Coral reefs represent some of the oldest and most complex communities of plants and animals on Earth. About 200–400 million years old, they cover about 231,660 mi² (600,000 km²) worldwide. (The most popular reefs range from 5,000–10,000 years old.) The primary structure of a coral reef is a calcareous skeleton formed by marine invertebrate organisms known as cnidarians, which are relatives of sea anemones. Corals are found in most of the oceans of the world, in deep as well as shallow seas and temperate as well as tropical waters. But corals are most abundant and diverse in relatively shallow tropical waters, where they have adapted to the constant temperatures provided by these waters. The reef-forming corals, or hermatypic corals, have their highest diversity in the Indian and Pacific Oceans, where over 700 species are found. By contrast, the Atlantic Ocean provides the habitat for less than 40 species. Other physical constraints needed...