What is the skeleton of corals composed of?

Corals are marine invertebrates in the class Anthozoa of phylum Cnidaria.

They typically live in compact colonies of many identical individual polyps. The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.

Corals can be found throughout the oceans, from deep, cold waters to shallow, tropical waters. Shallow coral reefs have optimal growth rates in warm water ranging from 70–85° F (21–29° C).

Corals also need salt water to survive, so they also grow poorly near river openings with fresh water runoff.

They are relatives of jellyfish and anemones. Corals can exist as individual polyps, or in colonies and communities that contain hundreds to hundreds of thousands of polyps.