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Hard coral surface
Hard coral surface, close-up. Corals are small colonial invertebrate animals. Individual polyps join together to make large structures, which are the basis of coral reefs. Hard corals differ from soft corals in that they secrete calcium minerals to form a hard skeleton structure. The coral polyps filter food from the water. Photographed in the Indian Ocean in the Maldives. Photo by ALEXANDER SEMENOV/SCIENCE PHOTO LIBRARY
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