how is this ecosystem formed?
They are formed of colonies of coral polyps held together by calcium carbonate.
Coral reef
Reefs are formed of colonies of coral polyps held together by calcium carbonate. 
where is the black mirror crocodile episode filmed Iceland when was it released? 29 December 2017 who is the director? John Hillcoat what is the star cast of the film? Andrea Riseborough, Andrew Gower, etc. what is the storyline? The episode centres on Mia who is distressed about having helped her friend Rob cover up a hit-and-run death. where is the director from? Queensland, Australia. can you name a few famous places to visit here? Brisbane, the Gold Coast, the Sunshine Coast and the Great Barrier Reef. why is the last mentioned place famous? It is the world's largest coral reef system in the Coral Sea. what is the meaning of this system? It is an underwater ecosystem characterized by reef-building corals.
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Coral reef [SEP] Introduction
A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. Coral belongs to the class Anthozoa in the animal phylum Cnidaria, which includes sea anemones and jellyfish. Unlike sea anemones, corals secrete hard carbonate exoskeletons that support and protect the coral. Most reefs grow best in warm, shallow, clear, sunny and agitated water. Coral reefs first appeared 485 million years ago, at the dawn of the Early Ordovician, displacing the microbial and sponge reefs of the Cambrian.