Effects of exposure, depth and aspect on sponge communities on a coral reef

Coral reef benthic communities include a wide range of taxa, but most attention has been given to hard coral assemblages, and how their cover and composition vary over strong spatial gradients. Much less is known about the spatial distribution and composition of coral reef sponge communities, which may become increasingly important on reefs with declining coral cover. Here, we examined the effects of exposure, depth, aspect and location on the cover and composition of sponge assemblages on a coral reef in Kimbe Bay, Papua New Guinea. We quantified sponge cover and species composition along replicate line transects on 6 inshore reefs, sampling exposed (seaward) and sheltered (landward) sides of reefs at 5, 10 and 15 m depth, with reef aspect subdivided into slopes or walls along each transect. Although the substratum was generally dominated by corals and algae, sponges ranked 3rd, with an average of 13.1% cover, including 63 recognisable species. Morphologically there were 38 encrusting, 21 erect and 4 massive sponge species, with the encrusting sponges Lamellodysidea cf. chlorea and Dysidea sp1 exhibiting the highest cover. Sponge cover, species richness and species composition all exhibited complex interactions among depth, exposure and location. Sponge cover and species richness increased in transects with higher percentages of wall aspects, and assemblage structure differed between slopes and walls. Sponges are a diverse component of the benthos, with exposure, depth and reef aspect all contributing to explain spatial variation in assemblage structure.