In the vast ocean, corals face many threats. One of the biggest is marine heatwaves. These events can cause coral bleaching, which happens when corals get too hot and lose the algae that give them food and color. Without these algae, corals can die, leaving behind bare skeletons. Often, these skeletons get taken over by turf algae, making it hard for new coral tissue to grow back. However, there is a rare but remarkable phenomenon called the Phoenix effect. This is when coral tissue deep within the skeleton grows back quickly, covering the dead parts. This has been observed in Hawaiian corals Porites compressa and Montipora capitata.

In 2015, a severe marine heatwave hit Kāne'ohe Bay, Hawai'i. Researchers monitored 237 coral individuals, both bleached and non-bleached, for several years. They found that nearly 16% of Porites compressa and 34% of Montipora capitata experienced significant partial mortality. About half of these affected corals had bleached during the heatwave. After the heatwave, turf algae colonized the exposed coral skeletons. However, some corals showed an incredible ability to bounce back. Six colonies, five Porites compressa and one Montipora capitata, recovered to over 90% live tissue within two years. Three more colonies showed similar recovery within four years.

The researchers defined rapid tissue recovery based on two criteria. First, the coral had to experience substantial partial mortality within the first 12 months after the heatwave. Second, the coral had to show some recovery of live tissue before 2022. Interestingly, only the corals that had bleached in 2015 showed this rapid recovery. This suggests that the stress of bleaching might trigger some corals to activate their recovery mechanisms. In 2019, another marine heatwave hit, but this time, the previously recovered colonies did not experience significant tissue loss. This shows that these corals had built some resilience to heat stress.

The Phoenix effect is a fascinating example of coral resilience. It shows that even after severe stress, some corals can bounce back. This could provide valuable insights into how corals regenerate tissue. However, it is important to note that not all corals show this remarkable recovery. Many corals struggle to survive after bleaching events. Therefore, while the Phoenix effect is a hopeful sign, it is not a guarantee of coral survival in the face of climate change. It is crucial to continue studying and protecting these vital ecosystems.

Hawaii's Corals Show Surprising Comeback After Heatwave

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