When you are staring at 60% of the coral reefs being bleached, and a 40% loss in coral reefs all over the world, as a scientist, you cannot just sit there and take it. Tens of scientists all around the world are working tirelessly to ensure that the reefs have a way to cope up with the large scale environmental changes that are occurring in the world’s oceans. However, so are the corals!

I mentioned in the previous post on coral bleaching about why it is happening, and what are the controls of coral bleaching. In this post, I’ll look at the the corals themselves are showing, and what kind of research is being done to understand this recovery process…

Coral’s resilience

Corals have survived 5 extinction events in the past. This shows that they are extremely resilient creatures. This resilience stems from the simplicity of their structure, as well as the huge diversity of coral forms in the world. Corals occur at different depths, host different organisms, have different structure and have different temperature controls. This is a good thing for the corals; it will, at least, ensure that some corals will have a chance to survive the current global extinction.

Some scientists believe that corals that have already recovered from a past bleaching event are more likely to be resilient to coral bleaching in the future. This was observed off the coast of Indonesia, where the reefs weathered in the 1998 bleaching event showed much more resilience in 2008-2010 bleaching event. Scientists believe that corals might be adapting fast enough to withstand the environmental changes.

However, this is not the case for corals all around the world. The corals that recovered a past bleaching event may remain vulnerable for a long time, before it reaches full health. As a consequence, further bleaching events will prove disastrous for them. This has been observed in the Caribbean. Scientists observed that the recovering corals are highly vulnerable to diseases and attack from coral-eating fishes.

Are some traits in corals better than others?

It appears to be so. There is a growing belief among coral scientists that corals that have a complex structure, and survive at greater depth, are more likely to weather the effects of a bleaching event.

A greater depth allows corals to escape much of the warm waters in the ocean. Deep waters are cooler and are expected to serve as coral refuges in the future.

There have been reports of coral recovery in the Middle East, where corals migrated to deeper waters when temperatures rose. Along with this migration, fishes that depend on these corals also migrated. In a few years after the 2008 bleaching event, the fishes that were extinct in the shallow waters showed good numbers deeper in the ocean, where the corals continued to thrive.

Is coral bleaching an adaptive mechanism?

It has been established that different strains of zooxanthellae exist, both within and around the coral structure. These strains are different in their ability to tolerate stresses. Research into the physiology and genetics of corals have suggested that coral bleaching may just be an adaptive response to changing environment.

The corals recognize that the temperature in the water is increasing. Therefore, they expel the algae within their body, which was until now, suited for cooler temperatures. It becomes white and devoid of algae, allowing other zooxanthellae species to colonize the coral structures. The new species are likely to be more resilient to greater temperatures. This could be a reason why the coral reefs off the coast of Indonesia did not suffer during the 2008-2010 bleaching event.

This could well be the case, because geological history suggests that corals have managed to survive much higher temperatures than this before. They may not have existed with the current species diversity, but they existed and thrived nevertheless.

 

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Coral bleaching may or may not be an adaptive mechanism. But the fact of the matter is that corals are trying their hardest to combat stressful environmental conditions to ensure their continued survival. But it isn’t just increasing acidity or temperature that is causing bleaching, is it? Overfishing, pollution and overexploitation of corals for economic benefits is by far the biggest contributor factor in coral bleaching. They may not directly cause coral bleaching, but they reduce the coral resilience to coral bleaching.

Is there something we should be doing to prevent coral bleaching?

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References-

1. Brown, B.E. 1997. Coral bleaching: causes and consequences. Coral Reefs (16): S129-S138.
2. Nature article
3. https://www.scientificamerican.com/article/coral-reefs-show-remarkable-ability-to-recover-from-near-death/