Corals and Reefs 2. What is a coral reef?

What is a coral reef?

In the last blog, I asked and hopefully answered the question “What is a coral?” My answer hopefully gave you some baseline knowledge about the biology of the amazing living structures that form reef systems. So, know we know what a coral is, it is time to ask another question: What is a coral reef?

A common answer to this question is ‘a collection of corals’. But coral reefs are made up of far more than the coral that provides a reef with structure. In fact, the very presence of coral on a reef enables a large diversity of life to thrive within a reef system. Picture a shopping mall, a complex building structure within which one can find a huge array of items. Corals provide a similar role on a reef. They are the reef builders, the foundation, the architects of the reef within which a huge diversity of life can be found.

In the last blog, we learnt that corals have a calcium carbonate skeleton, and it is this skeleton that forms the 3D structure and complexity we see on reef systems. Over generations, corals grow and lay down layer upon layer of calcium carbonate. But corals are not the only calcium carbonate ‘reef builders’ that can be found on the reef. Another example is Coralline Crustose Algae, or CCA. This algae contains calcium carbonate and provides an excellent settlement site for juvenile corals, making CCA an important component in the settlement and growth of coral on coral reefs. However, it doesn’t stop there; any organism that creates a calcium carbonate skeleton contributes to the structure of a coral reef. Why? Because this hard skeleton provides living space for other reef organisms. For example, juvenile fish will take refuge in the spines of sea urchins: On a coral reef, anything can be a home, including urchins, crabs, lobsters and even some snails! The video below is from my time in the Cayman Islands, where I came across a lobster happily living inside a huge barrel sponge!

So coral reefs are made up of a huge variety of calcium carbonate ‘reef builders’. Corals themselves come in a variety of different morphologies, or shapes, each of which contributes to the 3D complexity of coral reefs. There are eight common morphologies of coral:

·         Branching: Found in areas of high wave action, colonising quickly following storms.

·         Encrusting: form a layer over the substrate, harder to see than more structured corals.

·         Sub massive: Thick columns slow growing, found deeper than branching corals.

·         Plate: Table top, with a stalk connecting to the substrate, flat surface.

·         Mushroom: single polyp corals.

·         Massive: Spherical, boulder shapes, often look like brains, long lived, can be huge!

·         Foliose: Lettuce corals, leaf structure minimises sediment settlement.

The eight different coral morphologies, form L-R: Branching, Encrusting, Sub massive, Plate, Mushroom, Massive and Foliose

The eight different coral morphologies, form L-R: Branching, Encrusting, Sub massive, Plate, Mushroom, Massive and Foliose

So we have learned that a reef builder is anything that contributes a calcium carbonate skeleton to the reef system. We have also learned that these reef builders provide living space for reef organisms. Given the variety of coral morphology alone, combined with the numerous different organisms that make up reef builders, it starts to become clear that a coral reef is able to sustain a huge diversity of life. But if you need more evidence, let’s take a look at some facts and figures.

In the animal kingdom, there are 34 known ‘phyla’, or groups. Of these 34 phyla, 32 are found on coral reefs! Compare this to what we will consider here to be the terrestrial counterpart to coral reefs, rain forests, where only 9 phyla are found. Furthermore, coral reefs occupy just 2% of the ocean floor, but 1/3 of all marine species can be found on coral reefs, and 38% of these reef organisms have only ever been encountered once because they are so rare! Combine these facts and you can see that the diversity of life on coral reefs really is astounding!

Global distribution of coral reefs. Despite supporting a huge diversity of life, coral reefs cover just 2% of the ocean floor. Image from  ResearchGate .

Global distribution of coral reefs. Despite supporting a huge diversity of life, coral reefs cover just 2% of the ocean floor. Image from ResearchGate.

What is a coral reef? Hopefully, this blog has shown you that reefs are far more than just a collection of corals. Anything with a calcium carbonate skeleton contributes to reef structure and helps support 1/3 of all marine organisms within just 2% of the ocean floor. So why should you care? Unfortunately this incredible diversity is declining. Bleaching events, combined with overfishing, are resulting in coral being replaced with unstructured macroalgae, with is having a devastating effect on all types of marine organisms, from invertebrates all the way up to apex predators, such as sharks.

To learn more about why we should care about coral reefs and their importance, check out the next coral blog in the series: ‘Why should I care? The importance of coral reefs’, where we will look at some common reef organisms in more detail and address what will happen as species start to decline and disappear from reef systems and what this means for local, human, communities.

 

Rachel Louise Gunn