Can ‘good sounds’ help coral islands recover from bleaching?



Staghorn corals seen in the waters off Man Nai Island, near Rayong province, Thailand, February 28, 2024.

Staghorn corals seen in the waters off Man Nai Island, near Rayong province, Thailand, February 28, 2024.
| Photo Credit: Reuters

Climate change is the most serious environmental problem facing the world today. One of the main reasons for this is the global warming caused by the extraction of coal and oil, which have been underground for millennia, and burning them for industrial and non-industrial purposes to generate energy. The effects of these activities were exacerbated by other causes of global warming, such as deforestation.

Global warming has many consequences, including rising sea levels and changes in the frequency of extreme weather. Another is the destruction of coral reefs. India’s Lakshadweep islands for instance are islands formed on such reefs.

Coral reefs have started to die

Coral reefs are built by a type of organism that, when it accumulates, forms reefs and islands. They live in large colonies. Corals have a symbiotic relationship with microscopic algae called zooxanthellae, which live in the corals’ tissues. Life becomes tough when the temperature of the water around them rises. When it does, the corals expel the zooxanthellae. An important visual symptom of these effects is that the colour of the coral reef changes, and subsequently they die off in droves. This event is called bleaching.

However, not all coral bleaching events are due to warm water. Ocean acidification – which is when the pH of the water drops as more carbon dioxide dissolves in it – is another well-known cause. More acidic water reduces the availability of calcium minerals that the corals need to build and repair their exoskeletons.

In many parts of the world, coral reefs have started to die due to global warming experienced over the years. The U.S. lost half of its coral reefs in the Caribbean in one year in 2005 due to a large-scale bleaching event after warm waters around the Virgin Islands and Puerto Rico expanded southward. When scientists collated satellite data of the region from the previous 20 years, they were able to confirm that the 2005 event exerted more thermal stress than those in the previous 20 years combined.

A source of hope

The Great Barrier Reef in Australia is the largest coral reef in the world. Recently many people there protested strongly against a plan to open a new coal mine because they were worried about the survival of the reef – not because the mine would directly affect the reef but because of the effects of burning fossil fuels at large.

Many coral islands like our Lakshadweep are inhabited by people. The destruction of these coral reefs will naturally affect the people there. A rising sea level due to climate change is another problem for them. Many small island countries like Tuvalu and Vanuatu are facing the threat of submergence and have articulated that at international climate conferences. Their submergence is a certainty now; the only question is when it will happen. Therefore, it is very scary that the coral reefs themselves are being destroyed and, axiomatically, any chance of saving the corals can and does become a source of hope.

It is against this background that about 5,000 scientists gathered in New Orleans, in the U.S., in February this year for the American Geophysical Union’s biennial ‘Ocean Sciences’ meeting. Here, Amy Apprill, a scientist at the Woods Hole Oceanographic Institute in Massachusetts and who studies coral reefs, provided information about a study she had been part of.

The coral polyps

To understand this study, we need to first understand what a coral reef is. Coral reefs are built by an organism called a polyp. A coral group is a colony of a large number of genetically identical polyps. Each polyp is a sac-like animal, usually a few millimeters wide and a few centimeters tall. It has a set of tentacles surrounding a mouth-like opening. Polyps are gregarious: they like to socialise.

Every polyp also sheds an exoskeleton at its bottom and over many generations this discarded substance accumulates to form a large structure up to several metres wide. The polyps organise their lives around this structure. A coral reef is formed when a large number of polyps collects in this way.

Polyps use sound to communicate with each other. Naturally, the sound of a healthy colony will differ from that of an unhealthy colony. Dr. Apprill and her team used this distinction in their new study.

The sound of a health colony

The investigators set about examining whether a degraded reef could be enriched by replaying healthy reef sounds, and increasing the rate at which polyps settled in it. They conducted a series of field studies in the Virgin Islands of the U.S., where they quantified the settlement rates of three species of corals with two different breeding strategies.

They found that in degraded coral reefs, the settlement rate increased when such sounds were played (using a novel underwater speaker system). They also reported that the settlement rate was higher closer to the speaker and reduced as one moved away from the speaker, indicating the impact of the sounds played.

Finally, it would be a mistake to think the researchers have solved the reef settlement problem posed by climate change. Their study was just an experiment. If what they did is tested repeatedly and at least a good fraction of instances are successful, then we can decide that their technique can help in practice. But we still don’t know what problems might arise when this project is implemented on a large scale to save coral reefs. We also need to find out the volume at which the sound has to be played and if there are any other parameters to be controlled.

In sum, the new study is just a pointer, even if also a positive one.

V. Sasi Kumar is a scientist formerly at the Centre for Earth Science Studies, Thiruvananthapuram.


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