What really happens to corals during marine heatwaves

In a recent study, scientists show for the first time what really
happens to corals during marine heatwaves, and they reveal that it’s not just
coral animals that are affected – their skeletons start to decay within weeks,
too. This means that the 3D coral framework which provides home to many other
animals on the reef is also at risk.

The study by a team
of researchers from UNSW Sydney, The University of Newcastle, The University of
Technology Sydney, James Cook University and The National Oceanic and
Atmospheric Administration (NOAA), USA was
published in the journal Current Biology.

SIMS was fortunate to have many of these researchers coordinating the recent Science Week Corals event here at SIMS.  It was a wondeful opportunity for students and the broader public to learn from such an inspiring collaboration of scientists who were motivated by engaging the community in the message of coral conservation.

Read the full story from UNSW Media below.

“In 2016 the team’s
showed that
just a 0.5OC increase in ocean temperature changes the extent of mortality that
happens in coral during bleaching.

In this study, the team now find that severe marine heatwaves not only trigger
bleaching events as we have known them – a breakdown of symbiosis – but in fact
can lead to heat-induced mortality of the coral animal itself. They suggest
that severe heatwave-induced mortality events should therefore be considered a
distinct biological phenomenon, with more direct damage different from coral

“Until now, we have described coral bleaching as an event where the symbiotic
relationship between coral and its microbes breaks down and corals lose their
main source of nutrition, and the coral can die if the symbiosis is not
restored,” author Associate Professor Tracy Ainsworth from UNSW says.

“But what we are now seeing is that severe marine heatwave events can have a
far more severe impact than coral bleaching: the water temperatures are so warm
that the coral animal doesn’t bleach – in terms of a loss of its symbiosis –
the animal dies and its underlying skeleton is all that remains.”

“We find that the skeleton is immediately overgrown by rapid growth of algae
and bacteria,” says Associate Professor Bill Leggat of the University of
Newcastle, a co-author on the paper.

“We were able to study the consequences of this process of rapid colonisation
using CT scanning of the coral skeleton – as would be used in medical imaging.
We show that this process is devastating not just for the animal tissue, but
also for the skeleton that is left behind, which is rapidly eroded and

University of Technology Sydney scientists A/Professor David Suggett and Dr
Emma Camp
explain how they were also able to use novel bio-optical techniques
that allow them to visualise and study the rapid transition in the coral
microbiome for the first time.

“With this technique, we can see microbial communities go from symbionts to
harmful coral skeleton-dissolvers. Adopting these techniques more broadly will
be central to understanding how this process occurs on reefs globally – we
anticipate that heatwave mortality events, and rapid reef decay, will become
more frequent as the intensity of marine heatwaves increase.”

Dr Scott Heron from James Cook University says this rapid dissolving of coral
skeletons following severe heatwaves hasn’t been known to date.

“Climate scientists talk about ‘unknown unknowns’ – impacts that we haven’t
anticipated from existing knowledge and experience. This discovery fits
into this category. As we begin now to understand this impact, the
question is how many more of these ‘unknown unknowns’ might there still be that
could bring faster and greater damage to coral reefs from climate change,” he

Dr Mark Eakin, Coordinator of NOAA’s Coral Reef Watch, says such events are

“We already use climate models and satellite data to predict and detect
conditions that cause coral bleaching. By focusing on especially severe marine
heatwaves, we should be able to predict this direct coral death, too.”

A/Prof Ainsworth says that the team hopes that this research will motivate the
public to tell decision makers how important coral reefs are to them, and
voice the immediate need to preserve coral reefs now.

“Across the globe coral reefs are still a source of inspiration and awe of the
natural world, as well as being critically important to the communities that
rely upon them. Given that the degradation of coral reefs will result in the
collapse of ecosystem services that sustain over half a billion people, we
urgently need actions both globally and locally that protect and conserve these
truly wonderful places.”