Long-term effects of competition on coral growth and sweeper tentacle development

Outcomes of competition between corals vary temporally and spatially, and depend in part on the agonistic mechanism involved. Competition may impact coral growth, reproduction and energy reserves, however few experimental studies have quantified these effects. We conducted a 1 yr laboratory experiment on competition between 2 massive corals, Platygyra daedalea and Favites complanata. Colonies of P. daedalea developed sweeper tentacles and extensively damaged the F. complanata colonies, causing them to loose ca. 55% of their soft tissue and eventually killing 30% of F. complanata colonies. Skeletal accretion rate varied widely among corals within each treatment, and correlated negatively with the percent tissue damaged on competing colonies of F. complanata. On isolated control colonies, sweeper tentacles developed randomly throughout the year, and then reverted back to feeding tentacles. They appeared to serve as probes to detect the approach of competitors. Development of sweeper tentacles is a powerful aggressive/defensive mechanism that may enable brain corals to dominate some reef areas in the Indo-Pacific region.