Scott Reef: a story of hope and caution for coral genetic recovery

A new study of the remote Scott Reef, located between northwest Western Australia and Indonesia, has revealed a genus of coral with an inherent ability to recover from repeated cycles of collapse.

The catch? They need enough time to pass between disturbances to recover.

The Australian Institute of Marine Science (AIMS)-led research analysed data collected between 1994 and 2021, including years in which disturbances like tropical cyclones and mass bleaching events decreased coral cover.

“We were really surprised when we looked at the genetic data through time and we effectively found no change since the early 2000s to now,” Dr Luke Thomas, AIMS coral scientist and lead author of the paper in Communications Biology, told Cosmos.

“Through multiple severe disturbance events the genetic data showed that… the population still maintained high levels of genetic diversity. Those levels of genetic diversity were similar to more pristine reefs that haven’t suffered bleaching and mortality.

Disturbances such as mass bleaching events can cause associated coral mortality that can drastically reduce coral numbers on a reef. The research team was interested in studying how repeated disturbances impact the corals’ recovery.

Specifically, they were looking for signs of population bottlenecks – when a population’s size gets drastically reduced, decreasing its gene pool, and resulting in a very low level of genetic diversity in the remaining population.

“Genetic diversity is the fuel for adaptation. So, [the] more genetic variation, [the] more possible combinations that generate that suitable phenotype for the environment,” says Thomas.  

“As climate change progresses, maintaining that genetic variation in populations is crucial to safeguard their evolvability in a way.

“There’s no other research today that has really explored this concept in corals.”

Like most Indo-Pacific and Indian Ocean reefs, the primary reef-building genus at Scott Reef is Acropora. This genus of stony coral tends to drive a reef’s recovery process and is also thermally sensitive, making a perfect candidate for the study.

“The beauty of Scott Reef is that it’s remote, it’s isolated, so it’s what we refer to as a closed system – it’s reliant on local sources of larvae. So, it’s sort of like this little experiment, natural lab, to study the impacts of these disturbances,” says Thomas.

AIMS has been sending research vessels to measure coral cover and diversity at long-term monitoring sites at Scott Reef for 3 decades. Since 2004, divers have also collected tissue samples of the coral species of interest, Acropora tenuis, for genetic analysis.

“When we get back to the surface, we preserve those tissue samples in ethanol or we flash freeze them in liquid nitrogen,” says Thomas.

“And then we take them back to our lab in Perth, and that’s when we do the analysis on them. We effectively have a big room full of ethanol preserved samples from all over the place.”

According to Thomas this is unique in the coral research world, because normally genetic studies on corals are from samples collected at a single time point.

“It’s hard to understand how things have changed if you don’t have a historical baseline to compare.  We shouldn’t be looking at natural populations at just a single time point, we need to study them through decades and multiple disturbances to really understand how the genetics is changing.”

What they found was, unexpectedly, the genetics changed very little. Despite severe declines in coral cover, the data suggests that, over time, there have been enough remaining survivors to facilitate recovery and maintain moderate levels of genetic diversity in the population.

“Corals… [are] really large populations with a huge amount of genetic diversity… When we see these mass disturbances there’s still enough survivors and those survivors are diverse enough that, when recovery stems from just a few surviving populations, we don’t see declines,” Thomas explains.

And the researchers believe these findings are applicable to Acropora species the world over.

“The reason they were able to maintain genetic variation is because the genus of coral has a really high dispersal capacity, so their larvae can last in the water column for up to a week plus, and they can disperse across the reef-scape quite easily,” says Thomas.

“There might be some sites that were less impacted and because of that high conductivity those less impacted sites can facilitate recovery at distant sites that were impacted.

“There are things that are unique to Scott Reef… its isolation, its water quality, it’s removed from other stressors that, for instance, plague the Great Barrier Reef like poor water quality and overfishing. But the processes, we think, are sort of consistent across reefs.”

It’s some much needed good news on the coral front following confirmation of a mass bleaching event unfolding on the Great Barrier Reef in March – the 5th such event since 2016.

AIMS Research Program Director Dr David Wachenfeld said the “mass coral bleaching event on the Great Barrier Reef is part of a global pattern of extreme heat, caused by climate change.”

“Climate change remains the biggest threat to coral reefs globally. It causes more frequent and severe marine heatwaves, leading to shortened windows of recovery for corals from bleaching and other disturbances.”

But the key to Acropora’s ability to recover its genetic diversity is that, historically, there has been sufficient time between recurrent disturbances to allow for it.

“The results, in a sense, are hopeful because we know it’s not too late to act. That genetic diversity is maintained in the population. We haven’t lost any of that yet,” says Thomas.

But that doesn’t mean we can sit on our coat-tails and solely rely on Acropora’s natural resilience. Thomas warns that bleaching events are likely forecast to become an annual occurrence in the next couple of decades.

“If these bleaching events are annual, then that’s going to erode the natural resilience of these systems and [there will be] just not enough time for them to recover. So, it’s definitely a story of hope, but it’s also one of caution.”