Octocorals outcompete scleractinian corals in a degraded reef

Competition among benthic organisms plays an important role in coral population dynamics in degraded reefs. In this study, interspecific interactions between scleractinians and octocorals and larval settlement of scleractinians near selected octocorals in Lucero, a degraded reef in Bolinao, Pangasinan, northwestern Philippines were assessed. Competitive index (CI) was computed based on species interactions (i.e., direct interaction, overgrowth) observed on the reef. The coral settlement near alcyonacean octocorals (i.e., Sarcophyton, Sinularia), blue coral (Heliopora), and substrate without octocorals (control) were monitored by deployment and retrieval of settlement plates on the reef in February and May 2018. Results showed that octocorals were more aggressive towards other species (CI between 1.00 and 0.20). Heliopora was subordinate (CI = -0.10) to octocorals, but relatively competitive than scleractinians. Scleractinian coral settlement is inhibited by octocorals and is reduced near Heliopora. Interspecific interactions indicate that octocorals present in the study site are more tolerant than other species; hence, it might be attributed to its higher competitive ability. Settlement inhibition by octocorals and Heliopora is indicative of the production of allelochemicals, a substance known to deter settlement. Considering the growth advantage of Heliopora over Sarcophyton and Sinularia, it is likely that Heliopora will dominate over scleractinians in a degraded reef of Lucero. However, this necessitates long-term monitoring using permanent quadrats to detect the large-scale effects of competition for space on the community structure of disturbed reefs.