Symptoms of Coral Reef Bleaching (cont)

As previously mentioned, different species of zooxanthellae possess differing resistances to environmental stress, and in turn the symptoms of different species of zooxanthellae to stress also differ. It is thought that the resistance of zooxanthellae is largely genetically programmed and is linked to the species physiology and morphology (McClanahan et al., 2004). The resistance of corals to bleaching and death has also been shown to be affected by the environmental conditions that the corals are exposed to. The threshold conditions that corals can tolerate are thought to be greater if the environmental conditions in that habitat are more variable (McClanahan & Maina, 2003). A coral living in a reef with very small variation in SST is more likely to be affected by a rare warm water event than a coral living in an area with slightly higher variation in SST. McClanahan and Maina (2003) hypothesise that due to climate change, and the likely future trend of an increasing annual temperature range, corals will become more resilient to more extreme rare environmental disturbances such as ENSO events. However they also predict that diversity will decrease as a result, with only the most resistant species of coral surviving, and consequently recolonising areas of dead reef previously occupied by less resilient species, a process generally known as the adaptive bleaching hypothesis (Kinzie III et al., 2001). It has also been shown via molecular studies that there is a certain degree of flexibility within the partnerships that exist between zooxanthellae and their hosts, and that following bleaching recolonisation can occur by more resistant corals (Baker, 2003).