Size-dependent differences in the photophysiology of the reef coral Porites astreoides

The recruitment and survival of juvenile corals is central to the maintenance of coral reef communities and the repopulation of denuded reef substrata. Although it is widely accepted that the mortality of scleractinians is inversely proportional to size, the biotic and abiotic factors that drive this trend remain unclear. Here we measure the mortality of corals on the reefs of St. John (U.S. Virgin Islands) to demonstrate that small corals are more likely to die than their larger counterparts, and we explore whether the photophysiological performance of juveniles in two size classes can provide some insight into why smaller corals are so vulnerable. To evaluate photophysiological performance, we examined chlorophyll fluorescence in two size classes (mean diameters 15 mm and 45 mm) of juvenile colonies of Porites astreoides exposed for short periods to ambient and elevated temperatures. Our results show that the photophysiology of these size classes differs under ambient conditions, with dark-adapted quantum yield (F-v/F-m) being significantly higher in smaller compared to bigger juveniles. As expected, the photophysiology of both size classes is negatively impacted by thermal stress, and although size-related trends are evident in our data, the interaction between size and temperature is not statistically significant. Thus, while there is size dependency in the photophysiological performance of juvenile colonies of P. astreoides, the link between this aspect of scleractinian biology and the higher mortality of small juvenile corals in the face of high thermal stress remains unclear.