The rise of octocoral forests on Caribbean reefs

Coral reefs throughout the tropics have experienced large declines in the abundance of scleractinian corals over the last few decades, and some reefs are becoming functionally dominated by animal taxa other than scleractinians. This phenomenon is striking on many shallow reefs in the tropical western Atlantic, where arborescent octocorals now are numerically and functionally dominant. Octocorals are one of several taxa that have been overlooked for decades in analyses of coral reef community dynamics, and our understanding of why octocorals are favoured (whereas scleractinians are not) on some modern reefs, and how they will affect the function of future reef communities, is not commensurate with the task of scientifically responding to the coral reef crisis. We summarize the biological and ecological features predisposing octocorals for success under contemporary conditions, and focus on those features that could have generated resistance and resilience of octocoral populations to environmental change on modern reefs. There is a rich set of opportunities for rapid advancement in understanding the factors driving the success of octocorals on modern reefs, but we underscore three lines of inquiry: (1) the functional implications of strongly mixotrophic, polytrophic, and plastic nutrition, (2) the capacity to recruit at high densities and maintain rapid initial rates of vertical growth, and (3) the emergent properties associated with dense animal forests at high colony densities.