Interspecies And Spatial Diversity Of Symbiodinium Density In Coral Species From The Huangyan Island,and Its Ecological Significance

The Symbiodinium density is the key physiological indicator of coral bleaching.In this study,260 samples across 63 common coral species were collected in May 2015 at fringing reefs in the Huangyan Island(15°08'N?117°45'E),South China Sea(SCS),and their Symbiodinium density were measured.The main conclusions are as follows:1)The mean Symbiodinium density in corals in Huangyan Island was 3.16×106 cells/cm,varied from 0.76×106 to 7.19×106 cells/cm2.Symbiodinium densities were subjected to significant interspecies differences,with branching corals having a lower algal density(2.28±0.73 ×106 cells/cm2)than massive species(4.06±2.51 x106 cell/cm2);(2)Symbiodinium densities were also subjected to significant spatial differences.The mean Symbiodinium densities of the outer slope corals within 10 m depth(3.42±2.93 ×106 cells/cm2)were higher when compared to corals within 10-15 m depth(2.65± 1.31 ×106 cells/cm2),and the mean Symbiodinium densities of the lagoon corals at 5 m depth(1.95±1.13×106 cells/cm2)were higher when compared to corals at 2 m depth(2.988±1.90 ×1O6 cells/cm2);(3)The mean Symbiodinium densities of corals at the outer slope(3.25±2.67× 106 cells/cm2)was significantly higher than that in the lagoon(2.30±1.57 ×106cells/cm2)in the Huangyan Island;(4)The mean Symbiodinium density of corals in Huangyan Island was significantly higher when compared to the corals in other reefs in the SCS,such as the Nansha islands and the Sanya Bay.Although mass coral bleaching had been occurring' due to the strong E1 Nino in 2015,the Symbiodinium densities of corals in Huangyan Island still kept in a normal level and no bleaching was found when the sea surface temperature(SST)was as high as 31?,which suggested that corals in Huangyan Island with high levels of Symbimbiodinium density were more inclined to resist thermal stress.Therefore,Huangyan Island is likely to be an ideal growth area of corals in the SCS in the context of global warming.