Coral is a stone-like substance which is made 1) … tiny sea creatures called ‘polyps’. There are many different 2) … of coral, and it comes in a huge variety of shapes, sizes 3) … colours. Many corals together form a ‘reef’. Coral reefs rely on sunlight and warmth, so they 4) … found in the shallow waters of tropical seas worldwide. Reefs can be very old – some formed 25 million years 5) …! Coral reefs are sometimes called ‘underwater Amazons’. They are home to thousands of different species, 6) … the polyps themselves to fish, molluscs like octopuses and crustaceans like crabs and shrimps. Scientists often say we know 7) … about space than we do about the ocean, and 8) … are probably many undiscovered species living on and near reefs. Rising ocean temperatures are threatening the reefs by changing the delicate balance of this tropical habitat. Water pollution 9) … the situation worse, encouraging the growth of algae and the development of disease. The first steals sunlight from the polyps, while the 10) … infects and kills them. Coral reefs have their own natural cleaners. Coral crabs are tiny crustaceans 11) … live on coral. They eat any algae that falls on the coral and generally keep 12) … clean. The crab gets a meal and the polyps get more sunlight, so everyone benefits. But these tiny creatures can’t 13) … the job alone anymore – it’s just 14) … big. Only people, working together globally, can save coral reefs from dying out altogether. There are many reef conservation organisations which you can join. Why 15) … have a look at some of them and think about volunteering?​