Coral recruitment: consequences of settlement choice for early growth and survivorship in two scleractinians

Settlement, early survivorship and growth of two species of reef-building corals were studied in order to assess the importance of these processes in determining the distribution of adult corals. Experiments were conducted using larvae of Platygyra sinensis (Edwards and Haime), a shallow water species with a massive morphology, and Oxypora lacera (Verrill), a shade-loving or deep water species with a laminar morphology. Settlement orientations in experimental field enclosures varied according to depth, Oxypora choosing more cryptic microhabitats than Platygyra. Laboratory settlement experiments utilising different light intensities indicated that light level was the variable responsible for changes in settlement orientation with depth. Total numbers of larvae settled at a range of depths did not correspond with adult distribution for either species, though there were significant variations in settlement with depth. For juveniles of each species, variations in growth and survivorship with depth were inconsistent with adult distributions. These results, in combination with the settlement experiments, suggest that pre-settlement factors may play an important role in determining reef-scale distribution patterns. Settlement orientation on experimental substrata influenced growth rates, which were highly correlated with survivorship. In the first few months after settlement, mortality was highest on highly sedimented upper surfaces. However, growth and survivorship were highest on these upper surfaces during the following 5 months, presumably because of higher light levels there than on undersurfaces. There was no correlation between settlement density on experimental surfaces and subsequent survivorship. This may be due to the fact that the susceptibility of newly settled corals to different sources of mortality changes over time. No single settlement orientation provided a means to optimise survivorship through the course of these changes.