Small-scale distribution of Great Barrier Reef bioeroding sponges in shallow water

Data on bioeroding sponge ecological distributions are sparse despite their important role in tropical reef erosion. Difficulties with in situ species identification and estimation of biomass have impeded field studies and remain a problem. An extensive transect study has been conducted at Orpheus Island, Central Great Barrier Reef. Encrusting Cliona orientalis, Aka mucosa and Zyzzya criceta were identified in the field, other sponges were grouped according to colour and papillar diameter. The sponges were not host-specific and frequencies per substrate type varied. Encrusting C. orientalis was almost restricted to massive coral substrates (> 90 %), whereas all brown papillate sponges were also abundant in branching corals. The latter encompass papillate C. orientalis, Pione vastifica, Pione caesia and other species. Only A. mucosa and C. orientalis occurred adjacent to live coral. C. orientalis harbours photoactive zooxanthellae and requires high irradiation levels. A. mucosa has specialised in living in substrate buried in sediment, erect fistules reaching into the water. A. mucosa persists on the sandy zone, where no other species was successful. The entire species composition changed across the reef. Species richness increased with substrate availability, and biomass was highest at the reef edge, where water turbulence is most pronounced.