Observations on reef coral undermining by the Caribbean excavating sponge Cliona delitrix (Demospongiae, Hadromerida)

Sponges which simultaneously encrust and excavate calcareous substratum are strong space competitors in coral reefs, actively undermining and displacing live coral tissue. On Caribbean reefs, Cliona delitrix colonizes massive corals, encrusting, deeply excavating and aggressively killing entire coral heads. To establish the details of the process of colonization, excavation, undermining and death of corals by this sponge, we carried out observations on sponge-colonized corals at San Andrés Island (SW Caribbean Colombia), and obtained samples for microscopical observation. As it spreads sideward, C. delitrix removed the upper few mm of the coral skeleton, maintaining its surface slightly lower than the surrounding coral, following the curved outline of the coral head. Internal excavation resulted in a solid outer supporting frame and a strongly eroded lace-like internal network. The outer frame was perforated below inhalant papillae by narrow vertical tunnels and below the large oscules by wide and deep spaces. A band of dying or dead coral surrounded the sponge. The sponge sent out a front of tissue using pioneering filaments projecting underneath the coral polyps. Long filaments may surface farther off, forming new bodies that later fuse. From microscopical observations, physical detachment of polyps was ruled out as the cause of coral death, because coral tissue displacement occurred before significant erosion of the polypar skeletal support had taken place. When sponge tissue reached underneath coral tissue, the latter remained healthy when still separated by thin skeletal barriers, but appeared as debris when barriers were broken. Live sponge and coral tissue could occur in direct contact, in which case there was accumulation of granulous cells in the coral tissue. We hypothesize that the mechanism of coral death involves close-range tissue, cell and/or biochemical interactions rather than fluid- or mucus-borne allelochemicals.