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For corals, color is more than superficial. Across the tropical Pacific, one species of reef-building coral, known as rice coral, comes in two colors: dark red and rusty orange. And as new research shows, these colors lend the corals different superpowers.
In laboratory experiments, microbiologist Amanda Shore-Maggio, of the University of Maryland, Baltimore County, found that red rice corals grow faster and are more resistant to disease. The orange corals can better withstand warm temperatures without bleaching.
It’s common for one animal species to appear in multiple colors. For example, leopards may be spotted or entirely black; the shell of the grove snail may be yellow, brown, or pink. In some cases, different color morphs also differ in their physiology or behavior. Researchers have theorized that this variety helps a species thrive.
Unlike leopards and grove snails, however, rice corals don’t make their own color. Instead, they get their hue from one of two types of microscopic algae in their tissue.
Shore-Maggio and her colleagues discovered the algae’s colorful influence in the wake of a bleaching event in Hawai‘i’s Kāneʻohe Bay in October 2013. Twice in the previous seven years, the researchers had surveyed rice corals in the bay for signs of white syndrome, a disease that kills corals.
Each time they dove in, they found that red corals were more common than orange ones. In 2011, for instance, red corals made up almost 60 percent of rice coral colonies. The red morphs also seemed less susceptible to the disease. That year, only about three percent of red corals had the disease compared to about 16 percent of orange morphs.
But the red corals weren’t invincible. During the 2013 bleaching event, 69 percent turned ghostly white—more than twice the rate of bleaching in orange morphs.
The researchers collected corals of both colors and brought them back to the lab. They found that the red morphs grew faster than the orange ones. But when researchers raised the water temperature, the red corals bleached faster.
Shore-Maggio says the difference comes down to the algae. Some algae are better at resisting the disruption in photosynthesis that leads to bleaching, which could help explain why the orange corals bleach more slowly.
But less is known about how algae might help their hosts resist disease, she says. Byproducts from the algae might end up in the mucus that coats a coral, affecting the bacterial community that lives there. These bacteria help defend corals against disease, she says, much like the microbes living in a person’s gut.
“This is one of the first studies to link coral life history traits to differences in bleaching and disease susceptibility,” says Jamie Caldwell, an ecologist at Stanford University in California who has studied coral diseases in Hawai‘i and was not involved in the new research. Climate change is expected to make coral bleaching and disease outbreaks more common and severe, she says. The findings hint at how these threats will affect the members of a coral community differently.
Shore-Maggio notes that even though orange corals were slower to bleach in the warm tanks, both groups had high mortality by the end of the experiment. So neither hue gives sure-fire protection. “Corals will need more than just thermal tolerance to survive an increasingly warmer ocean,” she says.