Interactions between Corals and their Symbiotic Algae

Interactions between Corals and their Symbiotic Algae Gisèle Muller-Parker and Christopher F. D'Elia The mutualistic relationship between corals and their algal endosymbionts is a key factor in the evolutionary success of hermatypic (reef building) corals. Corals that harbor endosymbiotic algae exhibit faster rates of calcification, photoautotrophy and many other attributes that are believed to contribute to the persistence of coral reefs in geologic time. Consequently, environmental and physiological conditions that result in changes in the relationship between the animal host and symbiotic algae may have profound ecological effects. This chapter discusses the coral-zooxanthella symbiosis from the perspective of the nutrient dynamics and energetics of the association and in the context of the stability and adaptability of the symbiosis in the coral-reef ecosystem in which they are found. We examine aspects of the structure and function of the symbiosis that are believed to account for the high rates of calcification and productivity exhibited by reef corals. We discuss some of the factors that are believed to influence the density of symbionts and hence the physiological balance between the symbiotic partners. We consider the possible effects of both natural and anthropogenic events on coral reef ecosystems and how they might be expected to affect the stability and survival of the symbiosis. We also consider the possible costs and benefits associated with the symbiotic condition. We conclude by speculating about the value of using the coral symbiotic association as a measure of the "health" of coral reef ecosystems.