Corals, like many representatives of marine life, are known to be strikingly colourful creatures. However, understanding the nature of those colours can be tricky. The most problematic part: there's no such thing as colour outside your head. Light differs in wavelength and those differences can be picked up by various receptors in the eye. Colours, in contrast, are born in the secondary visual cortex in the brain after a lot of information is processed. The brain decides how to render the information from the retina based on the context of the whole image. In other words, there's no easy correspondence between wavelength and colour. Every photographer is familiar with this concept as part of white balance correction.
Our brains have been formed in the terrestrial environment and our colour vision is adapted to it, but not to the underwater one. Water and air have dramatically different light transmitting properties. Water absorbs visible light of longer wavelength (reds) faster than that of shorter wavelength (blues). As a result, the deeper you dive, the less reds you will see, and the more immersed in blue the environment becomes. If you turn on a torch (at depth), you might get surprised as the light coming from it appears red. That's because the brain tries to compensate for the missing red part of the spectrum and corrects the internal white balance, turning what would be white light on surface into red. Plus, the torch will actually bring out the red colours confusing the brain even further.
The brain fails miserably at rendering colours below 10 meters and playing around with white balance of underwater shots can quickly show that. To bring out the hidden colours (yellow, orange, red) divers, especially underwater photographers, use white light and strobes. A simple torch makes marine life amazingly more vivid and colourful. That's why the vast majority of underwater pictures were taken with the aid of electronic flashes.
Unfortunately, such seemingly logical approach doesn't go well with corals for they have fluorescent pigments and their colouration depends on the light they are exposed to (1, 2). To illustrate this, below is a picture of a coral under "normal" white light into which we are immersed in our out-of-water world:
The coral (Favites pentagona) looks brown and colorless.
Now let's take a look at this specimen under wide-spectrum light (imitating ambient spectrum at depth of 10m):
What a change! The colours pop up because coral's pigments don't simply reflect light, like normal materials do. For example, grass is green because it absorbs all spectra of light (such as reds and blues) but it doesn't absorb greens and reflects them. Fluorescent materials (molecules or proteins to be specific), unlike normal ones, absorb it and emit light of different wavelength. They make the wavelength longer (shift the colour towards the red part of the spectrum), thus creating a colour that might be absent in the source of light.
To make it simpler: that what happens if you expose the same coral to blue light (to excite fluorescence) and put an orange barrier filter in front of the eyes to remove the reflected blue light. The resulting image shows only fluorescence.
The barrier filter simply doesn't let any (reflected) blue light through, but it is transparent to other wavelengths. That's how photographers and fluoro divers detect fluorescence.
Colours of marine life have been puzzling scientists for a long time. Only research of the last decade shed some light on purpose of these captivating colours and fluorescence.
First of all, fluorescent pigments can shield the symbiotic zooxanthellae algae from excess of UV light (3). UV light gets absorbed before it harms algae and transformed into something more useful.
Since fluorescence transforms the light spectrum, it can be used to create the appropriate light environment at depth. At depth the symbiotic algae would be confined to the ambient light. Corals solve the spectral problems and light accumulation with the pigments, thus allowing algae to live and photosynthesize inside coral tissues and provide the hosts with nutrients (4 - alternative opinions cited inside the article).
The magnificent coral colours are just beautiful to us. To the algae and corals they are vital.
When corals are unwell, their fluorescent pigment composition and abundance changes. Under environmental stressors such as increased temperatures or decreased salinity corals totally lose their symbionts and become white. Bleaching threatens coral reefs across the world. Corals lose fluorescent pigments with declining health as well. Right before bleaching, however (and perhaps contra-intuitive), fluorescence can rise in brightness--as zooxanthellae leave tissues, more fluorescent pigments become exposed (5).
To illustrate colouration differences let's look at this hard coral (possibly Pocillopora):
Another branch of this coral under white light visually looks healthy too:
Because the light that falls on this branch has too many green and red components, the fluorescence is invisible. Thus the coral looks brown. Fluorescence reveals an interesting picture. For the shot below I used an orange barrier filter to make the glow more apparent (by means of removing the reflected light).
Now it's visible that something is different about the tissue on top. Notice that multiple polyps and parts of the tissue are green. Although it might be natural as this coral is known to form green patterns at the tips, to me the tissue doesn't look very healthy.Without fluorescent filters, these differences are impossible to spot.
Green is not always an indicator of bad health at all. Some corals are naturally green under blue light. The sample on the picture below is perfectly healthy and the image is taken in white light. The green is still apparent.
On the reef this coral stands out as it is dazzling green. Not surprisingly, if we check out how many pigments it has with fluoro diving kit we will see a lot of green:
Aquarists are perfectly aware of fluorescence and what spectra corals need. Professional aquariums sometimes reveal more interesting colours compared to what you will see on a dive. Mostly because aquarium lamps exaggerate the violet-blue light and make the corals glow brighter. Plus you don't get ideal conditions on every dive and don't study corals with a magnifying glass. But photographers, unfortunately, still use white flashes for corals. It works well for numerous objects that are colourful even without fluorescence (see stars, nudis, etc). But I imagine how much is missed when we dive with torches and electronic flashes. The vast majority of coral photos that I see lack colors, especially those taken on the Great Barrier Reef. After spending months looking at coral fluorescence I can often guess which colours are patterns are missing on even the most spectacular and high-end underwater shots.
One of my favorite activities is swimming and free diving at night watching bioluminescence, fluorescence (with fluoro diving kits), and trying to understand what those mesmerizing glows and colours mean. I have to say that this activity is very addictive and I'll surely will never get tired of it! Sometimes I feel envious when I realize how complex the vision systems of marine animals are. Especially those of mantis shrimps. There's so much out there our brains and eyes cannot put together.