Functional Traits and Community Assembly in Deep-Sea Octocorals Along a Depth Gradient in the Northern Gulf of Mexico

Deep-water corals represent abundant and highly diverse taxa with important functional and structural roles. Climate change can impact these ecological roles by altering coral community composition as response to changes in temperature, seawater chemistry, and food supply among other factors. Our aim is to understand processes of community assembly by integrating species’ traits and environmental information into an evolutionary context. Particularly we examined whether depth and the factors that vary with it are important mechanisms in structuring deep-sea octocoral assemblages in the Gulf of Mexico. Collections were conducted on hardbottom from 250–2500 m depth across 27 sites using remotely operated vehicles. A total of 188 colonies spanning 54 different species where sampled from which 11 morphological traits were measured. The ensuing species-by-traits matrix was used as the basis for multivariate analyses performed on three different depth categories: 250–800 m, 800–1100 m, and 1100–2500 m. Principal coordinates analyses revealed that the traits of the octocoral community in the Gulf of Mexico segregate according to depth, where the first two components explained 79.8% of the variation in species’ traits. Axis type (calcified - non-calcified), polyp shape and polyp retraction were highly correlated with PCo1, while polyp density, polyp arrangement (solitary - whorls), and type of sclerites were highly correlated with PCo2. Permutation tests showed statistical differences between depths (pseudo-F_2,108=4.84, p<0.01), where the shallowest assemblage differed from the deepest one. Polyp size and inter-polyp distance showed significant positive relationships with depth, with higher variability in shallower communities, which highlight the labile nature of these traits. Functional diversity was higher in the shallowest and deepest depth zones; however, there was no significant difference (F_2,32=1.33 p=0.27), suggesting that a wide range of traits are important in resource use and interacting with abiotic factors at the different depths. These results highlight the importance of including functional traits when attempting to make predictions of assembly mechanisms as well as for future responses of this significant taxonomic group as climate and ocean change progress.