The reduction of harmful algae on Caribbean coral reefs through the reintroduction of a keystone herbivore, the long‐spined sea urchin <scp> <i>Diadema antillarum</i> </scp>

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