Coral reefs are underwater structures formed from calcium carbonate secreted by coral polyps that live together in colonies. Corals can live over 4,000 years, longer than any other ocean animal. Corals are animals, not plants, that obtain nutrients through photosynthesis via algae living inside them or by catching food with stinging tentacles. Coral reefs are important habitats that are home to 25% of marine species and support tourism, fishing and coastal protection for many communities. However, coral reefs are threatened by pollution, climate change and other human impacts.