Corals are animals. They sit in groups in a fixed place in the water, which is called “colonies”. Most corals live in the sea. All corals belong to the cnidarian phylum, as do jellyfish and many other animals. Some of the different corals are not particularly closely related. The best known are the hard corals that can form coral reefs.
Corals are beautiful to look at. Many of them are very colorful and therefore popular with divers. Some of them you like to take as jewelry. However, the corals are more important for nature: around a quarter of all sea fish live between corals. They find shelter there and raise their eggs and young there.
Corals only like a certain temperature. As soon as it gets too warm, they die. They then lose their color and only the white skeleton of lime remains. This is happening more and more often in different places lately.
A likely cause is climate change caused by humans. This increases the temperature in the oceans. The amount of carbon dioxide that humans are releasing into the atmosphere is making the oceans more and more acidic. This makes it harder for the corals to build up their skeleton. In the well-known Great Barrier Reef off Australia, more than half of the coral is already badly damaged. Some have already died.
Another enemy of corals is trawled from large fishing boats that are towed across the seabed. They just break off the coral. Many corals are also destroyed by the extraction of oil and natural gas. The same happens when electrical lines are laid on the seabed.
Corals have other enemies besides humans: Various fish, starfish, and snails like to eat coral polyps. Boring sponges burrow into the coral skeleton and hide there. Also, some mussels, worms, and algae build cavities in the skeletons of the corals so that they can live inside.
How do hard corals live?
Hard corals live in tropical seas. The warmth of the water is right for them there. You also absolutely need salt water. They live together with small algae, each consisting of only a single cell. This coexistence is called “symbiosis”.
Every hard coral consists of two parts: The upper part is called a “polyp”. It is in the shape of a cup. At the top are the tentacles, in the middle is a mouth opening, and below is the space for digestion. Hard corals suck in seawater, filter out what they can use, and return the rest to the sea. This is part of their diet. They get the rest of their food from the algae they live with.
The stone corals get lime from the seawater, which they excrete through the foot disk. This forms the second part of the coral, the “corallite”. This part dies off, forming a coral reef. This keeps growing. This is how many islands were formed, such as the Bahamas, Bermuda, the Maldives, Tuvalu, and many others.
A hard coral can be male or female or both at the same time. They give up their sperm and egg cells. Fertilization can take place in the sea or in the mother. From this arise larvae. They float in the water for a maximum of six weeks, then settle and form a new polyp.
But it also happens that females reproduce alone. Some species of coral can grow any fragment on its own after being broken.