Hakai Magazine

Coastal science and societies

A diver surveys the damage done to a coral reef by bleaching. Photo by Ethan Daniels/Alamy Stock Photo

The Hangover of Coral Bleaching

Even if a reef recovers from bleaching, the damage is already done to future generations.

Authored by

by Alex Riley

Article body copy

The wave of destruction started in October 2014. As the shallow water surrounding the Hawaiʻian Islands warmed, fueled by El Niño, the coral began to blanch. Once a kaleidoscope of color, the coral reef in Kāneʻohe Bay, on the northeast coast of Oʻahu, faded to a skeletal white. “It was like watching a train wreck in slow motion,” says Mary Hagedorn, a marine biologist with the Smithsonian Institution. “It was really awful to see.”

When coral are bleached by warm water, the algae that live within them are forced out, depriving the animals of more than just their color. Without their algal lodgers to convert the Sun’s rays into edible sugars, coral lose their main source of food. If the warm conditions persist, the algae may never return home. The corals remain bleached. Hunger turns to starvation. It is known as “white death.”

But as new research is showing, death is only one outcome of coral bleaching. Even if the adult coral survive, bleaching can have dire long-term effects.

By December 2014, the coral that survived the initial bleaching seemed to be back to normal: full of color and well fed. “They looked perfect,” says Hagedorn. But beneath the surface, the damage was done.

Coral spend nine months of the year laying down fats and proteins to feed their growing eggs and sperm, preparing for their seasonal spawn. “Think of it as a human baby, in a way,” says Hagedorn. If the coral are bleached during this nine-month window, “the early parts of development can be tragically disrupted.”

Collecting the sperm and eggs released by two coral species (Fungia scutaria and Montipora capitata) in the spawning season after the 2014 bleaching, Hagedorn and her colleagues found that, compared to cooler, non-bleaching years, the number of swimming sperm was cut in half. Eggs were significantly smaller. And, as a result, fertilization success fell from 83 percent to 47 percent.

The bad news didn’t stop there. When the researchers managed to successfully fertilize coral eggs back in the lab, the larval survival dropped by more than 40 percent. For Montipora capitata, the larvae’s ability to transform from free-swimming babies into anchored adults was cut in half. For Fungia scutaria, it didn’t happen at all.

Coral bleaching starves coral living today, but it also crosses generational boundaries, weakening the corals of tomorrow. “It’s very clear that the stress of warming and the actual bleaching had long-lasting effects,” says Hagedorn.

For Hagedorn, this is cause for immediate concern, and immediate action. She and her colleagues have taken to feeding certain coral during times of heat stress, hoping to mitigate some of the intergenerational effects. They’re also planning long term.

Hagedorn is working on a project to cryopreserve the eggs and sperm from coral species from around the world, hoping to save as much of their genetic diversity as possible. This frozen ark, she says, could then be used to replenish ailing reefs with healthy stocks. It’s an insurance policy similar to captive breeding programs.

Only the healthiest samples—the “stars” as Hagedorn calls them—make it into the freezer. But following the 2014 bleaching, there was nothing worth saving. For her to take them into the ark, coral sperm need motility above 50 percent. For the Kāneʻohe Bay coral, sperm motility was as low as 30 percent. “We didn’t get any stars,” says Hagedorn.

In October 2015, warm water bleached Kāneʻohe Bay’s coral once more. “So this year will be the same,” she says—another year without any stars.

Bleaching is becoming more frequent. From 1998 to 2010, the US National Oceanic and Atmospheric Administration recorded 3,700 events worldwide. What’s happening in Hawaiʻi, says Hagedorn, is likely happening everywhere. “It’s a window to the world.”