SKELETAL PROXIES FOR BLEACHING-RELATED STRESS IN A SCLERACTINIAN CORAL

Scleractinian reef-building corals are affected today by environmental stresses, both natural and anthropogenic. Reef corals are sessile organisms, which produce incremental structures with the potential to preserve information about their growth and interaction with the surrounding environment. The skeletal density, stable isotope fraction content, and trace metal content of scleractinian coral skeletons have been used as proxies for paleoclimatic conditions, but these same indicators may also record evidence of stress, disturbance, and/or disease in both modern and fossil corals.

One of the responses of living reef-building corals to stress is bleaching, caused by the loss of their symbiotic photosynthesizing algae. Reef-building corals may sequester excess trace elements, such as strontium, selenium, and silver, acquired in the processes of metabolism, in their calcium carbonate skeletons. Differences in these elements between skeletal material from bleached and unbleached sites on the colonies may therefore be a signal for stress related to bleaching.

To test this hypothesis, skeletal material from bleached and unbleached portions of Recent Porites porites colonies were examined for trace metal and morphological indicators. Statistically significant differences in amounts of strontium, selenium and silver were found in skeletal material from bleached and unbleached sites on the colonies.

Morphological differences are also evident in skeletal material from bleached and unbleached sites on the colonies, when examined with light and scanning electron microscopy. Corallites from unbleached sites appeared normal, with well-defined septal denticles. Corallites from bleached sites on the colonies had a "melted" appearance, with septal denticles which were weakly defined and appeared rudimentary.