Relative Contributions of Size and Shape to Coral Demography

AbstractIt has been 40 years since T. P. Hughes put forward the idea that the size of modular corals is a better predictor of demographic fates than age. However, colonies of similar size may exhibit different shapes, and shape holds great ecological and evolutionary significance. This study used orthomosaics of coral reefs to track changes in 796 Pocillopora acuta colonies in Kenting National Park, Taiwan, over 2 years. We quantified relationships between coral demographic fates and three morphological traits: planar area (size), circularity (shape), and perimeter-to-area ratio, which integrates size and shape. Together, area and circularity consistently explained the most variation for all modular processes except shrinkage, which was explained best by area alone. Including circularity with area significantly improved the capacity to predict survival and fission, with large and circular colonies surviving better and with large and irregular colonies more prone to fission. Circularity also improved predictions of proportional area change, with smaller circular colonies experiencing higher rates of change. Fusion was unrelated to any single morphological trait, presumably because it relies on proximity in space. Perimeter-to-area ratio is the best single trait for survival prediction. Our results highlight that size and shape should both be considered for the demographic modeling of modular organisms.