Octocoral barrier to grazing sea urchins allows macroalgal recovery on barrens ground

Abstract Experimental exclusion of marine herbivores is a powerful approach to reveal their effects on the structure and function of ecosystems. However, such manipulations can be logistically challenging, particularly for subtidal reef systems. Here we report novel observations of a natural barrier to grazing sea urchins formed by the octocoral Erythropodium hicksoni (Family: Anthothelidae, Utinomi, 1971), which allowed localised growth of standing macroalgae on barrens ground despite sea urchin (Heliocidaris erythrogramma) densities locally exceeding 40 individuals m−2. Effectiveness of the octocoral barrier was experimentally tested by cutting 200 mm by 200 mm plots within the centre of octocoral colonies (providing clean reef surfaces completely enclosed by octocoral), which were otherwise surrounded by sea urchin barrens. Within 1 month, macroalgae completely colonised reef surfaces inside octocoral plots, which continued to proliferate over 12 months relative to background barren ground plots. Furthermore, a procedurally-controlled experiment (whereby barren control plots were also scraped clean of all biota to the same dimensions as octocoral plots), revealed octocoral-barriers to again exclude urchins and enable greater recovery of macroalgae relative to the procedural-control. In all octocoral plots, algae grew to the octocoral edge, indicating a lack of allelopathic/ shading effects by octocoral, which forms a thin encrusting veneer over reef surfaces