Healthy coral reef ecosystems are like bustling cities, with buildings made of coral and thousands of marine inhabitants coming and going, interacting with one another, carrying out their business. In this sense, coral reefs are the sea’s metropolises. Coral reefs provide shelter for nearly one quarter of all known marine species. And over the last 240 million years, reefs have evolved into one of the largest and most complex ecosystems on the planet. They are home to more than 4,000 species of fish, 700 species of coral, and thousands of other species of plants and animals. Scientists estimate that, in total, more than one million species of plants and animals are associated with the coral reef ecosystem.