Clonal Variation for Phenotypic Plasticity in the Coral Madracis Mirabilis

Morphological plasticity is common among clonal organisms, including scleractinian corals, yet the role of phenotypic plasticity in coral ecology and evolution is largely unexplored. Additionally, it is unclear how much variation in plastic responses exists among individuals, populations, and species, and thus how much potential there is for natural selection to act on coral reaction norms. In the branching coral Madracis mirabilis, corallite architecture and density, branch diameter and spacing, and overall aggregate morphology all vary among environments. To examine the role of phenotypic plasticity in generating these patterns, clonal replicates of five genotypes of M. mirabilis were transplanted from each of two source populations into four treatment environments on the north coast of Jamaica. Flow rate, sedimentation, irradiance, water temperature, and salinity all varied among these environments. DNA fingerprinting was used to ensure that the 10 transplanted genotypes were genetically distinct. Six morphological traits (intersepta area, septa length, columella area, corallite area, corallite spacing, and branch tip diameter) were measured after transplantation to determine whether they were altered in response to environmental conditions. Because these traits were correlated, principal components analysis was used to define new, uncorrelated traits for analysis. Four of the five corallite traits and branch diameter were significantly affected by the environment, demonstrating that morphological variation among environments in M. mirabilis is due in large part to phenotypic plasticity. No difference was detected between the two source populations in the magnitude or direction of their plastic responses, but there was substantial variation among genotypes (genotype × environment interaction). Many of the phenotypic changes of both populations resulted in the transplants becoming morphologically similar to resident conspecifics in each treatment environment. Genotypes from both populations were able to maintain similar growth rates under diverse environmental conditions. Such morphological convergence by phenotypic plasticity may expand the ecological range of this species by enabling genotypes to tolerate spatially and temporally variable environments.