Macroalgae and zoanthids require physical contact to harm corals in Southwestern Atlantic

Space can be limited in reef ecosystems leading to competitive interactions among sessile organisms. Some competitive mechanisms can require physical contact while others operate with proximity between organisms. We investigated how the scleractinian coral Siderastrea stellata and the hydrocoral Millepora alcicornis respond to physical contact and proximity to the macroalga Dictyopteris delicatula and the zoanthid Palythoa caribaeorum, common pairs of interactions in Southwestern Atlantic reefs in Northeast Brazil. We held two colonies of the same species within the same tank, one in physical contact and the other within 5 cm of the competitor for four days with macroalgae and three days with the zoanthid. We monitored the corals for 26 days taking photographs and measuring their photosynthetic efficiency (PE) at the point of contact and on the side where competitors were in proximity. Corals were only affected when physically contacted by the competitors, with the macroalga causing a slight damage on M. alcicornis and almost no effect on S. stellata. Contacts with P. caribaeorum reduced the PE and caused tissue discoloration in both corals. M. alcicornis recovered after 26 days but the contacted areas of S. stellata remained discolored and with low PE. Macroalgae and zoanthids required physical contact to damage corals, with the zoanthid causing more damage than the macroalgae. These results suggest that if local and global stressors make corals weaker competitors (e.g. warming) or favor macroalgae and zoanthid growth (e.g. nutrients), corals could be replaced by either of these organisms affecting ecosystem structure and functioning.