More than 70% of the surface of the Earth is covered in water. Almost all of this enormous body of water consists of oceans. They are home to a tremendous variety of species – from microscopic plankton to the largest marine animal – the blue whale. Corals are a very important part of the ocean life. Contrary to what a lot of people may think, corals are animals. Usually they live in colonies; an individual coral is called a polyp and is just a couple of centimeters in length. Corals have tentacles which help them capture their food. They also have one more very interesting way of getting energy – through symbiosis. Corals host in their tissue an one-cell alga called zooxanthellae. The algae get energy from the sunlight and pass it through to the corals. Corals provide the algae with nutrients and shelter in return. Zooxanthellae also give the corals a variety of beautiful colours.

   There are a lot of different coral species which explains the diversity of their colours and shapes.  Some of them are hard corals. They are also called reef builders and are able to secrete calcium carbonate. It forms a skeleton and the coral actually lives on its outside. Soft corals do not have this ability, as their name suggests. Usually corals thrive in warm shallow water, with temperature around 29°C. The easy access to sunlight gives plenty of energy to the zooxanthellae which gives the corals the opportunity to grow immensely. When a lot of coral skeletons grow together, they form a reef! There are fringing reefs, barrier reefs and attols. It is due to the fact that reefs provide a habitat for a great diversity of marine life that they are considered to be rainforests of the oceans. Probably the most famous one is the Great Barrier Reef. It is situated in the Coral Sea, along the coast of Queensland, Australia. It is so big that it can be seen form space! With its 2500 individual reefs and more than 900 islands, it is the largest living structure on the planet! Corals grow very slowly, just a couple of centimeters per year and to get to its present-day size, the Great Barrier Reef began forming as early as 20 million years ago! There are fossilized corals dating from 500 million years ago.

   Considered one of the 7 natural wonders of the world, the Great Barrier Reef is a very popular attraction. However, this impressive ecosystem is also a home to more than 1500 fish species, sea turtles and birds. Predators like dolphins and sharks hunt the smaller fish that live there. The range of biodiversity there is one of a kind.

   There is one immense threat to the reefs and that is global warming. The increasing ocean water temperature causes the zooxanthellae to die, which leads to discolouration of the hard skeleton of the corals, better known as bleaching.



plankton – small plant or animal drifting on the surface of seas or lakes

alga, pl. algae – a red, brown or green plant-like organism that usually grow in water

symbiosis – the living together of two organisms, usually this relationship benefits both of them

tentacles – a narrow, flexible part of the body of an animal,such  as an octopus or jellyfish

calcium  carbonate – a chemical compound consisted of carbon, oxygen and calcium. It can be found in rocks and the shells of eggs, marine animals, snails

habitat – the natural home of an animal or a plant


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