Why Do Corals Glow in the Dark?

If you have ever seen a photograph of a coral reef glowing bright green, orange, or red beneath blue light, you may have wondered how it was producing such vivid colours. At first glance, it can look as though the coral is generating its own light. In reality, something rather different is happening.

What you are seeing is called fluorescence, a phenomenon that allows certain organisms to absorb light of one colour and re-emit it as another. Corals are among the best-known examples of fluorescence in the ocean, creating spectacular displays of colour when viewed under the right conditions.

Fluorescence occurs when a substance absorbs light at one wavelength and then releases it at a different, usually longer, wavelength. In simple terms, a coral might absorb blue light and re-emit it as green, orange, or red light. This is why fluorescent organisms can appear to glow when illuminated by certain types of light.

Unlike a torch, a light bulb, or a firefly, fluorescent organisms are not producing their own light. Instead, they are transforming light that is already present.

This is one of the reasons fluorescence is often confused with bioluminescence. Bioluminescent animals create light through chemical reactions inside their bodies. Fluorescent organisms need an external source of light in order to glow. Without light to absorb first, there is nothing for them to re-emit.

Because fluorescent colours are created from absorbed light, they often become much more obvious when viewed under blue illumination. Divers using specialised blue lights and yellow viewing filters can reveal colours that are almost invisible under normal white light, transforming an ordinary-looking reef into a landscape of bright greens, oranges, and reds.

The answer lies in the way light behaves underwater. As sunlight passes through water, different colours are filtered out at different depths. Red light disappears relatively quickly, while blue light is able to travel much further. As a result, many marine organisms live in environments where blue light dominates. Fluorescent pigments can absorb this available blue light and convert it into colours that would otherwise be largely absent from their surroundings.

This is why reefs viewed under blue or ultraviolet light can appear dramatically different from the way they look during the day.

Corals are among the best-known fluorescent organisms in the ocean, and some of the most spectacular examples can be found on tropical reefs. Under blue or ultraviolet light, corals can appear bright green, red, orange, or yellow. These colours are produced by fluorescent proteins within their tissues, which absorb one wavelength of light and re-emit it at another. For many years, scientists assumed these proteins were simply an interesting feature of coral biology. Research now suggests they may play a much more important role.

Most reef-building corals depend on microscopic algae known as zooxanthellae living within their tissues. These algae use sunlight to perform photosynthesis, providing much of the energy that allows corals to grow and build reefs. However, managing light is not always straightforward. Too much sunlight can damage these algae, while too little can limit the amount of energy they can produce.

Scientists now think that fluorescent proteins may help corals manage this balance. In shallow water, some fluorescent pigments appear to help regulate excess light reaching coral tissues, potentially reducing light-related stress. In deeper water, where light becomes increasingly limited, fluorescence may help redistribute available light within coral tissues, potentially making it easier for the algae living within them to photosynthesise.

In other words, corals may not simply be glowing for the sake of it. They may be actively modifying the light environment around their symbiotic algae, helping them cope with very different lighting conditions across the reef.

While corals are among the most famous fluorescent organisms, they are not the only ones. Scientists have also discovered fluorescence in fish, sharks, and even hawksbill turtles, suggesting that the ability has evolved multiple times across different groups of marine life. Exactly why these animals fluoresce is still being investigated, but the discoveries highlight just how much of the ocean's visual world remains hidden from us.

Although some fluorescent colours can be visible under natural conditions, the effect is usually much more dramatic when organisms are illuminated with blue or ultraviolet light. This is why divers using specialised lights and filters are often able to reveal colours and patterns that appear completely hidden during the day.

The next time you see a photograph of a glowing coral reef, it is worth remembering that the corals are not producing their own light. Instead, they are transforming the light around them, revealing a hidden layer of colour that is usually invisible to human eyes.