Interactions between morphological and physiological plasticity optimize energy acquisition in corals

Morphological plasticity in response to environmental heterogeneity may be performance enhancing or may simply result from an intrinsic instability in morphology during development. Although patterns of morphological change are well documented for numerous taxa, it is often unclear whether this plasticity enhances the performance of organisms in the habitat to which they have acclimatized. Reef-building corals are an ideal model system in which to investigate this question. We here develop a three-dimensional geometric model and present a comprehensive photosynthesis data set with experimentally calibrated photosynthesis models that predicts energy acquisition by foliose corals as a function of colony shape. This allows us to assess the extent to which changes in colony morphology along an environmental gradient track the predicted optimal colony morphologies. Our results provide strong evidence that phenotypic plasticity in foliose corals optimizes photosynthetic energy acquisition and is not simply a mechanism to increase light capture. We show that the optimal morphology is constrained at the boundaries of the environmental gradient, with non-optimal morphologies in these habitats having greatly reduced energy acquisition. However, at the center of the environmental gradient, flexibility in photophysiology allows energy acquisition to be very similar for multiple morphologies. Our results highlight the importance of phenotypic plasticity at multiple scales. Variation in overall morphology is important at niche boundaries at which conditions are consistently more stressful, whereas physiological flexibility is important in intermediate and less predictable habitats in which a rapid and reversible response to environmental fluctuations is required.