Boston might be getting chillier as autumn settles in, but for coral reefs in the Caribbean Sea over 4,000 miles south of here, temperatures are anything but cool. After a summer that brought devastation to reefs around Florida—with experts reporting some of the worst coral bleaching they’ve ever seen—intense heat is continuing to take a toll on marine environments in Panama, Colombia, El Salvador, Costa Rica, Mexico, and countries in the Caribbean.   

When Boston University coral expert Sarah Davies heard reports of coral bleaching on the Caribbean coast of Panama where she and her team studied last year, she raced to send three researchers back there as quickly as possible to see how they’re doing. 

“We saw a lot of bleaching,” says Hannah Aichelman, a postdoctoral researcher in Davies’ lab who led the team. 

These two bleaching corals were seen at a reef near Cristóbal Island in Bocas del Toro, Panama, on August 29, 2023. The white color indicates the bleached parts, where the coral has lost its symbiotic algae. The sheet of gray squares placed above the light pink and white coral is used to standardize photos across different light environments. Photos courtesy of Hannah Aichelman and the Davies Lab

What Is Coral Bleaching?

Bleaching happens when corals and their symbiotic algae, called Symbiodiniaceae (sometimes called zooxanthellae), part ways due to hotter-than-normal temperatures. The coral expels the algae as a stress response, which robs them of their color, turning them white. This is a problem, because the mutualistic relationship between coral and algae is essential for their survival—algae provide food for the coral, and the coral provides shelter for the algae. Bleaching doesn’t instantly kill coral, but if it goes on too long, it’s eventually lethal. 

Aichelman (GRS’23) and her team visited four reef sites at the end of August, the same spots where Davies, a BU College of Arts & Sciences assistant professor of biology, took DNA samples and tagged coral colonies on the reef in June 2022. The researchers were able to reidentify the tagged coral colonies—by diving to the coral sites, and locating the tiny metal tags using underwater metal detectors—and compare their observations from what they saw last year to now. They were accompanied by a scientific diver from the Smithsonian Tropical Research Institute, which is based in Panama.  

Viviana Guerra, a Panamanian scientific diver at the Smithsonian Tropical Research Institute, worked with the Davies Lab researchers to photograph and monitor the corals. The yellow oval she’s tucked under her arm, as she approaches bleaching brain coral, is one of the metal detectors the team used to locate the tagged coral colonies. Photo courtesy of Hannah Aichelman and the Davies Lab

“Reefs are really complicated places,” Aichelman says. Light conditions, salinity, and temperature can vary on very small spatial scales, and even neighboring corals that experience similar environments can respond differently. 

Aichelman, Davies, and their team will be analyzing temperature data and the samples they collected on the trip to get a clearer picture of how specific reefs are doing. But, unfortunately, they don’t see conditions improving in the near future.  

“In Panama, the water is going to continue to get hotter. We’re going into what is usually the warmest months of the year for this area and the temperatures were already so hot,” Aichelman says. 

Temperature Is Everything  

Far from Panama, Davies has seen similar patterns in Palau, an island country in the western Pacific Ocean. One focus of her work is distinguishing corals that look identical, but are genetically distinct (these are called cryptic coral lineages, and the only way to know who is who is by sequencing their DNA). Aichelman and Davies have identified three cryptic coral lineages around Panama in the last few years, as well as three lineages in Palau.

“In Palau, we would see two cryptic corals next to each other, and one was bleached, and the other one wasn’t,” Davies says. Her lab, the Davies Marine Population Genomics Lab, is working to figure out why.

Scientists don’t know the molecular mechanisms behind coral bleaching, so understanding why is key to predicting the extent climate change will impact different reef ecosystems and how to best protect them. Despite coral reefs taking up only a small percentage of the ocean, about a quarter of all marine life depends on reefs at some point in their lives, so widespread bleaching is a big deal. Bleaching affects a coral’s ability to take in nutrients and reproduce—which are both essential for their resilience.  

Earlier in the summer, Davies and her team, led by postdoctoral researcher Carsten Grupstra, started analyzing coral from Palau called lobe coral (Porites lobata). This coral is able to withstand higher water temperatures compared to most other types, making it a prime candidate to study how it acclimates under different climate scenarios. In the lab, they take pieces of the coral and put them in different environments with varying temperatures that simulate conditions in the wild and the predicted changes due to the warming climate.  

“I’ve been thinking a lot about why some corals are more resilient than others,” Davies says. “Why is it that some corals can live through intense heating events, what is it about them?”

Despite the constant threats being hurled at them, some corals can live to be hundreds of years old—such as “Big Momma,” a coral in the Pacific that’s over 500 years old. Scientists, including Davies, think the organisms doing well under climate change are the ones that can adjust to changing circumstances and reproduce consistently over many years. 

“Maybe these old, bouldering corals are doing relatively well under climate change, because that’s their life strategy. They’ve always been playing the long game,” Davies says. “These corals that live for so long have to expect that their environment will change, because change is normal—it’s just the rate of change right now that is not normal.” 

Immunity and Resilience 

Coral reefs are also getting sick as a result of bleaching and drastic environmental changes.

“Corals aren’t just dying because they’re bleaching, they’re also getting infected by a lot of new bacteria and viruses, probably because their immune systems are being altered by environmental influences,” says Thomas Gilmore, a CAS professor of biology. In a paper published in Communications Biology, he and Davies found that a lack of nutrition makes sea anemones—a group of tiny marine invertebrates related to coral—more susceptible to bacterial diseases in the lab. 

All creatures, like us, need to eat a healthy diet to keep their immune systems strong. So, it’s not surprising that a starved, bleached coral is at risk of getting sick. What is surprising, however, is that the same genes and proteins that regulate immunity in humans are also found in corals, sea anemones, and sponges—a protein pathway called nuclear factor kappa B (NF-κB). 

For us, the NF-κB pathway gets turned on only when we encounter a pathogen, but Gilmore and his team are finding that, in the ocean, the defense mechanism is always switched on, since, he says, “they’re living in a soup of bacteria and viruses.” He spent many years studying NF-κB in humans, and then decided to apply the same techniques to study it in corals and anemones.

“We want to understand the basis of the immune system in a lot of these simpler organisms that are under environmental stress,” he says, because many of the diseases that affect marine invertebrates might have to do with effects on NF-κB. “Our primary motivation as basic scientists is to understand the diversity of biological processes across many organisms. But the more we understand how different organisms combat pathogens, the better we may be able to develop new techniques to kill viruses and bacteria in humans.” 

This is one of the coral lineages that Aichelman studies, seen at the reef point near the Smithsonian Tropical Research Institute in Panama. The metal tag sits on top of the coral; another sponge (in red) grows around the coral. This coral is not bleaching, and appears healthy, despite warm temperatures. Photo courtesy of Hannah Aichelman and the Davies Lab

Coral Reproduction 

A bleached coral is also one that might have a hard time spawning. Reproduction is an essential part of reefs rebounding, Davies says—and it’s what got her into the field.

“Spawning is the reason why I became a coral biologist, because it’s so cool,” she says. “We assume because they’re basal metazoans and they look like rocks that they don’t have a ton of interesting behaviors. And they don’t, for the most part, but they have very interesting reproductive biology.”

Coral sex requires a delicate combination of water temperatures being just right, and cues from the moon and sun that tell coral when it’s time for them to reproduce. And, for most types of coral, this only happens one night a year during the warmest month, a set number of days after the full moon. 

On that day, timed just after sunset, corals release bundles of eggs and sperm, a process called broadcast spawning. (Most corals make both eggs and sperm bundles, but some have separate sexes.) The bundles float to the surface, basically sparking a coral sex party. “It’s external fertilization at the water surface,” she says. “They have to hope they released their gametes at the same time as another coral of the same species so there’s someone’s gametes to fertilize at the surface.”

Davies has studied this phenomena in many different oceans, and it can be so synchronous that she can predict which corals will spawn next before they even start. She says that a stressed-out coral will often not release the bundles to conserve energy, or send up whatever they can as a last Hail Mary. 

About three to five days after fertilization, “they become these cute little planula larvae that look like ovals covered in sensory hairs that allow them to receive cues emanating from the reef,” she says.  

Many of the offspring get eaten by other sea creatures on the way down, but the surviving larvae return to the ocean floor where they will hopefully mature into adult coral. It’s not clear why they end up exactly where they do, but research from Davies and BU Marine Program faculty and students estimated that thin-finger coral (Porites divaricata) larvae settle a few meters from the parent colonies. They suggest that, as a result, coral conservation should account for the protection of the entire coral habitat, and not just reefs, with regulations and protections.

Will Coral Be All Right? It Depends on Who You Ask 

Last year, the Great Barrier Reef in the Pacific Ocean showed promising signs of rebounding, with local experts reporting the largest amount of coral cover in 36 years. Davies wasn’t surprised. 

“Pacific reefs, including the Great Barrier Reef, are more resilient than Caribbean reefs since they have much higher species diversity and they have a lot of fast-growing corals,” she says. “But it’s kind of like a boxing match where someone gets knocked down, but they get up. They get down, they get up again, but for how long can they do that?” 

A persistent problem in the field is the lack of consensus on how to monitor and analyze reef health, since not all scientists assess them the same way—higher coral cover can be good news to a biologist, while a chemist analyzing the same reef may find stress hormones that indicate bad news. To usher in some agreement, earlier this year Davies led 60 other scientists in authoring a paper on building consensus around assessing coral symbionts, calling for agreement on how to genetically sequence properly, and to sequence more robustly and consistently in research. 

“The symbionts have really complicated genomes, so we need basic experiments to confirm what sequences mean. We put forward these types of experiments, and hope this paper is a framework for moving the field forward,” she says. “My favorite part of this paper highlights ways to make the research community more inclusive, because we need more diverse perspectives at the table.” 

In her own work, Davies has shone a light on the staggering genetic diversity among corals, but says she worries “about the loss of diversity in each of these bleaching events, and I don’t think people are really monitoring that.” 

She says her attitude and hopefulness depends on the day. “If the glass is half full, I’m like, ‘oh wow, there’s so much diversity and more potential for adaptation!’ If the glass is half empty, then I’m thinking about how these bleaching events are probably wiping out entire species that we don’t even know about yet.”