Coral reefs are among the most biodiverse ecosystems on Earth, rivaling tropical rainforests in their complexity. Built over millennia by colonies of tiny polyps that secrete calcium carbonate skeletons, reefs provide shelter, food, and breeding grounds for an estimated 25 percent of all marine species. The Great Barrier Reef alone hosts over 1,500 species of fish, 400 species of coral, and countless invertebrates. Reefs also protect coastlines from storm surge and erosion, providing an estimated $375 billion annually in ecosystem services worldwide.