Potential drivers of pocilloporid coral extirpations in Singapore

The reason why four out of five historically recorded pocilloporid species in Singapore went extinct remains unclear. However, potential causes include urbanization-related stressors such as sedimentation and low light. In this study, we conducted two ex-situ experiments to examine the effects of light limitation and sediment load on the survival and health of two extirpated (Stylophora pistillata, and Seriatopora hystrix), one extant (Pocillopora acuta), and one regional (Pocillopora meandrina) pocilloporid species. All were able to photoacclimate to high sedimentation and low light conditions. However, P. acuta and Se. hystrix exhibited reduced growth under low light, and mortality was significantly higher under increased sedimentation, especially for St. pistillata. While our results indicate that sedimentation and low light characteristic in Singapore's urban reefs are unlikely to be the sole drivers of pocilloporid extirpations, these variables probably contributed to the overall stress burden, pushing already uncommon species into functional, and then actual, extinction.