Coral reefs have been built up from the skeletons of reef-building coral a small primitive marine animal, and other marine animals and algae over thousands of years. They occur in clear, shallow and sunlit seas. Coral reefs are one of the most productive and diverse ecosystems and are estimated to yield about 12% of the world's fish catch. They are very vulnerable to any change in their environment, especially pollution, because it makes the water opaque. They must have light in order that photosyntesis by the algae can take place. Like trees, corals reflect the environmental conditions in which they grow, indicating marine pollution, sea-surface temperature and other aquatic conditions.