Living on the edge: the sponge fauna of Australia’s southwestern and northwestern deep continental margin

This first assessment of sponges on Australia’s deep western continental margin (100–1,100 m) found that highly species-rich sponge assemblages dominate the megabenthic invertebrate biomass in both southwestern (86%) and northwestern (35%) areas. The demosponge orders Poecilosclerida, Dictyoceratida, Haplosclerida, and Astrophorida are dominant, while the presence of the order Agelasida, lithistid sponges, and the Verongida are noteworthy in providing contrasts to other studies from the deep temperate Australian margin. Most sponge species appeared to be rare as two-thirds were present in only one or two samples—a finding consistent with studies of the shallow Australian sponge fauna. The Demospongiae and Calcarea had similar distribution and abundance patterns being found in the greatest numbers in the south on the outer shelf and shelf edge in hard substrates. In contrast, the Hexactinellida were more abundant at deeper depths and in soft substrates, and were more common in the north. Although the environmental factors that influence sponge distributions on the western margin cannot be completely understood from the physical covariates analyzed in this study, the data suggest depth-related factors, substrate type, and current regimes are the most influential. Incompletely documented historic demersal trawling may partly account for the lower sponge biomass found in the north. The potentially high importance of sponges to benthic ecosystems, as well as the potential for high impacts on sponges by bottom trawling, indicates that maintaining healthy sponge assemblages should be an important consideration for marine conservation planners. Successful management will need to be under-pinned by additional research that better identifies the ecological roles of sponges, and their distributions over local and broad environmental scales.