A new study conducted across the world has finally revealed exactly why corals appear to be glowing during heatwaves – the answer is survival.

Colorful Struggles for Survival

Avid divers, snorkelers, and/or admirers of the deep might have noticed that some coral reefs appear to glow during warmer days. They put on a dazzling exhibition of colorful shimmers, almost in a choreographed manner. Check out this pretty cool short video that captures a few of these reefs showing off their advanced-evolutionary shimmer dancing. Although beautiful and to witness it must be quite bewildering, this display can be likened to a desperate attempt by the corals to protect themselves against the harsh temperatures of our ever-warming seas.

Scientists from the University of Southampton’s Coral Reef Laboratory found evidence that this instinct occurs specifically within corals going through bleaching. Through the act of shimmering, corals have adapted to create a protective layer for themselves against the sun’s penetrating rays. Although both the water surface and the water column filter out much of the sun’s more harmful light waves, the surrounding water itself gradually heats up. By creating this illuminating glow, they essentially lather themselves up with sunscreen. This extra layer of protection is needed during heatwaves because the stress of the heat causes them to lose their natural protection, symbiotic micro-algae. During stressful events, critical algal endosymbionts are expelled from within the tissues of the coral. Since the algae provide the corals with most of their nutrition, the loss of these symbionts leads to starvation and ultimately death, where the corals turn a dismal grayish-white, i.e., “bleaching”; that is, unless the symbiotic relationship can be reestablished. Additionally, the vivid light show performed by the corals acts as a flashing neon sign to draw algae towards them. The Southampton study revealed that corals have demonstrated glowing as quick as two to three weeks after being exposed to heat stress, even at mild or temporary temperature fluctuations.

Last Line of Defense

In some cases, coral reefs are able to rebuild themselves after mass bleaching events. Usually this has only occurred where there have been considerable conservation efforts in conjunction with a slowly re-establishing marine ecosystem. There is an awesome example of a reef that came back from extinction following a 2001 hurricane in the Caribbean, where a 10-year restoration project assisted with bringing life back into the reef ecosystem.

Unfortunately, though corals are slowly adapting to drastically shifting climates and warming sea temperatures, the current rate at which we are losing reefs infers that the pace at which climate change is at work is outcompeting the natural adaptation rate for corals. Because algae are the primary food source for corals, when they do not return quickly enough, reefs continue to face environmental stress without having the fuel to fight it. Within just a few years, an entire reef can break down piece by piece, slowly losing critical biodiversity, and ultimately being disrupted beyond repair. Hence why this shimmering glow is playing such an important role in coral rehabilitation. If they can protect themselves—even fractionally—and attract the integral algae back, they are then capable of reestablishing their own ecosystem.

The head scientist of Southampton’s Coral Lab explained it as this: if coral tissues and their inner cellular makeup are capable of carrying out basic life functions even throughout environmental stress events, then their internal light levels will be able to boost production of photoprotective pigments. In this way, they create a virtuous cycle for themselves whereby they maintain critical life support while simultaneously drawing in the necessary organisms for survival.

Perseverance is Key

Even if you are not an ocean lover, or the smell of the sea on your skin and in your hair does not particularly float your boat, it is almost an undeniable fact that corals are freaking cool. Coral reefs alone support a quarter of all marine life, which is an estimated whopping 9 million different species. In the same way that urban cities serve as civilization hubs above ground, coral reefs do this for below-surface wildlife: they are the underwater city centers connecting the highways—i.e., currents—of our oceans. In fact, the natural underwater cities that coral reefs have created serve such a vastly more abundant array of life than any human man-made city, that if compared by scale, reefs would dwarf us. They serve as breeding, nursing, feeding, and cleaning grounds for critical ecosystem service species.

Corals are also not just a pretty face in terms of human appreciation, they have so much more depth—pun intended. In addition to providing for a huge tourism industry, they stand as the front line workers for coastline protection, quelling swells, slowing down floods, and diffusing strong winds. They also provide many coastal communities with a range of livelihoods, including education activities, tours, and fishing. In some areas, the existence of coral reefs is the sole reason communities are food secure. This of course is in places where the system is managed sustainably and individuals are fully informed of proper catch allowances to allow for a healthily-functioning marine ecosystem.

Aqua Studies

In order to gain this valuable insight into the inner workings of corals and how they are adapting every day to their chaotic surroundings, the Southampton Coral Lab scientists conducted a series of controlled experiments within a coral aquarium. One of their findings was that when the seemingly colorful bleaching is occurring, an optical feedback loop is taking place that involves both symbiosis partners—i.e., the coral together with the algae. In healthy corals, algae are responsible for absorbing the photosynthetic pigments from sunlight. But when undergoing heat stress, and corals expel their algae, the excess light gets trapped inside their cellular tissue, bouncing back and forth within the organism and reflecting off of the harsh white coral skeleton, creating the shimmery glow.

The major findings of the study were obviously that some bleached corals have evolved to glow in luminescent colors in hues of blue, yellow, and purple, but also that they do this to attract their critical symbiotic partners back. Their phenomenal ability to shimmer recently sparked UNEP to partner with The Ocean Agency through the Glowing Glowing Gone Campaign. The campaign aims to create awareness and draw in citizen attention to together be “the first generation to save an entire ecosystem”. It has been such a great effort that the process was showcased in an Emmy Award-winning Netflix documentary, Chasing Coral.

Not only are these pretty profound findings for coral research, a deeper understanding of the glowing also serves as an ecosystem indicator. Measuring how coral fluorescence fluctuates could potentially act as an early warning system for an ecosystem in distress. By monitoring reef habitats, we could essentially be alerted to a coral on the edge of destruction, and respond dutifully with suitable conservation behaviors quicker than ever before.