Interactions in the canopy among Caribbean reef octocorals

The incidence of octocoral-octocoral interactions and the physical effects of the interactions were determined in surveys of 29 branching octocoral species at 2 sites in St. John, U.S. Virgin Islands. Close proximity, defined as the bases or branches of octocorals being within 5 cm of each other, creates the potential for direct competitive interactions. Cases of close proximity were common (63 and 34% of the colonies at the 2 sites), and were more abundant at the site with higher colony density and proportionately more colonies with tall, highly branched morphologies. Damaged branches associated with neighboring colonies were found in 19% of the colonies in close proximity. Eunicea flexuosa colonies exhibited less damage than the 5 most common species it interacted with (Antillogorgia americana, Eunicea mammosa, Gorgonia ventalina, Pseudoplexaura crucis, and Pseudoplexaura wagenaari). Of the colonies in close proximity, 23% featured asymmetric colony forms, representing the effects of competition through the preemption of canopy space. G. ventalina had the highest incidence of asymmetry. Nearest-neighbor analyses detected a weak but significant signal of competition in the combined sizes of neighboring colonies. Competition among octocorals in the canopy probably has limited effects on their spatial distribution, but tissue damage and altered colony form could affect growth and fitness.