Habitat differentiation in the early life stages of simultaneously mass-spawning corals

The settlement process of coral larvae following simultaneous mass-spawning remains poorly understood, particularly in terms of population and community parameters. Here, the larval settlement patterns of Acropora corals, which are the most diverse genera of scleractinian corals at the species (haplotype) level, were investigated within a single subtropical reef. Across a 4-year period (2007–2010), the mitochondrial and nuclear molecular markers of 1,073 larval settlers were analyzed. Of the 11 dominant haplotypes of recruited populations, nine exhibited non-random patterns of settlement distribution. This result suggests that the actual habitat segregation starts during the early swimming larval stages of their life history, rather than by natural selection after random settlement. In addition, the presence of a depth-related settlement pattern supports that species-specific vertical zonation of coral larvae may play a role in the establishment of habitat segregation. Moreover, in some species that showed a preference toward the shoreward area of the bay, the settlement pattern was consistent with that of the adult distribution. This result indicates that the gametes were not mixed between fore and back reefs in the period from fertilization to settlement during the mass-spawning event, even within a single small reef. Another compatible hypothesis of this pattern is that the larvae are able to recognize various types of environmental information, facilitating the selection of optimal micro-habitats. Overall, Acropora coral larvae that are produced from a simultaneous mass-spawning event may have adapted to complex reef topography by means of multi-step habitat selection at settlement, corresponding to different spatial scales.