Density-associated recruitment in octocoral communities in St. John, US Virgin Islands

To evaluate the possibility that density-associated effects modulate octocoral abundance on a Caribbean coral reef, we tested the hypothesis that the density of octocoral recruits (colonies ≤ 4 cm tall) and adult colonies are positively associated on shallow reefs (≤ 14 m depth) in St. John, US Virgin Islands. Both life stages were censused for density at 8-10 sites along 7 km of shore in 2013 and 2014, and a correlative approach was used to evaluate the extent to which the densities were associated using sites as replicates. For 8 sites censused in both years, mean densities of adults (pooled among taxa) varied from 2.95 ± 1.16 colonies m− 2 to 20.60 ± 2.62 colonies m− 2 in 2013, and from 3.20 ± 0.75 colonies m− 2 to 13.00 ± 1.04 colonies m− 2 in 2014; for recruits, mean densities varied from 1.05 ± 0.34 colonies m− 2 to 4.25 ± 0.81 colonies m− 2 in 2013, and from 0.60 ± 0.31 colonies m− 2 to 1.44 ± 0.40 colonies m− 2 in 2014 (all ± SE). The most common taxa in both years among all sites were Antillogorgia spp., Gorgonia spp., and plexaurids. Density of recruits was significantly and positively correlated with population density of adult octocorals (pooled among taxa) and plexaurids in both years, and for Gorgonia spp. in 2013 (with a similar trend in 2014). Densities of recruits and adults of Antillogorgia spp. were not associated in either year. Together, these data suggest that densities of adult octocorals positively influence the density of co-occurring octocoral recruits, thereby potentially promoting population growth.