Researchers' Zone:

The majority of the methods for coral restoration are associated with high costs and the predominant methods focus solely on transplanting specific coral species rather than considering the ecosystem as a whole. A new discovery might change that.

Baby corals are attracted to the sound of a healthy reef

And that could be key to saving our struggling coral reefs.

Ayla M. Kroon M.Sc. student, Department of Ecology at Lund University and Center for Oceanic Research and Education, Thailand
Nick J. Burlet Ph.d. student at Department of Biotechnology and Biomedicine at Technical University of Denmark (DTU)

When you think of coral reefs, you might imagine an underwater forest-like area filled with vibrant colours and full of marine life.

In real life, however, around 14 percent of the world’s coral was lost between 2009 and 2018 due to climate change, overfishing, and pollution amongst others, making it increasingly likely that instead of a colourful underwater forest, you will find a vast collection of white-bleached coral.

These bleaching events that are now turning global are very daunting, especially considering that coral reefs support at least 25 percent of all known marine species at some point in their life and are the nursing ground for many others.

Several attempts have been made to rescue suffering coral reefs, including coral transplants and gardening, with mixed success.

But a new and somewhat surprising solution has been discovered – or heard, you might say. Scientists have found that baby corals are attracted to the sound of a healthy coral reef.

How are baby corals' sound preferences going to save coral reefs, you might wonder. That's what we explain here.

We need new restoration methods

In 2020, 7 methods for coral restoration were proposed by the United Nations Environment Programme and the International Coral Reef Initiative.

The majority of these methods are associated with high costs, potentially limiting widespread implementation, especially in low-income countries.

Furthermore, the two predominant methods, direct transplantation and coral gardening, which together comprise over two-thirds of all reef restoration efforts, focus solely on transplanting specific coral species rather than considering the ecosystem as a whole.

So what should we do instead?

The sound of a new solution?

New research is exploring the use of healthy coral reef sounds to attract the settlement of new corals to a reef.

Potentially we could use the unique sound preferences of baby corals to guide them towards reefs in need or areas of the reef where conditions are more optimal for their survival and the survival of the reef.

To understand how we can lure a creature without ears with the sound of something we often associate with silence (the ocean floor), we need to dive into how corals live and, more importantly, reproduce.

Floridas Failed rescue attempt

Even though multimillion-dollar rescue efforts have been set up to restore coral reefs, they do not seem to be yielding the desired result.

For example ‘Mission: Iconic Reefs’ in southern Florida aimed to transplant half a million grown coral colonies to seven ailing coral reefs.

Sadly, the record-breaking underwater heat wave from 2023 ended up bleaching and killing most of the transplanted corals.

With climate change leading to increasingly extreme ocean temperatures, confidence in decades-old reef restoration methods, such as coral transplantation, is likely to diminish. Thus, a drastic call for action is needed to keep these natural wonders safe.

 

Corals are animals with plant-friends living inside

You would not be the first to assume that corals are plants, as they do look similar at first glance. But actually they are animals.

Corals are so called ‘immobile animals’ that are permanently attached to the ocean floor, almost as if they are ‘taking root’.

They have tiny tentacles which are used to scoop food from the ocean into their mouths.

These tentacles also house the ‘zooxanthellae’ (an algae), with whom the corals have a symbiotic relationship, meaning that they have a partnership that benefits them both.

Why do corals bleach?

The very small algae live inside the coral’s tissues, where they get protection and can use the coral’s metabolic waste products of the coral for photosynthesis (the process that plants use to make their own food from sunlight, water, and air).

In return, the algae removes the coral’s waste, provides it with nutrients from photosynthesis that the coral needs to grow, and supplies oxygen.

The bleaching of corals, where the once colorful creatures turn pale white, happens when corals expell the symbiotic algae that live in their tissues.

This typically occurs when the algae become toxic to the coral, resulting from experienced stress due to changes in conditions such as temperature and light.

However, bleached corals are not dead (yet), as they are known to be capable of recovery.

Floating baby corals

Corals have a life stage that they spend floating in the water (steps 2-5 in the figure below), followed by a stage where they permanently attach themselves on the reef (step 1 and 6 in the figure).

Adult corals release egg and sperm cells that form larvae which can swim around in the water freely (the pelagic stage).

Using their small hair-like tentacles (exterior cilia) the larvae create small water currents to steer them to where they want to go.

Once they find an area on the reef that they like, the coral larvae settle there and attach themselves on the reef to form new coral recruits that eventually grow to be the adult corals that contribute to the structures of the reef.

You can see the process in the illustration below.

Various stages in the coral lifecycle. 1) Adult coral colonies of the Lobophyllia (right) and Acropora (left) genus, 2) Egg and sperm cells, 3) Coral embryos, 4) Planular larvae, 5) Coral settler, 6) Coral recruits of Lobophyllia (left) and Acropora (right).

The attractive sound of a healthy reef

Back in 2010, researchers found that coral larvae from various species were attracted by sounds that you typically tend to find in healthy coral reefs.

These reef sounds included the crackling sounds of snapping shrimps, the pops and grunts of fish, and sounds of animal calling, feeding, and movement.

The results even suggested that larvae favoured following these sounds over their tendency to swim down for settlement.

This may seem weird, as coral larvae do not have specialized anatomical features to detect sounds (like our ears).

However, they may be able to register and respond to underwater sounds through their exterior cilia.

Why do healthy reefs sound different?

More recently, in 2018, a field experiment showed that exposure to healthy coral reef sounds increased the degree of larval settlement of a particular species of coral larvae in the Caribbean.

Interestingly, the main difference in sound between a healthy coral reef and the other test locations (which included an ailing reef and a sand bank) was the presence of low-frequency sounds (which typically includes fish calls and the snapping of shrimps) on healthy reefs, which was lacking at the other test locations.

This suggests a link between the habitat quality, the sound production from the reef, and coral settlement.

However, it is still unknown which specific sounds and combinations thereof may be important and whether sound can also be used as a settlement cue for other coral species and locations.

Should we start playing ‘music’ for corals?

These findings suggest that reefs in poor health could struggle to attract the coral larvae needed for recovery.

If the specific sounds that promote coral larvae settlement are identified, artificially playing back low-frequency reef fish sounds may be useful in coral reef restoration.

The use of reef sounds for coral restoration is still in the early stages of development.

However, this could potentially become a more accessible and affordable tool to add to the arsenal of coral restoration methods, instead of the inefficient, expensive and invasive solutions, such as transplantation and propagation as mentioned earlier.

Moreover, it would offer the benefit of enhancing the natural attraction of reefs for all coral species simultaneously, rather than focusing solely on the restoration of a single species, as is the case with the majority of current efforts.

Thus, just like the pop icon Taylor Swift can cause people to travel long distances for her music, some well positioned concerts of healthy reef sounds might be able to help coral larvae find a new home in the places where they can survive and are needed the most.

    Sources

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