Warming Oceans Mean Extinction for Reef-Building Corals, Study Finds

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A school of small fish swims around two scuba divers hovering close to a bleached coral reef. Divers swimming over a bleached coral reef. 
Photos courtesy of James Reimer 

New research led by scientists from the Museum and Harvey Mudd College (HMC), suggests that without a significant change in human-caused carbon emissions, reef-building corals will go extinct while softer corals and sea anemones persist.

The research team traced the evolution of corals, sea anemones, and their relatives through deep time and found that changes in ocean chemistry and temperature dramatically affected the group’s diversity and will likely continue to do so in the future.

Portraits of Catherine McFadden, Estefanía Rodríguez, and Andrea Quattrini.
The lead researchers on the study. (From left to right) Catherine McFadden, a biologist at Harvey Mudd College, Estefanía Rodríguez, a curator at the American Museum of Natural History, and Andrea Quattrini, research zoologist and curator of corals at the National Museum of Natural History. 
Courtesy of Andrea Quattrini 

“This study shows us how nature through evolution is able to adapt, survive and reinvent itself, so when hard corals are not able to survive, their soft-bodied relatives such as sea anemones will thrive instead,” said Estefanía Rodríguez, curator in the Museum’s Division of Invertebrate Zoology and a co-author on the study, published today in the journal Nature Ecology and Evolution. “The question is whether we will be able to adapt and reinvent ourselves once nature, as we currently know it, is not there anymore.” 

New genomic analyses reported in the study show that corals, which together with sea anemones and other relatives make up a class of animals known as anthozoans, have been on the planet for 770 million years. That’s 250 million years before the earliest undisputed fossil evidence of their existence—and long enough to experience massive shifts in climate, fluctuations in ocean chemistry, and several mass extinctions.

The researchers examined how these past conditions impacted anthozoan diversity with a new molecular approach developed by Andrea Quattrini, formerly at HMC, now curator of corals at the Smithsonian’s National Museum of Natural History, Catherine McFadden, the Vivian and D. Kenneth Baker Professor of Biology at Harvey Mudd College, and Rodríguez. The method allowed the team to compare nearly 2,000 key regions of anthozoan genomes to discern the evolutionary relationships between species.

Hundreds of anthozoan specimens collected around the world and now stored in museum collections were included in their analysis. When this molecular data was aligned with fossil evidence of anthozoan history, it revealed how these diverse animals evolved over geologic time.

Over the Earth’s history, changes in acidity and ion concentrations have shifted the ocean’s chemical composition between two states, known as aragonite and calcite seas. These changes, as well as changes in ocean water temperature, appear to have played an important role in determining what kinds of skeletons corals were able to produce.

Spherical and fan-like corals next to one another.
A healthy coral reef with both hard corals and softer-bodied relatives at Chinchorro Reef in the Mexican Caribbean Sea. 
Photo courtesy of David Paz-Garcia 

Stony corals —the type that build massive reefs that support complex marine ecosystems—take up minerals from the water to construct hard skeletons from a form of calcium carbonate known as aragonite. Other corals, such as sea fans and black corals, build their softer skeletons from protein or calcite (a less soluble form of calcium carbonate), whereas sea anemones have no skeleton at all.

The researchers found that reef-building corals emerged only when conditions favored the construction of their aragonite skeletons—periods of aragonite seas, when ocean temperatures were relatively cool. During periods of calcite seas, when carbon dioxide is more abundant in the atmosphere and oceans are more acidic, evolution favored anemones and corals that built their skeletons from protein or calcite.

A fish swims among sea fans and other corals.
Sea fans—softer-bodied coral relatives—colonizing a dead stony coral framework. 
Photo courtesy of David Paz-Garcia 

It was the soft anthozoans that fared best after reef crises—times when up to 90 percent of reef-building organisms died off as oceans warmed and became more acidic.

“Unfortunately, although these softer bodied species may adapt better to climate change than the stony corals, they don’t form large reefs. So in the future, reefs may be replaced by different marine communities,” McFadden said. “This already appears to be happening in the Caribbean where stony corals are being replaced by ‘forests’ of sea fans. Although stony corals have recovered from past reef crises and re-diversified, those recoveries have taken millions of years. That's a long time to wait for coral reefs as we know them to reappear.”

Today, about 1,300 species of stony coral inhabit the ocean, favored by aragonite sea conditions. But rising levels of carbon dioxide in the atmosphere are warming and acidifying the waters, making them less hospitable for these and other organisms whose shells and skeletons are made from aragonite. The scientists say that the loss of reef-building corals will have devastating consequences for communities who depend on reefs and the rich, complex ecosystems they support for fishing, shoreline protection, and tourism. 

“Aragonite is expected to dissolve under ocean acidification,” Quattrini said. “As our seas are becoming more acidic and warmer, it’s likely that the skeletons of corals will dissolve or not be able to grow. The best way to protect them is to curb our carbon emissions.”