The skeletal isotopic composition as an indicator of ecological and physiological plasticity in the coral genus Madracis

Abstract

Three species of the reef coral genus Madracis display skeletal isotopic characteristics that relate to depth, colony topography, and consequently to coral physiology. The joint interpretation of skeletal δ13C and δ18O provides information on the ecological plasticity and adaptation to depth of a coral species. Isotopic results are most easily understood in terms of "kinetic" effects, which reduce both δ18O and δ13C below isotopic equilibrium values, and "metabolic" effects, which only influence the skeletal δ13C. Madracis mirabilis is adapted to depths shallower than 20 m, and shows the greatest range in kinetic effects and the strongest metabolic 13C enrichments caused by symbiont photosynthesis. Madracis formosa lives deeper than 40 m, and shows a reduced range of kinetic effects and relatively weak metabolic 13C enrichments. Madracis pharensis inhabits depths from 5 to >60 m, and does not attain the strength of kinetic effects of either of the other two species, apparently because it is not quite as well adapted to rapid growth at either extreme.