Here we are at the first episode of a story called Coral Reef! Let's start to know something more about her. First of all...
Why is she famous?
“I am considered one of the most biodiverse places in the world. Biodiversity is the variety of all the organisms that live in a certain place, from the smallest shrimp to the grey reef shark!”
Where is the coral reef located?
The coral reef is not found everywhere; in fact it is distributed only in the equatorial and tropical areas of the Earth. Here it finds favorable environmental conditions to grow:
Water temperature from 21 to 30 degrees. maximum;
Intense sunlight all year round;
Constant salinity
Clear waters play a fundamental role in the development of corals as they allow the filtration of solar rays, then used by plants to produce oxygen, through photosynthesis.
Shallow waters, up to 50760 m.
What is coral?
"I am often confused with a stone or a plant but I am actually an animal! Or rather a large group of tiny animals that can do extraordinary things!"
Let's imagine the coral reef as a huge city, full of skyscrapers and buildings of all shapes and sizes, each building (coral) has many apartments, studios to be precise made of calcium carbonate! Who lives in these apartments? The polyps!
Polyps, extraordinary engineers
Polyps are animals belonging to the large group of Cnidarians. Inside, we find organisms that share a specific characteristic: having cnidocytes.
Cnidocytes are very particular cells with a stinging filament inside them, used both for hunting and for defense!
“Let’s continue…”
Cnidarians include a huge number of animals, very different from each other, among the most famous are:
Anemones: they are very large solitary polyps;
"Nemo's house, to be clear!";
Jellyfish: famous for their stinging power given by the cnidocytes
“you've surely heard about it”;
Corals of all types, made up of polyps;
What a polyps looks like
The polyp can vary in size from microscopic to large up to a meter (in the case of anemones). Therefore we do not always see them with the naked eye.
How we recognize them:
Their most obvious characteristic is to have a crown of tentacles around their mouth, thanks to which they filter the food and defend themselves. These tentacles can be introflexed or extroflexed, precisely when they filter the food.
They have a mouth in the center of the crown of tentacles
They have a small tube-like body, which is often not seen because it is enclosed inside their external skeleton.
So far everything seems normal, but… These little animals are very special for their ability to produce calcium carbonate and use it as an external skeleton (called corallite) for their thin body. When the polyp dies, this skeleton will remain and another polyp can build its new skeleton on it!
“This is the extraordinary keystone of the coral reef. Although we polyps are born, grow, reproduce and die, like all other animals; throughout our lives, we produce this skeleton, precisely the coral, which will contribute to the formation of the coral reef!”
“It is thanks to the work of millions of polyps that this wonder exists!”
Inseparable companions
There are hermatypic and ahermatypic corals: the first ones have guests inside them, the others do not. A fundamental contribution to the life of hermatypic corals in fact, is the presence of tiny algae called zooxanthellae, inside the tentacles of the polyp.
Zooxanthellae satisfy a large part of the nutritional needs of the polyps and above all produce oxygen thanks to photosynthesis, of vital importance for the entire coral reef.
They remove carbon dioxide dissolved in water, facilitating the deposition of calcium carbonate for the development of the external skeleton.
These algae also give color to the coral itself, thanks to their colored pigments. If it were not for them, they would be completely white as calcium carbonate is white.
For these excellent services, zooxanthellae receive protection between the stinging tentacles of the polyp and benefit from the metabolites produced by the polyp itself.
"This extraordinary relationship, very common in Nature, is called symbiosis."
Not all builder corals are zooxanthellate, in fact there are aermatypic corals, which contribute to the growth of the coral reef, without the help of algae. Corals that live deeper for example, where light cannot filter and algae would be of no help.
Corals are not all the same
Polyps live in colonies and the quantity of accumulated skeletons is such as to produce imposing structures with infinitely different shapes.
"We are more than 800 species of building corals, distributed in coral reefs around the world"
When we put our heads under water, we are immediately captivated by the colors of this unique environment but let's linger for a moment on the structure and its shapes.
With a little attention, our eyes will surely rest on...
Some rounded mounds resembling a large mushroom, these are porites, among the oldest corals in the world.
A tabular coral, which expands horizontally and provides shelter to fish that love to stay in the shade;
To the classic all-branching corals that contribute vertically to the reef and can become large in size, for example the staghorn corals.
Not just corals
Other organisms that add their skeletal remains to the reef are mollusks and echinoderms with their shells and skeletons, made of calcium carbonate, which is then used by the polyps.
Also contributing are grazers and perforators, which reduce the skeletons of dead corals into sand that fills the interstices.
Algae and other encrusting organisms help cement together sand and coral fragments, sometimes growing to considerable size.
The growth of coral
“But don’t think that the birth of the coral reef is fast! We alternate periods of faster growth with slower periods, it depends on the environmental conditions!”
All the beauty of the coral reef was not created overnight. It was a very very slow process, lasting thousands of years. Consider that most corals grow a few centimeters if not millimeters per year! Growth is also influenced by many environmental variables.
The biodiversity of the coral reef
This extraordinary variety of shapes, vertical and horizontal structures, crevices, shaded and sunny areas, added to the extraordinary environmental conditions of the reef, allows the life and development of equally varied animals and plants. Although coral reefs occupy 0.5% of the oceans, they host 25% of marine species. This says a lot about their importance. If they did not exist, we would lose a quarter of all marine animals on the planet.
“It is almost impossible to describe all our inhabitants, but we assure you that there are hundreds of thousands! In every corner of the coral reef a different organism is hosted, with its very curious history.”
As you may have understood, the coral reef is a complex and delicate place, it requires the meeting of many environmental variables, unfortunately it is not doing very well now; in fact it is subjected to considerable stress. We are talking about Bleaching, do you know what it is?
