Coral reefs are created by carnivorous coral polyps that capture prey with stinging cells called nematocysts. Zooxanthellae algae live inside the cell walls of each polyp. The polyp and the algae have a symbiotic relationship. Algae provide the polyp with byproducts of photosynthesis that become proteins, fats and carbohydrates. The polyp produces carbon, nitrates and phosphates that algae require for photosynthesis. Ninety percent of the energy produced by photosynthesis is given back to the coral [source: NOAA]. The energy is used by the polyp to produce limestone that forms the coral's skeleton. This skeleton features a hollow area called the "cup" where the polyps hide from predators. Coral polyps join with other polyps and form colonies that, over hundreds or thousands of years, form a coral reef.
Thousands of individual polyps cluster together to form this single branching coral. (Jeff Foott/Discovery Channel Images/Getty Images)
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