Coral Reefs: A Reef Resilience Toolkit Module

Stress Tolerance

Coral communities that are exposed at low tide are often composed of species known to be more tolerant to stress. Photo © R. Salm

Although corals are very sensitive organisms that generally require narrow ranges of certain conditions to survive (e.g., temperature, salinity, light), some corals have adapted to exist in highly stressful conditions at the outer limits of these ranges. A history of exposure to high temperatures can influence the thermal tolerance of corals and their resistance to bleaching. Parts of reefs that regularly experience heat stress conditions, such as reef flats and crests, may be populated by corals that are more tolerant and resistant to bleaching.

For example, corals on reef flats are exposed to air during the lowest tides. These corals may be accustomed to extreme stress from heat, desiccation, and great fluctuations in salinity. This may help to explain why corals in some inner reefs appear less susceptible to bleaching than the same species growing in deeper waters.

Guidelines for identifying stress tolerant corals include the following:

• Check for living corals on reef flats and reef crests. Important areas to protect are those with multiple species and a wide range of sizes.
• Size often relates directly to the age of the coral, and thus a wide range of sizes may indicate a wide range of ages and serve as a proxy for survival prospects through bleaching events.
• Coral species that tend to be more resistant can still bleach, but surveying species composition of a reef may indicate past history of exposure to bleaching, possible resistance, and provide indications of future bleaching risk.