Octocorals have increased in abundance on a number of Caribbean reefs, but this trend has largely been reported with functional group or genus resolution. A species-level analysis of octocoral communities in St. John, US Virgin Islands was conducted to better understand how this taxon will respond to changing conditions based on their synecology at two sites that are 1.5 km apart and differ in physical conditions. East Cabritte is characterized by moderate wave energy, low sedimentation, and clear water, while contrasting conditions characterize Europa Bay. Surveys conducted in 2014 and 2015 showed that the abundance and size of adult octocorals differed between sites, with taller and denser communities at East Cabritte than Europa Bay (mean height of 32 versus 20 cm; 18 versus 8 colonies m−2). Octocoral diversity and evenness were similar between sites, although multivariate octocoral community structure differed between sites regardless of whether octocorals were resolved to genera or species. Genus-resolution masked differences between sites for speciose genera like Eunicea. The broad overlap in species representation at both sites suggests that diversity is less responsive than community structure to differing environmental conditions, perhaps because the ecological niches of these species are broad. With 35 octocoral species, and diversity and abundances comparable to those studied > 40 years ago on shallow Caribbean reefs, the dense octocoral communities appearing on some present-day reefs reflect expanded benthic occupancy by a well-known (rather than a novel) community type.
Compiled abundances of juvenile corals revealed no change over time in the Pacific, but a decline in the Caribbean. Using these analyses as a rationale, we explored recruitment and post-settlement success in determining coral cover using studies in the Caribbean (St John, Bonaire) and Pacific (Moorea, Okinawa). Juvenile corals, coral recruits, and coral cover have been censused in these locations for years, and the ratio of juvenile (J) to recruiting (R) corals was used to measure post-settlement success. In St John and Bonaire, coral cover was stable but different between studies, with the ratio of the density of juveniles to density of recruits (J : R) ~0.10; in Moorea, declines in coral cover were followed by recovery that was related to the density of juvenile corals 3 years before, with J : R ~0.40; and in Okinawa, a decline in coral cover in 1998 was followed by a slow recovery with J/R ~0.01. Coral cover was associated positively with juvenile corals in St John, and in Okinawa, the density of juvenile corals was associated positively with recruits the year before. J : R varied among studies, and standardised densities of juvenile corals declined in the Caribbean, but increased in the Pacific. These results suggest that differences in the post-settlement success may drive variation in coral community structure.