Coral reefs are made up of tiny animals like sea anemones called polyps. They stay all their lives fixed in one place, attached to a rock or to dead polyps, and grow a cup-shaped skeleton. The skeleton becomes hard coral when they die. Coral reefs are colonies made up from millions of these polyps and their skeletons, and can stretch for thousands of kilometres. Fringing coral reefs grow near shores.
Barrier reefs form a little way offshore.
Coral atolls form around the edge of an old volcano. As the volcano sinks or the sea level rises, so the coral grows up and eventually just a ring of coral, the atoll, is left at the surface. Anemones tend to stay in one place, but can slide along. Jellyfish float freely, drifting with the currents. Like both corals and anemones, jellyfish have tentacles with stinging cells called nematocysts.