Coral larval metamorphosis, but not attachment, is influenced by high natural light intensity

The coral larval settlement process is a pivotal phase in the life cycle of reef-building corals, significantly affecting the structure and resilience of coral assemblages and reef ecosystems. It involves two distinct phases: 1) the descent of competent motile larvae to actively search and test substrates and initially attach; and 2) metamorphosis involving extensive tissue transformation into a polyp as it settles and initiates the sessile benthic phase of coral lifecycles. The precise effects of various biological and physico-chemical cues on these two stages of coral settlement are not well understood. This study investigated the effect of natural light on the attachment and metamorphosis phases of settlement among Acropora millepora coral larvae every three hours for 51 hours. A multinomial regression model of the different larval categories showed a strong positive effect of light intensity on the metamorphosis probability but not on attachment probability. The results suggest that light affects attachment and larval metamorphosis phases differently. This study provides new perspectives and insights into the significant role of light in the success of coral larval settlement and advances our understanding of the complex early life history of corals. These insights can assist development of effective strategies in coral conservation efforts in the face of global environmental change.