The reduction of harmful algae on Caribbean coral reefs through the reintroduction of a keystone herbivore, the long‐spined sea urchin Diadema antillarum.

Herbivores play an essential role in the health and recovery of a coral reef ecosystem. The lack of recovery of the keystone herbivore Diadema antillarum has had long‐lasting effects, evidenced with many reefs persisting in algal dominance. This study restocked 756 lab‐reared D. antillarum to four coral reefs on the east and south coast of Puerto Rico. Sea urchins were placed in experimental plots ("corrals") for 2 months, and the change in benthic composition was measured. Significant changes in the benthic structure were observed during the first week after the restocking. Significant reductions of fleshy macroalgae (Dictyota spp.) and thick turf algal/sediment mats (TAS), both unsuitable substrates (e.g. coral settlement), contributed to this change. Also, restocked D. antillarum significantly reduced the cover of encrusting red algae, Ramicrusta spp. By the end of the study, the abundance of fleshy macroalgae decreased by a mean of 77% (max of 100%) and Ramicrusta and TAS by 53% (max 71%) and 56% (max 100%), respectively. Clean substrate ("pavement"), crustose coralline algae (CCA), and filamentous turf algae increased between one to two orders of magnitude. The restoration of native sea urchins is a non‐invasive and useful approach to aid in the mitigation of algae, especially potentially dangerous alga like Ramicrusta. The results of this study highlight the importance of herbivores in improving the conditions on coral reefs. [ABSTRACT FROM AUTHOR]