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Corals: a story of death and rebirth

Updated: Jun 15, 2023

As the corals continue dying, who is to blame? Earth's rising temperature.

Corals before and after bleaching by Nivedita Bansal

What are corals?

Coral is not a singular animal but rather a colony of many individual animals called polyps. The polyps cover a skeleton made of calcium carbonate. Within the soft tissue of the polyps live photosynthetic algae called zooxanthellae that provide nutrition for the coral. A coral reef is an ecosystem in which stony corals are the foundational organisms. The corals provide physical structure and complexity to the reef, as well as play an integral role in the food web within the ecosystem.

Free swimming larval (baby) coral polyps tend to settle on top of other dead/dying corals and hard substrate where they remain for the rest of their life. That is why there is an increase in structural complexity over time. As the physical structure of the reef becomes more complex, so does the biological structure. The corals create niches, little pockets of space, in which other organisms can exist. Many corals have some sort of mutually beneficial relationship with some other reef-dwelling animals (cleaner shrimp, crabs, cryptic snails, etc.) Some animals feed on the microbial organisms and algae that grow on the corals, some eat the corals themselves. Without the corals, there can be no reef. Without the reef, there would be a total ecological collapse.

(Also Read: Feeding the Future)

How does climate change affect corals?

Corals are particularly sensitive to thermal stress. Keep in mind that the majority of corals evolved in a tropical, equatorial environment. The equator is an environment that is very, very stable. Therefore, corals aren’t equipped to handle a wide range of temperatures.

Global climate change is warming the ocean at an alarming rate, putting corals under more and more physical stress. When a coral is physically stressed, they undergo a process known as ‘bleaching.’ When this happens, the coral has detected something going wrong, and as an attempt to save itself, ejects the photosynthetic zooxanthellae, like when your body alters its temperature to burn off a pathogen. With the green algal cells gone, all that remains is the white skeleton of the coral. Bleaching is a natural stress response. And corals have the capacity to reuptake zooxanthellae when conditions have normalized. But conditions are becoming more and more abnormal. As they endure this stress, they continue rejecting the zooxanthellae and become more susceptible to disease, starvation, and death.

In reefs that have faced mass bleaching events from abnormally high ocean temperatures, we’re seeing a phase shift from a coral-dominated system to a macroalgal-dominated (seaweed) system. Large mats of red, brown, and green algae are growing over the skeletons of dead corals. And as we lose that physical complexity, we lose the biological diversity. This is a global phenomenon and one that has dire consequences on fisheries, coastal resilience, and many people’s livelihoods.

How can we save corals?

There is a massive push to save the reefs, such as selective breeding of thermal stress-resistant corals, coral reef nurseries and restoration projects, and coral translocation. All of these practices show promise. Combinations of restoration projects and marine protected areas seem to be some of the most effective methods of maintaining coral reefs. Many reefs have been badly damaged and many across the world are hanging by a thread. But it is not too far gone and hope remains. Scientific understanding of corals throughout the world continues to grow. It is critical that this knowledge be implemented into policy and practice if we are to stop the degradation of our reefs. It can be done. It must be done.

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