A calcium carbonate exoskeleton provides an excellent defence[1] but limits the size of the individual polyp. Many tiny polyps living close together tend to make lots of calcium carbonate which forms into the massive chunks we call reefs.

These grow and grow and grow, very slowly but inexorably, so that the longest reef today is the Great Barrier Reef, which is two and a half thousand kilometres long. The biggest reef area is the South China Sea Reef Bank, nearly nine thousand square kilometers (half the size of Wales).

Other species of coral create smaller, distinctly structured shapes, such as Brain Coral or Fan Coral. Some have colonised the deep ocean, hundreds of meters down, where they may have developed other energy gathering approaches such as methane collection.

In fact, the only thing in two billion years to trouble your coral family is the rising water temperature of the late 20th and 21st centuries, caused by humans burning carbon deposits. The warm water dissolves carbon dioxide into it, making it slightly acidic, which in turn makes calcium carbonate more soluable. This may, quite soon, dissolve your entire ecosystem.

[1] at least until parrotfish evolve