They live shallow because of the sunlight they require. The corals are a home for sea life. Corals have lived on Earth for about 400 million years, but reef building corals need shallow waters. Coral reefs we see today have been growing since the glaciers melted, and sea level rose and flooded the coastlines. In mid-latitudes, that would be generally about 5,000 years ago.
Corals reproduce sexually, often in mass spawning events.
Fertilization is external and results in a free swimming planula larvae which disperses the coral through out the sea. Corals are made of dead organism's skeletons inside, and on the outside are living organisms called polyps. They are embedded in a hard exoskeleton of calcium carbonate secreted by the external covering of the polyp. To feed, the polyp extends it's tentacles into the water. The polyp tentacles contain stinging cells called nematocysts, which catch prey, mainly zooplankton.