Introduction into the Fascinating Biology of Coral Reefs

Sony HX50 Unterwasser

Last year, in 2013, when I went to Dahab/Egypt at the Red Sea to do my internship at the Red Sea Environmental Centre (RSEC), I didn´t know much about corals. But I was fascinated by marine life and curious about what´s going on in the reef.

The 6 weeks of diving and exploration in Dahab/Egypt as well as exciting lectures held by RSEC member Nina Milton gave me detailed insights into the biocoenosis of coral reefs – and it was mindchanging.

More than before I understand how vulnerable this ecosystem is and that it needs our help because coral reefs are threatened by many factors such as global warming (-> rising water temperatures), rising CO2 emissions (-> decrase of pH-value of the seawater), overfishing and of course pollution.

I want to give you a first introduction into the  topic „Biology of Coral Reefs“ and I want to share my fascination for this vulnerable ecosystem.

Snorcling in the Red Sea
Enjoying the beauty of coral reefs

Let me start with a basic question:
What is a coral? Stupid question? Well, I´m sure not everybody knows the answer! A first look at a coral might suggest that it is a plant – or a stone.
But it´s not!
Corals belong to the taxon of „Cnidarians“ which also includes anemones and jelly fishes – so corals are animals!
But if you look at a coral you (usually – there are exceptions) do not see a single animal, no, you see a whole colony of hundreds and thousands of animals, so called „polyps“! And all those single polyps are  connected by living tissue.
But every colony of corals starts with a single polyp which develops out of a fertilized egg cell. If this young polyp finds a adequate place it settles and starts to cleavage into new polyps – the colony starts to grow (and sometimes reaches huge sizes -> see the pictures).
Depending on which taxon the coral belongs to – there is a important differentiation between hard corals and soft corals – the polyps produce a limestone skeleton which is THE basic building material of coral reefs! Guess, which of these 2 taxons build the skeleton 😉 If you have a closer look at soft corals you will not find a real limestone skeleton but small skeletal elements which stabilize the coral. These small elements are also important for the reef growth – when the coral colony dies they settle down and build a layer of calcium carbonate.

Soft Coral
A Tree-Soft Coral guarded by a damselfish.

Now imagine what happens when the pH-value decreases (-> the water becomes acetous) caused by rising CO2 emission  – ever used vinegar or citric acid to remove limescale from your tap?! Acid solves calcium carbonate and the same might happen  (of course much slower in a long period of time) to the reefs – that might be a big issue for the future.
Now let´s have a closer look at corals!
You already know that corals are animals, living in colonies and that they have a limestone skeleton or at least skeletal elements.
But what do these animals feed on?
As I wrote, corals belong to the cnidarians and like most cnidarians coral polyps have tentacles which enable them to filter plankton out of the water. But that´s not all – corals live in a symbiosis with micro algae, so called „Zooxanthellae“ which provide important products of metabolism to the coral. These micro algae are responsible for the beautiful colors of the corals.
But there is a problem: If the coral is under stress (which can be caused by rising water temperatures or sedimentation for example) this can lead to the release of the symbiotic algae. This release of the algae is linked with a loss of the corals color – the so called „Coral Bleaching“ – maybe you have heard of it? If the stress lasts a longer period of time, and therefore the bleaching, the coral will die! There have been big incidents of coral bleaching in the last years (for example in the reefs of the Maledives) which led to the death of big parts of the reefs ( Of course there are coral species which are more resistant against stress factors like high water temperatures, strong UV radiation etc. (they evolved heat shock proteins or UV protection pigments to protect the symbiotic algae) but imagine what will happen when we keep going on with pollution or rising CO2 emission! I can´t say if this will destroy all coral reefs (but I don´t think so – Evolution maybe will find solutions) but there are many species which will not keep up with the speed of the changings (caused by humans) -those might extinct. In either case, there will be big changes in the ecosystem coral reef if we do not change our way of life!

Massive Porites Coral
Massive Colony of Porites Coral

Now I want to give you a short overview over the coral taxonomy – which is complex! Only in the Red Sea there have been found about 300 hard coral species (200 of them along the egyptian coastline with over 50 genera). And different coral species are often not close related to each other. There are corals which are related to the soft corals ( as part of the Octocorallia) but which build a limestone skeleton like hard corals (such as the genus Tubipora), there are the well known „Fire Corals“ which actually belong to the Hydrozoa and not to Hexacorallia or Octocorallia… Like I said, it´s complex!
But there are a few typical genera (distinguish species is sometimes even hard for experts) which can be easily distinguished and which are quite common in the coral reefs.
As you know there are soft- and hard corals as a approximate differentiation.
They can be distinguished by the existence or non-existence of a limestone skeleton. But there is another way:  by counting the tentacles of a polyp!
Most hard corals have six or multiple of six tentacles (Hexacorallia), soft corals do have eight tentacles (Octocorallia).
2 Typical genera of hard corals are Acropora and Porites (shown on the pictures). Usually Acropora has a branching growth but also digit or tabulate growth is quite common!

Branches Acropora Coral
Coral of the genus Acropora

The genus Porites is known for huge and very old colonies. There are massive growth forms as well as columnar growth forms!
Common types of soft corals are for example tree soft corals (on the pictures) or the leather corals (also on the pictures) – their names tell you a lot about how they look and how they feel (but of course: Do not touch corals – you could harm them and touching a FIRE coral would harm you!!!).

The diversity of colors and growth forms in the reef is breathtaking – if you ever have the chance of going snorcling or diving in a reef – take this chance (but do it with caution)!

To come to an end of this short introduction:
Why is it important to protect the coral reefs?
Well, first of all: It´s home of thousands of species (fishes, invertebrates, turtles and many more…) – Do you want your home to be destroyed? These animals definitely don´t want to be homeless.
Furthermore, millions of people are living at the coast –  and reef protection is coast protection – a healthy reef prevents the coast from erosion!
For many people dive tourism is a important source of income (of course this has to be ecological compatible diving) – no reef – no dive tourism – no income!
And: a healthy coral reef is pure beauty! It´s hard to describe if you haven´t seen it – but once you have experienced it you won´t forget!
A coral reef is nursery ground for different species and it´s livelihood for many people.
There are several more reasons…

If you are interested in having such an experience why not check out the link to the Red Sea Environmental Centre?!
For RSEC on Facebook click here.

Please excuse the low image quality – last year I only used a cheap point-and-shoot camera for these underwater pictures!

If you have questions or criticism – don´t hesitate to write me!

Check also out:
Interesting overview!
Interesting page about the deep sea cold water coral reefs!

Pohlmann Photography auf Facebook
Pohlmann Photography auf 500px
Pohlmann Photography auf Flickr


  1. Schöne Bilder, die Du am Korallenriff in Ägypten gemacht hast. Ich hoffe nur, dass das Riff noch lange so farbenfroh bleibt. Dort kann man noch einige Tierarten sehen, die es in anderen Riffen schon lange nicht mehr gibt.

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    1. Hallo Nils,
      vielen Dank! Das hoffe ich auch. Die Riffe in Dahab sind zu meist noch in recht gutem Zustand. Korallenbleiche und Krankheiten sind recht selten bei den Korallen dort. Ein größeres Problem sind leider unachtsame Taucher, die das Riff beschädigen und Verschmutzung mit z.B. weggeworfenen Plastiktüten oder durch alte Fischernetze, die sich in den Korallen verfangen. Das Red Sea Environmental Centre macht vor Ort regelmäßig Reefchecks und Coral Projects, um den Zustand der Riffe dauerhaft zu überwachen und steht in Kontakt mit den ägyptischen Behörden. Hoffen wir das Beste für die Zukunft!

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