Dishon, G., Grossowicz, M., Krom, M. et al. Evolutionary Traits that Enable Scleractinian Corals to Survive Mass Extinction Events. Sci Rep 10, 3903 (2020). https://doi.org/10.1038/s41598-020-60605-2
What lies beneath
Imagine snorkeling in crystal-clear waters, surrounded by a symphony of fish as you gaze on the colors and intricacies of a large coral reef. Coral reef ecosystems, although representing only a fraction of a percent of the ocean floor, are vital for the survival of many marine species.
In particular, Scleractinian (Skler-ract-tin-e-an) corals, which are known as “stony” or “hard” corals, form the framework for the entire reef ecosystem. By building a skeleton made of calcium carbonate (hence “hard” corals), these corals create an ecosystem that acts as a habitat and food source for fish and invertebrates. If that’s not enough, this skeleton also produces well-preserved fossils making Scleractinian corals easy to trace back throughout history. By examining coral skeletons in the fossil record, scientists have traced corals all the way back to the Triassic period (roughly 241 million years ago!)
Despite the importance of coral reefs, the International Union for Conservation of Nature (IUCN) has reported that one third of the world’s reefs are currently threatened with mass extinction. Threats from human influences like overfishing, eutrophication (too much runoff from the land) and bleaching events (high water temperatures which cause coral to lose their color) have devastated coral reef ecosystems globally.
Clues from the past
Throughout earth’s history, five mass extinction events have wiped out many species’ populations. Now, as we start another decade of the 21st century, we enter a new era: the Anthropocene. Ever since the Industrial Revolution, humans have altered our environment with greenhouse gas emissions, pollution, and habitat destruction. Rates of species extinction are higher than before, which likely means the sixth mass extinction is already underway. Many scientists have raised concerns over the future health of our oceans, especially the coral reef communities within them.
But what about Scleractinian corals? Since they have been around for over 240 million years and recovered from mass extinctions in the past, such as the Cretaceous-Tertiary extinction 66 million years ago, could they survive in the future?
To find out, scientists compared trends in Scleractinian corals from the Cretaceous-Tertiary mass extinction to the current status of the Anthropocene extinction. Scientists examined fossil occurrence data together with coral trait data over the last 250 million years. The average time that traits were in each period was examined and “winners and losers” determined using statistical analysis. The goal was to figure out if specific traits had helped the corals survive in the past and if these would help corals in the future.
The future for corals.. and us
Scientists found that corals which survived and recovered from mass extinctions in the past had similar ‘survival’ traits. The coral ‘survival’ traits are:
- living in deep water,
- living in lots of areas,
- having no zooxanthellae (a specific type of algae),
- living solitary or in smaller colonies and
- being resistant to bleaching or high temperatures.
Currently, modern corals that have these ‘survival’ traits have stable populations and therefore might fare well in the future. However, corals without these traits (ie. more bleaching susceptible or in shallower water), are decreasing in diversity and could potentially die out.
The findings from this study suggest that some coral reefs stand a good chance in surviving the next mass extinction caused by the Anthropocene. Although many coral reefs are already dying and will continue to decrease, some corals ‘survival’ traits shed hope that they may live and re-establish in the future. The future trajectory of coral reefs is important to understand, not only because of the ecosystem services corals provide to humans, but because of the benefits they provide to many marine species and the environment.
Interestingly, humans may not even be around to witness corals in the future. It has become increasingly clear that humans face our own set of threats including environmental disasters, drought, food insecurity and extreme weather events. Humans haven’t survived mass extinctions like corals have, and we don’t have the same survival traits and adaptability. We are also threatened by the current mass extinction, and turns out, we may not be as prepared as corals.