Role of light intensity and spectral quality in coral settlement: implications for depth-dependent settlement?

On coral reefs scleractinian corals show strong patterns of vertical zonation, yet the underlying mechanisms creating and maintaining vertical zonation are poorly understood. Here we examine the potential contribution of light-dependent settlement in scleractinian coral planulae to patterns of vertical zonation. The effect of intensity and spectral quality of light on the settlement of six species of scleractinian corals (Goniastrea favulus Dana, Goniastrea aspera Verrill, Acropora tenuis Dana, Oxypora lacera Verrill, Montipora peltiformis Bernard, and Platygyra daedalea Ellis and Solander) with contrasting depth distributions was examined in laboratory trials. Light-dependent settlement was shown by planulae from five of the six species examined. Planulae from individual species showed a response to either light quality or light quantity, but not both. Settlement patterns shown by planulae from all six species were consistent with the vertical distribution patterns of adults in the field. Settlement of planulae from con-generic species with similar adult distribution patterns did not respond to variation in light intensity or spectral quality in a uniform manner, indicating the optimal light environment for settlement is species specific. The settlement patterns shown by planulae from five of the six species examined were more complex than required for selection of cryptic or exposed micro-habitats at settlement. The ecological function of such complex responses to light at settlement may be to identify optimum habitats for adult survival.