Competition and epibiosis by the sponge Pseudoceratina purpurea (Carter, 1880) on scleractinian corals at a tourism hotspot in the Gulf of Thailand

Competition for space on coral reefs is fierce, however when external natural and anthropogenic drivers disproportionately benefit certain organisms, reefs can quickly be conquered. Sponges in particular, have been shown to be competitive benthic colonisers and incidences of their epibiosis on scleractinian corals in the Indo-Pacific have been growing. Here we document the competitive capacity, frequency, and overgrowth interactions of the sponge Pseudoceratina purpurea on the island of Koh Tao, Gulf of Thailand. A comparison between four chosen sites revealed remarkable variability in the extent of sponge competition with two sites showing high incidences of P. purpurea epibiosis and two others showing low incidences. A total of 303 coral colonies were found to be overgrown by the sponge, with indications that mushroom corals and Pavona spp. may be disproportionally affected compared to other corals. Smaller P. purpurea individuals were found most often to colonise​ rock and larger individuals most often covering mushroom corals. Individual size and spatial distribution of the sponge suggest highly localised factors determine growth of both individuals and populations as a whole. No records of extensive overgrowth of any pseudoceratinid sponge is currently known from elsewhere, making this study a novel baseline for the Gulf of Thailand.