New evidence suggests some of them may survive climate change better than the rest
Those rich, colorful pics of corals we are used to may soon fade to black and white if climate change continues unchecked. The phenomenon is called bleaching.
The frequency and severity of coral bleaching events are increasing as the effects of climate change become more apparent. A recent study out of the Florida State University has provided more details about how one group of corals, Pocillopora, in particular is handling these mass mortality events. The study shows how corals that are closely related genetically and visibly indistinguishable may differ in their response to heat stress. These findings are important to better understand coral bleaching, so that we can better mitigate and prepare for climate change.
However, often such findings may be misconstrued to imply that species are adapting and may survive the threats of climate change. In this article we will cover what this study found, explain the context of these findings, and explore their wider implications. What we really wanted to know, was how the team did this research, what it means for this field, and how the information can be used in practical ways. So, we sat down with the paper’s primary author, Scott Burgess, an associate professor at the Florida State University, to get a behind-the-science look at these new findings.
What is coral bleaching all about?
Corals are colonial animals that are closely related to sea anemones and jellyfish. As such, they have very thin skin and clear body tissues. Living inside these tissues are millions of unicellular algae cells called zooxanthellae. These algae live symbiotically with the coral and produce about 80-95% of the coral’s energy, allowing corals to produce the limestone skeletons that build the reef and create the ecosystem. These algae also give the corals their beautiful and vibrant colors that inspires so much awe and wonder.
These algae are similar to the leaves of a tree, using the sun’s energy to produce sugars and carbohydrates through photosynthesis, which they share with the coral animal. However, when water temperatures increase by only a small amount (1-2 degrees Celsius, or 1.8 to 3.6 degrees Fahrenheit) for a few days this symbiotic relationship breaks down, the algae are expelled from the coral tissue, and the corals lose their color (and their energy production). When this occurs, we see white corals, and refer to it as coral bleaching.
Today, every respectable scientist agrees that coral bleaching is one of the top threats to the survival of reefs, and is a direct result of climate change. We have already lost 50% of the Great Barrier Reef and 90% of the Caribbean corals. If action is not taken soon, reefs could be functionally extinct by the year 2050.
What the study found
In 2019, Scott Burgess and his team were conducting research on corals as part of a grant from the National Science Foundation They were on the island of Moorea, in French Polynesia, to study the genetic diversity within the genus of corals known as Pocillopora, to get baseline data that could be compared against future data, including growth rates, survival, and reproduction. But, as often happens in the ocean, the sea had a different agenda.
While collecting the first round of samples for genetic analysis in February, the team noticed that the corals they were observing were starting to lose color. This was despite satellite data from the NOAA Coral Reef Watch maps showing no bleaching threat in the region. The team immediately saw an opportunity here. Normally, it is very rare to find a bleaching event just starting, given how difficult it is to predict these events in any given year.
By May, the bleaching event was in full swing, and Burgess’ colleague and co-author of the study, Peter Edmunds, was there to capture it at its peak. In August, after it was over, the team returned to assess how many corals survived the event. Typically, genetic work on corals to identify cryptic species and assess diversity is only carried out during or after a bleaching event, when it is hard to sample randomly as the weakest corals may already be dead and gone.
Furthermore, data taken in long-term reef monitoring programs is usually limited to only percent cover data at the genus level, so there is very little data available to show the differences in bleaching by closely related species. This study identified the corals to the species level, but through the genetic work could look at haplotypes. Haplotypes are similar genetic markers that indicate differences in cryptic species of corals (cryptic species are those that have been lumped together based on similar appearance but are actually genetically distinct species that are yet to be differentiated).
How to assess coral bleaching
The study took genetic material from about 462 corals during the survey, starting with the initial 68 in February. They used photos of the reef to assess over 1,000 colonies of the corals, and to measure their size in photo-analysis software. Thousands of hours of tedious work went into this. Not only do the photos need to be taken while SCUBA diving in the open ocean, but hundreds of hours go into analyzing them. For each photo, the angles and dimensions have to be corrected, a scale set, the coral measured. It is a very long, arduous and tiring process, one managed predominantly Burgess’ undergraduates and graduate students. The process, which took several months, resulted in two undergraduate thesis projects.
According to Burgess, while the image processing was tedious, the most difficult part of this study was getting the genetic samples out of Moorea and back to the lab in Florida.
Corals are protected under the Convention for the International Trade in Endangered Species (CITES) and so there are complex regulations involved in moving them between countries. As principal investigator on the grant, instead of a lawyer up on international wildlife law, it was up to Burgess to navigate the process. He not only had to get permission to import the samples into the U.S., but also to get the French High Commission to give him export permits. While arduous, these international rules are important to protect against the illegal collection and trade of vulnerable corals.
Finally, once the photos were analyzed, and the genetic information compared, the researchers had a better understanding of how corals, that looked the same, fared differently in a bleaching event. At first, it appeared that the corals in larger colonies suffered more bleaching and subsequent mortality, which was counter to many previous studies that suggested larger colonies fared better than smaller colonies. But this was quickly explained by the genetic data. Although most of the corals had settled over the same period of time, following almost complete loss of corals from Crown of Thorns Sea Star invasions and tropical storms around 2010, one kind had grown to larger sizes than all the rest. Burgess and his team see this group, known uncharismatically as haplotype 11 (and its closely related counterpart, haplotype 2), as a distinct cryptic species more susceptible to bleaching when temperatures rise.
How does this study help?
The first big finding here is that corals that appear to be very similar, or even the same species, may be differently resilient to bleaching. Genus level monitoring, which is used so widely in coral reef studies, may not tell us the whole story; in fact, it may be misleading.
This paper showed that small differences in genetics can result in big differences in resilience. Haplotype 11 was larger, suggesting that it grew faster, than the other corals in its genus, but fared worse as the temperature rose. While it may be tempting to think that the other haplotypes are adapting well to warmer temperatures, the researchers did not see that. In the larger context, the corals that did better still suffered, and with a slightly hotter event, might even have perished.
The second finding is that the satellite-based bleaching tool made publicly available from NOAA may not be as accurate as believed. Widely used by reef managers around the world, it is only based on average sea surface temperatures averaged over a large area (2° by 2° area, or about 19,000 square miles) and time (12 weeks) scales. The long-term monitoring program at Moorea uses very accurate temperature probes, deployed permanently at the same depth and location as the corals. These could collect much more accurate data on what the corals were actually experiencing than a satellite could.
The researchers could then explain why they were observing coral bleaching while the satellite tools said they should not, and also explain why some areas within the reef bleached more than others not too many miles away. The researchers suspect that much of this discrepancy could be accounted for by the presence of internal waves – plumes of colder, denser water welling up from the deeper ocean and ‘bathing’ the corals at depth, but not at the surface. Such findings may eventually refine this tool to improve its accuracy and value to scientists and local reef managers.
Although the slower-growing species did survive better, and are likely to reproduce and seed future reefs, it is hard to argue that these other haplotypes are heat-resistant corals. The first issue is that disturbances of reefs are becoming so frequent that we need a high proportion of fast-growing corals to maintain the structural and functional diversity of a reef. But, it is vital to remember that Pocillopora, while prolific, is just one type of coral. To sustain the millions of organisms that inhabit reefs, we need a high diversity of different types of coral, each with different functions in the ecosystem. Today, in many areas like Moorea and the Gulf of Thailand, Pocillopora is replacing many of the historic, and more ecologically valuable, species.
Pocillopora may be able to survive the Anthropocene – a geological period defined by changes produced by human activity – better than other some coral – such as the branching Acroporides. But it does not mean that corals are necessarily adapting to change. This study covered only a few species of a single family of corals.
As Burgess puts it, “With increased climate change these differences between species will not matter much, because every coral will experience increased stress.”
Chad Scott is program director for the New Heaven Reef Conservation Program and the project coordinator for the Save Koh Tao Marine Branch in Thailand.