The Ecological Role And Environmental Implications Of Sea Urchin At The Luhuitou Fringing Reef

Sea urchin is an important member of benthic zones of coral reefs,playing a dual role in coral reef ecology.On the one hand,by controlling algal overgrowth,sea urchins provide space for coral development and juvenile coral recruitment,thereby maintaining the health and stability of coral reef.On the other hand,high sea urchin population densities contribute to coral reef degeneration,by hindering adult coral development and reducing juvenile coral abundance.However,research in this field has focused mainly on Caribbean reefs,while the South China Sea,the largest marginal sea in the western Pacific Ocean,has largely been overlooked.Luhuitou fringing reef,located on the north coast of the South China Sea,has experienced severe degradation as a result of anthropogenic activities.Although sea urchin is the dominant species in the benthic zone of Luhuitou fringing reef,its ecological characteristics and ecological role in this ecosystem have not been studied to date.Consequently,in this study,we investigated sea urchin densities and community structure,coral cover,juvenile coral densities,algal community composition,and algal cover to estimate the ecological role of the sea urchin in Luhuitou fringing reef.Additionally,we analyzed the gut contents and gut bacterial community composition of sea urchin and measured the concentrations of polycyclic aromatic hydrocarbons?PAHs?and organophosphorus esters?OPEs?.The aims were to discuss the ecological characteristics of the sea urchin and its environmental implications for the Luhuitou fringing reef ecosystem.The main conclusions of our study were as follows:?1?Five sea urchin species were identified in Luhuitou fringing reef,including Diadema setosum,Diadema savignyi,Stomopneustes variolaris,Echinothrix calamaris,and Tripneustes gratilla.Among them,D.setosum was the most abundant and only species in the counting zone,with a mean density was 0.39±0.12individuals m^-2,similar to the medium density for coral reefs worldwide.Coral and sea urchin abundance exhibited a significant positive relationship,suggesting that the sea urchin could maintain coral development and facilitate juvenile coral recruitment at Luhuitou fringing reef.?2?The gut contents of T.gratilla consisted almost entirely of macroalgal fragments,with no evidence of coral tissue or its calcium carbonate skeleton,while those of the other four sea urchin species were dominated by coral and their calcium carbonate skeleton,turf algae,filamentous algae,silt,and calcareous algae.These results indicated that D.setosum,D.savignyi,S.varilaris,and E.calamaris were bioeroders,whereas T.gratilla was not.The bioerosion rate for D.setosum was 1.68±0.78 g CaCO₃ individual^-1 day^-1,while for the total sea urchin population in Luhuitou fringing reef,the bioerosion rate was 120.4±55.6 g CaCO₃ m^-2 yr^-1.?3?The gut bacterial communities of sea urchins in Luhuitou fringing reef were dominated by three bacterial phyla,namely,Fusobacteria,Proteobacteria,and Bacteroidetes,while Propionigenium?49.09±4.79%?,Prolixibacter?10.48±3.92%?,and Photobacterium?7.17±3.92%?were the most abundant bacterial genera.Because Fusobacteria have been reported to favor stressful host conditions and propionate,a fermentation metabolite of Photobacterium,has been found to exhibit health-promoting effects,we inferred that sea urchin in Luhuitou fringing reef might be experiencing some type of stress.?4?The gut bacterial composition of five sea urchin species in Luhuitou fringing reef was markedly different from those previously reported for sea urchins.Therefore,we inferred that Luhuitou fringing reef was not a suitable habitat for sea urchins due to its severely degraded environment.The stress experienced by sea urchins in Luhuitou fringing reef may be primarily due to a lack of appropriate shelter,heavy sediment input.?5?A total of 16 PAHs and 11 OPEs were detected among all sea urchin samples from Luhuitou fringing reef,with mean total concentrations of 656±195 ng g^-1 dry weight and 2036±653 ng g^-1 lipid weight,respectively;the mean total PAH concentration in our study was at a medium level in coral reef worldwide,indicating that PAHs contributed chemical population to Luhuitou fringing reef.?6?Given the positive effect of sea urchin on the maintenance of coral development and juvenile coral recruitment,the control effect of sea urchin on turf algae cover,their erosion effect on coral skeletons,and the low densities of sea urchin,our study inferred that the severely degraded environment in Luhuitou fringing reef prevented the positive effect of sea urchin,and sea urchin could not control algal overgrowth,and thus contributed little to the resilience of Luhuitou fringing reef.