Role of post-settlement mortality in the establishment of Acropora reef slope zonation in Ishigaki Island, Japan

Abstract

The relative contributions of larval recruitment and post-settlement mortality to the establishment of the zonation of Acropora communities, a dominant group in reef corals, remain unknown. Our field research revealed that the relative abundances of juveniles between two morphological categories of acroporid corals were different on shallow and deep slopes: tabular species dominated in shallow waters (1.5-2.5m), whereas arborescent species dominated in deep waters (5.5-11m). These abundances were consistent with the distributional patterns of the adults, suggesting that the zonation structure is formed during the initial life stages of settlement and early post-settlement periods. To determine to what extent Acropora zonation is determined by depth-related differences in initial mortality among species, a transplant experiment using artificially settled corals (10 days after settlement) of four dominant species (the shallow-dominants Acropora digitifera and A. selago and the deep-dominants A. muricata and A. tenuis) was performed at four depths (1.5, 2.5, 5.5, and 11m). Survivorship of all four species was lower at 5.5m than the other depths and that the survival rate of A. muricata was much higher than that of other species at all depths. However, there was no species-specific survival related to adaptation to a particular habitat (i.e., depth), implying that Acropora spats do not possess species-specific tolerance for the environmental stresses of their own particular habitat, at least during the first 6 months after settlement. Post-settlement survival rate during this period is therefore not a major factor in forming the vertical zonation pattern of these Acropora species in subtropical reefs around Japan.