Reef corals are remarkably variable in growth form and skeletal configuration. This high degree of phenetic diversity has contributed greatly to the success of the hermatypic scleractinians as widespread reef builders in tropical oceans. Although phenetic variability or “plasticity” is common, its cause remains obscure, because the species are genetically stable. The extent of phenetic variability is surprising in view of the morphologic simplicity of the animal itself. Carbon isotope ratios have been used to determine changes in the relative proportions of skeletal carbon in the form of carbonate derived from sea-water bicarbonate and metabolic carbon dioxide sources. Increasing amounts of respiratory CO2 incorporated into skeletal aragonite are attributed to decreasing efficiency of metabolite excretion. Isotope data indicate that zooxanthellae are effectively removing metabolic waste products from the tissues of hermatypic scleractinians, and that, for these corals, diffusion is not an important excretion mechanism. Ahermatypic corals, however, depend on diffusion processes to eliminate metabolic waste products. 13C/12C ratios indicate that, among the ahermatypes, small changes in corallum geometry that result in greater coarctation of the polyps and (or) a decrease in the size of the polyp surface exposed to sea water have a large effect on the isotopic composition of the skeletal carbonate. These findings suggest that corals that do not build reefs are restricted to small and relatively simple forms because of serious difficulties in the metabolite excretion they would encounter in more complex corallum geometries. In the case of the hermatypes, however, the metabolite excretion problems that would limit the range of possible skeletal configurations have been resolved by zooxanthellae. The spectacular phenetic variability in reef corals is thus achieved through a symbiotic association of the hermatypic corals with zooxanthellae.