This section provides information about biological and physical characteristics that affect whether or not a coral bleaches during a warm water event. Individual corals vary in their responses to light and heat stress. Such differences in sensitivity in corals and zooxanthellae are affected by characteristics such as:
- species differences
- genetic differences
- other factors affecting bleaching susceptibility (e.g., fluorescent tissue proteins, heat-shock proteins, colony integration, changes in feedingbehavior in response to thermal stress, tissue thickness, and history of exposure)
Not all coral species are equally susceptible to bleaching. In response to elevated sea temperatures, some corals may bleach, while other coral species in the same location may not. Some corals are able to acclimatize to local temperature increases over time. In general, coral species that are more resistant to bleaching can be characterized by massive growth forms, thick or less-integrated tissues and slow growth rates. Examples of coral genera recognized as more resistant to thermal stress include:
At the coral colony level, fast-growing species that are characterized by fine-structured, branching or tabular growth forms tend to be more susceptible to bleaching. These more susceptible coral genera include:
It is important to note that no species are completely immune from bleaching-induced mortality and nearly all genera have suffered high mortality during severe bleaching events in one location or another. ref A general hierarchy of resistance to bleaching provides a reasonable indication of susceptibility to heat stress. This table helps managers understand what to look for when monitoring their reefs – i.e., managers can assess coral genera in their area to determine which are likely to be the most/least bleaching resistant.
The term “zooxanthellae” refers to a wide variety of algae of the genus Symbiodiniaceae, previously referred to as Symbiodinium. Zooxanthellae is a genetically diverse group of dinoflagellates, including nine phylogenetic types, distinguished as clades A-I. These genetically distinct clades have different environmental, ecological, and geographic characteristics which influence the resistance and resilience of corals to thermal stress. Studies have revealed that the different clades of zooxanthellae have different susceptibilities to thermal and light stress.
Clade D Zooxanthellae
Clade D zooxanthellae are thermally tolerant and increase the resistance of corals that harbor them to elevated SSTs. ref Clade D zooxanthellae are found in a diverse range of coral species. Clade D zooxanthellae are present in higher abundances on some reefs than others, and these are often reefs exposed to relatively high levels of thermal stress or local stressors (e.g., sedimentation on reefs) with a history of coral bleaching. For example, clade D zooxanthellae are more abundant in acroporid corals from back reef lagoons in American Samoa, which experience higher maximum temperatures than the fore reef environments, where Acropora primarily hosts clade C. ref Because they are often found in increased abundance on reefs that are exposed to environmental stressors, the presence of clade D symbionts can be a biological indicator of negative changes in coral health. However, this is not always the case; sometimes clade D symbionts indicate positive acclimatization to stressful conditions. Information on abundance of clade D zooxanthellae can help managers understand the susceptibility of specific corals to thermal stress and to identify changes in coral reef health.
The ability to associate with multiple zooxanthellae clades is common in corals. ref The selective exchange of zooxanthellae is a potential mechanism by which corals might survive climate stressors, such as increased sea-surface temperatures. Changes in the dominant zooxanthellae types of a coral colony may occur through two processes:
- Shuffling – changes in the relative abundance of zooxanthellae clades that are already present in the coral tissue
- Switching – uptake of new zooxanthellae clades from the environment
In the short term, corals with flexible symbioses may shuffle or switch zooxanthellae; and an increase in the abundance of thermally tolerant zooxanthellae strains (such as those of clade D) is expected with an increasing frequency of bleaching conditions. The potential to adapt to increasing sea-surface temperatures depends on the extent of genetic variation for heat tolerance, the generation time of the coral host and zooxanthellae, and the strength of selection.
Knowledge of biological characteristics of individual corals enhances the ability to predict stress responses to a bleaching event. Several biological and physical characteristics of corals may contribute to their ability to resist bleaching, including:
- Heat-shock proteins: Many different heat-shock proteins are found in coral tissues and their activity influences the bleaching response. Heat-shock proteins help maintain protein structure and cell function, following stress. ref For example, in one study, high-light-acclimatized tissues of the coral Goniastrea aspera had higher concentrations of heat shock proteins and these tissues did not bleach, unlike areas of the same colony that had not acclimatized to high light. ref
- Fluorescent tissue proteins: Corals are known for their bright colors, due primarily to fluorescent proteins in their tissues. Fluorescent proteins provide a system for regulating light; they protect the coral from broad-spectrum solar radiation by filtering out damaging UVA rays. The protective capacity of these proteins provides an internal defense mechanism that may have important implications for long-term survival of corals exposed to thermal stress. Corals containing fluorescent proteins have been found to bleach significantly less than non-fluorescent colonies of the same species. Furthermore, a recent study ref identified an additional role of fluorescent proteins as antioxidants, which may help to prevent stress in coral. Concentrations of fluorescent proteins vary among species (e.g., pocilloporids and acroporids have relatively low densities, while poritids, faviids and other slow-growing massive corals have high densities).
- Change in feeding behavior in response to thermal stress: Some corals rely heavily on food particles captured from the water column to supplement their energy requirements. These corals may be less dependent on the energy provided by their zooxanthellae, and thus less prone to starvation during a bleaching event when zooxanthellae are expelled from the coral. Additionally, some corals are able to change their feeding behavior in response to bleaching. Evidence suggests that coral species which can change their feeding behavior may survive bleaching better than species which cannot. ref
- Tissue thickness: The thickness of coral tissues may contribute to the level of susceptibility to bleaching. Thin tissue is found in coral species that are more susceptible to bleaching. Thicker tissue may help shade zooxanthellae from intense light, reducing thermal stress, and thus decreasing the chance of bleaching.
- Shading: The presence of shading is likely to increase resistance to bleaching. When shade is present, either due to weather conditions (persistent cloud cover) or physical location of a coral (e.g., under high island shadow or overhanging vegetation), bleaching may be less likely due to reduced solar radiation.
- History of exposure: Corals generally require narrow ranges of certain conditions to survive (e.g., temperature, salinity, light), but some corals have acclimatized to highly stressful conditions at the outer limits of their ranges. A history of exposure to high temperatures can influence the thermal tolerance of corals and enhance their resilience. For example, corals subjected to warmer than average temperatures prior to a bleaching event can be more thermally tolerant compared to corals that have not been pre-stressed. ref Healthy corals in areas where thermal variability is high (e.g., in back reef and lagoons) may also be more resistant to thermal stress. ref Additionally, parts of reefs that regularly experience heat stress conditions, such as reef flats and crests, may be populated by corals that are more tolerant of and resistant to stresses.
Guidelines for identifying stress tolerant corals include the following recommendations: ref
Compile existing data or local knowledge of composition of coral communities at sites. Identify dominant coral groups and rank their bleaching tolerance based on morphology (massive>encrusting>branching/tabular).
Conduct surveys of coral community composition at sites and assess dominance of coral types known to be more resistant or tolerant to bleaching.
If data are available, use physiological studies of dominant corals to assess likely resistance and tolerance based on zooxanthellae type, photo-protective pigments, or tissue condition (lipid levels), and/or heterotrophic capacity.
Once managers have assessed the stress tolerance of corals at sites based on the actions listed above, they can use this information to inform MPA design and management. For example, areas that are dominated by stress tolerant corals may be considered priorities for protection in MPAs. Sites that contain corals exhibiting resistance properties serve as refuges and sources of seed and may be vital to connectivity and other ecological dynamics at larger scales. Areas dominated by highly susceptible species will be critical to monitor following thermal stress events to assess the ecological responses of the corals to bleaching.