Density and Bleaching of Corals and Their Relationship to the Coral Symbiotic Community

Abstract

Corals provide an important habitat for diverse fauna. The habitat is especially rich in branching species. Each colony harbors several species and dozens of individuals, forming an integrated symbiotic community. Factors affecting the diversity and abundance of coral symbionts are poorly understood. The present study tested experimentally the relationship between the population density of coral Pocillopora verrucosa and the diversity and abundance of symbionts in planted coral fragments exposed for three months. Colony size and bleaching were also considered as two additional factors. We demonstrated that, even at the early stage of colony formation, the diversity of obligate symbionts on planted corals in Nha Trang Bay was higher than the diversity of any other studied local faunas of the world’s ocean. Species richness and the abundance of symbionts were positively related to colony size and density, while no impact of bleaching was found. Species richness and the abundance of obligate symbionts increased with host population density, while in facultative symbionts, these indices decreased. We hypothesized that the negative effect of host population density on species richness and the abundance of facultative symbionts is caused by the competitive pressure of obligates, which increases with their abundance. The reasons for the different responses of obligate and facultative symbionts to host density were considered.