Corals and Reefs 1. What is a coral?

What is a coral?  


Let us start with a question. If you look at the image below, would you assume the organism in the image is a plant or an animal? Or something else entirely? Here is a clue. The organism is a hard, or scleractinian, coral. Does that help at all? Has your answer changed?

You may be surprised to hear that corals are animals. Not only that, within the animal kingdom, they belong to a group called cnidarians, meaning they are relatives of the jellyfish! Furthermore, corals are the architects of coral reef ecosystems. To put this into perspective, a reef without corals is analogous to having a forest without trees. If there were no trees in a forest, there is nowhere for anything to live and biodiversity would be diminished. It is exactly the same for coral reefs- without corals; there would be very little life within a reef ecosystem.

Hopefully that brief introduction has convinced you that corals are both amazing and important. But let’s not stop there. As animals, corals growth, reproduce and feed. But how?

Now we have established that corals are animals, the next important note is that corals are colonial. The huge coral structures that can be seen on reefs (below left) are made up of hundreds, thousands, even tens of thousands of individual animals, all working together (below right). If you were to the cross section diagram of an individual coral animal (below centre), you can see that corals have numerous important features. Firstly, the individual animal part of the coral is referred to as a polyp. This is the fleshy part of the organism and all necessary metabolic and physiological processes occur within the polyp. The corallite is the hard calcium carbonate skeleton, secreted by the polyps to form the scleractinian coral structures that we see on the reefs. Finally, the coenosarc is a layer of tissue that connects all the individual polyps together and allows the entire coral structure to benefit from feeding processes.

Coral diagram-1.jpg


So, corals are colonial animals. But how does a colonial organism reproduce? In the case of corals, reproduction can occur asexually, which allows the polyps to grow into colonies, and sexually, to maintain a healthy genetic diversity. Sexual reproduction often takes the form of external fertilisation via simultaneous hermaphroditism, which is quite a mouthful. In other words, a single colony releases gametes into the water and these gametes are met by those from other colonies , because all coral colonies tend to spawn at the same time of year, although no one is really sure why! Fertilisation then occurs within the water column and larvae begin to form. The larvae eventually settles on a suitable substrate and a new coral colony can begin to form. The extent to which the larvae disperse depends on whether a coral is a brooder (short dispersal) or a broadcaster (longer dispersal). Asexual reproduction in corals is far less complicated. Corals can either bud, whereby a smaller polyp develops within an existing polyp and then buds off, or divide, where a single polyp grows and splits into two. This allows colonies to reproduce and grow rapidly, whereas sexual reproduction, whilst fundamental to coral survival, is a much slower process.


Following settlement on a suitable substrate, a newly established coral starts to grow. This is a very, very, slow process. The process is so slow that some of the large coral structures on reefs can be thousands of years old! The polyps themselves are not thousands of years old, rather the first generation that formed the coral structure first settled thousands of years ago. In other words, corals grow over successive generations. When one generation dies, a bare skeleton is left behind, and this skeleton is the perfect substrate for a new generation of larvae to settle on. This process is repeated over very long periods of time, eventually building an entire reef ecosystem!


We have now covered how corals reproduce and how they grow. There is one more aspect left to address, and it is arguably the most fascinating- How corals feed. Feeding occurs via two different strategies: Autotrophy, whereby corals produce their own food from within their structure, and Heterotrophy, whereby corals predate on other small reef organisms.

Let us begin with Autotrophy.  To do so, we must cast our minds back to coral reproduction. It was mentioned that when corals reproduce via sexual reproduction, the larvae remains within the water column prior to settlement on a suitable substrate. During this time in the water column, the coral larvae assimilate free living microalgae, or zooplankton. This assimilation forms a symbiotic relationship whereby both the coral and the zooplankton gain benefits. Once the assimilation has occurred, the larvae, complete with microalgae within the structure, can settle. Once settled, the microalgae are termed zooxanthellae. Without these tiny zooxanthellae within each polyp, corals cannot survive, but why are they so important?

Both the coral and the zooxanthellae benefit from the symbiotic relationship. The zooxanthellae gain environmental protection by living within the coral structure. Furthermore, the coral provides the zooxanthellae the compounds they require for photosynthesis, mainly carbon dioxide and ammonia. In return, corals also gain useful compounds required for growth and development, mainly Glucose and amino acids. Furthermore, corals gain oxygen as a by product of zooxanthellae photosynthesis, which is put into meeting the energetic demands required to produce the calcium carbonate coral skeleton. There is one more ‘benefit’ that the zooxanthellae provides to corals: Their colour! As photosynthetic organisms, zooxanthellae take in and reflect light, resulting in corals existing in various different colours according to the light wavelengths required by the zooxanthellae for photosynthesis. The process of autotrophy can provide corals with up to 85% of their energetic requirements, all from tiny free living microalgae!

Without Zooxanthellae, corals lose both their colour and 85% of their energy source.

Without Zooxanthellae, corals lose both their colour and 85% of their energy source.

The remaining energetic demand required by corals comes from heterotrophy in the form of predation. It was mentioned previously that corals are part of a group of organisms called Cnidaria. Cnidaria are defined by the presence of Cnidocytes, or specialised stinging cells. So how do corals go about obtaining a much needed snack? They go fishing! Cnidocytes are found along the outside of each polyp and they are sensitive to movement in the water. Triggering of the Cnidocytes by a small reef fish passing by, results in the deployment of an organelle called a nematocyst. The nematocyst is barbed and sticks to the prey. There are often toxins within the nematocyst that disorientates the prey. The prey item can then be taken into the coral and digested. This process is similar to that of anemones, so check out the video below!

And there we have it, a whirlwind tour of coral biology. My hope is that this has perked your interest in corals. This post has focussed in on the corals themselves. But, as mentioned at the start, corals are the architects of coral reef ecosystems. To learn more about reef ecosystems as a whole, check out the next blog in the series, coming soon!




Rachel Louise Gunn