A Colourful Reef

closeup of coral polyp, brown lines show where these little algal cells reside for this particular species of coral (Bubble coral).

A closeup of the battery packs (algal cells) inside the coral polyp tissues that are ~ 100th of a millimetre in diameter.

How Corals Get Their Colour

Corals form a symbiotic relationship with tiny, microscopic algae known as Symbiodiniaceae. Even for a biologist this is a challenging word to say, and we commonly use the old colloquial name “Zooxanthellae”. Similar to plants, these algae photosynthesise, that is they produce their own energy from sunlight, carbon dioxide and nutrients. When corals form this relationship, they hold hundreds of thousands of these little solar powered battery packs in just one centre meter of surface area within their tissue layers. They can reach incredibly high densities, like a half million in the area the size of your thumbnail. These battery packs provide substantial energy to the coral polyp so it can grow and make the hard exoskeleton (coral). This relationship is responsible for the creation of the vast and beautiful reef systems we see today, like the Great Barrier Reef.

Symbiotic meaning: involving interaction between two different organisms living in close physical association.

Special Pigments

Light is the driving force behind these little algal cells, and it becomes important to be able to choose colours that will soak up the right amount of sun. Both coral polyps and the tiny algae produce different pigments to achieve this. The coral polyp is responsible for producing special pigments created from a family of green fluorescent proteins, scientifically known as pocilloporin’s. These green fluorescent-like proteins are accessory proteins produced by the coral polyp are responsible for many coral variations such as red, cyan, green, blues and purples.

Symbiotic Algae

The brown pigments that appear commonly in corals come from the symbiotic algae. High concentrations of symbiotic algae, means the browner the coral and the healthier it is. (More battery cells = more energy produced). Because they are so important to the construction, evolution of coral reefs, these tiny organisms are regarded as keystone species on coral reefs. Basically, this means an organism that helps hold the system together.

But Why Are Corals Colourful?

This has become an exciting field in coral biology, and it looks like there are quite a few reasons why coral polyps use different coloured pigments to build and protect their relationship with their symbiotic algae.

Light Harvesting

This is where coral polyps want to create an ideal light environment for their symbiotic friends, because high ultraviolet light can damage the algae and too little light would not produce as much energy. The colourful pigments produced by green fluorescent proteins can convert non-photosynthetic harmful ultraviolet radiation to blue green light (longer wavelengths) which can extend the range of which photosynthesis can occur. This increases the efficiency of the algae to actively photosynthesise in high and low light conditions and the corals with their symbionts act as super-efficient solar panels.

Corals practice the process of fluorescence which is the absorption of light in one colour and the emission in another. Most corals possess several combinations of these green fluorescent proteins produced by genetic code. The variation in combinations is why light harvesting capabilities can change between location, species and even between colonies of the same species and for different sections of the coral polyp. For example, in some species of branching coral, the tentacles can fluoresce green, purple, pink, red and yellow under blue, light conditions, but green under ultraviolet conditions. Human eyes cannot detect ultraviolet light so what we see when we shine a UV torch at a coral colony, its just the green proteins reflecting back that we see. When equipped with yellow mask filters and blue flashlight we get to see a whole other realm of colour.

Coral and Wobbegong shark under normal light conditions.

Coral and Wobbegong shark under ultraviolet light. This cute little shark is one of the many creatures that are also able to fluoresce!

Small coral colony under blue light, with yellow filter. Here the blue light excites other pigments, so we get to see the reds, yellows and greens reflect.

Sun Protection

The little algal cells (just like plants) produce oxygen as a by-product of photosynthesis. This is normally good thing for the coral polyp host as it also contributes to the chemical reaction that produces the coral exoskeleton. However, when there is too much light these little algal cells start to produce too much oxygen, which in turn can damage the symbionts and host polyp. Think of rust on metal, this in the chemistry field is known as an oxidation. So, the coral colonies must find ways to reduce the amount of oxygen produced, and they do this in a couple of different ways.

(a) Firstly, they can position the symbiotic cells in different parts of the tissue layers on the colony, for example on the sides and underside of a branching coral that receives the right amount of light. The polyps are also able to regulate the internal environment by adjusting the internal pH within the tissue layers, this stops the algal cells from reproducing, since fewer cells means less amounts of oxygen produced.

(b) Secondly, they produce different pigments that enables only certain wavelengths of light to be used in photosynthesis. When this happens, the corals appear super colourful, they will look bright purple and even fluorescent green under normal daylight conditions. Whilst this looks spectacular, it’s a sign of stress for the colony. But this process is a form of natural sunscreen, it changes the amount and wavelengths of light that the algal cells receive.

(a)

(b)

Colours That Attract Fish

Fish don’t see colour the way humans do, some see only specific wavelengths of light for example, most plankton feeding damsel fish see ultraviolet wavelengths and even have ultraviolet markings on their head. We know this by looking at the internal structure of the eyes. So, corals have developed pigments that can attract fish. Fisherman have long used these same techniques, by using different coloured lures to attract and target a select species of fish. Scientists can determine which colours are more appealing to certain fish by setting up experiments with corals of different colours and seeing which coloured colony the fish swim to, this is done over and over again with different individual fish and coral colonies.

Fish and other marine species can benefit coral growth, since just like us, they pee and poop. The coral produces mass amounts of mucus which traps this excrement and enables the coral colonies to take up important nutrients for the growth of the polyp and of course their symbiotic algae. Small reef fish like Damsel fish make individual coral colonies their home. The coral structure provides them with shelter from currents and other environmental conditions, the intricate branches of a coral also protects the fish from predators. In return the fish will eat close to home dropping particles of food and of course go to the toilet. The mucus with all these delicious, trapped nutrients can then be ingested by the coral polyp. Studies have shown that a colony with lots of fish living in and around it will grow quicker and are generally healthier than those that don’t have fish.

Next time you’re at the reef, see if you can notice any particular coral colours that these or other fish are living in or feeding on like these butterflyfish.

Colours to Deter Polyp Predators

Corals using colour to hide in plain sight from fish predators appears as a selective strategy. There are over 160 different species of reef fish and invertebrates that prey on corals. Just as colouration can be used to attract beneficial species, it can also be used to deter others. It has been shown that the Picasso Triggerfish (Rhinecanthus aculeatus) can see the exact same wavelengths of light as a humans can see, they also prefer to prey on red and green items when compared with yellow, blue and monochromes. Some species of Butterflyfish are known to prey on corals that were more green and yellow in colour. These behaviours suggest that some colours may appear less vivid than others. However, there may be other factors besides colouration that attract or deter other organisms, for example, size and shape or even chemical compounds, so this idea requires more observation and research.