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

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 <20 mm height. However, octocoral regrew to infill the excised portion of the colony at a mean rate of 0.23 mm ± 0.01SE day⁻¹, which was routinely trimmed to maintain original plot dimensions. Competition for space between octocoral and macroalgae was also evident when recovered macroalgae overhung octocoral barriers and scoured surrounding octocoral. Results demonstrate the existence of natural biological barriers to grazing urchins, which can be utilized to efficiently replicate experimental exclusions, taking only ~10 min to construct per plot. While bio-chemical/physical mechanism(s) of the barrier were not determined, the potential for such biological barriers to form important local refugia for macroalgae and applications over larger scales remains largely unexplored.