foundation species in the deep-sea coral and deep-sea sponge habitat. Hence, even less abundant, or non-representative (indicator), species can be important for food-web integrity via trophic and non-trophic interactions.