Coral Bleaching Biology
Elevated seawater temperatures in combination with strong sunlight cause thermal stress in corals. This stress can cause disruption of normal photosynthetic processes in the coral’s zooxanthellae which leads to coral bleaching.
When a coral is bleached, the colony will have a shiny white appearance as the polyps are free of the zooxanthellae and therefore transparent. A bleached coral is alive but functioning with less than 95% of his nutritional source. Corals can survive for several days or months without their zooxanthellae, but their ability to survive depends upon the level and type of stress and the sensitivity of the coral. If stressors persist, corals may starve and die. If stressors do not persist, then the corals may regain zooxanthellae and recover. However, even if corals survive bleaching, they may suffer sub-lethal impacts such as reduced coral growth rates and reproduction and increased susceptibility to disease.
Source: National Oceanic and Atmospheric Administration
Variations in Bleaching Susceptibility
Corals vary in their susceptibility to bleaching. While trends vary depending on colony size and stress intensity, ref consistent patterns of susceptibility can be seen among coral species, with a general trend of higher susceptibility in more intricate, branching forms and lower susceptibility in massive species, especially those with fleshy polyps. ref
Coral reefs’ susceptibility to bleaching may also vary based on the conditions to which a reef has adapted, its species composition, and reef zonation. Corals on reef flats, for example, will often be able to tolerate much higher water temperatures than colonies of the same species inhabiting reef slopes.
The type of zooxanthellae can also influence bleaching susceptibility. Various species of zooxanthellae (from the genus Symbiodiniacea), previously referred to as “clades”, ref have different sensitivities to stressors. Their paring with host-corals therefore influences the coral’s ability to be more resistant to higher temperatures. However, corals with heat-resistant symbionts tend to grow more slowly, ref which is perhaps one reason why slower-growing coral species are more tolerant of thermal stress.
Table highlighting a general hierarchy of resistance to bleaching according to growth forms. Source: Loya et al. 2001, McClanahan et al. 2004, Marshall and Schuttenberg 2006