Corals are marine animals made up of small, soft-bodied polyps, often living together in colonies. Many shallow-water corals host microscopic algae that provide energy through photosynthesis, which is why reef-building corals thrive in clear, sunlit seas. Over time, these corals deposit calcium carbonate skeletons that can form reefs-complex habitats sheltering many kinds of marine life. Not all corals build reefs: soft corals lack the same hard framework, and some species live in deep, dark waters. Together, coral groups show a range of ways these animals grow and live in the ocean.