The role of arborescent octocorals as ecosystem engineers in shaping the diversity and composition of shelf-dwelling benthic assemblages.

Biodiversity plays a key role in regulating ecosystem functioning, thus controlling ecosystem stability. One of the main variables known to affect species diversity is habitat heterogeneity, which can be generated both by geological and biological structures. In this sense, autogenic ecosystem engineers, such as scleractinian corals, are known to enhance biodiversity at local scales. Although there is convincing evidence suggesting a pattern of increased faunistic diversity should be observed in arborescent octocoral (previously referred as gorgonians) aggregations, little empirical data is currently available. In this study, we quantitatively evaluated through underwater video analyses how octocoral density can influence the diversity and taxonomic composition of benthic habitats from continental shelves. Seafloor images were recorded along the continental shelves of two extremely contrasting regions: the NW Mediterranean Sea and Antarctica. Octocoral density was positively correlated with species richness and diversity, likely shaping the taxonomic composition of the invertebrate assemblages in both regions. The effects produced by octocoral density on the surrounding fauna probably derive from an increase in predator-free spaces, habitat amelioration and spatial heterogeneity produced by the three-dimensional structures generated by these organisms. Moreover, dense coral gardens likely promote the settlement of many species that would otherwise not be present, considering the homogeneity of sedimentary shelf environments. Species richness and diversity steadily increased until the octocoral population reached a density threshold (i.e., ~10 col. m–2), beyond which these values remained stable. These results can have implications for the management of continental shelf marine protected areas and be relevant when planning restoration actions focussed on the transplantation of arborescent octocorals.