Which marine invertebrates build the coral reef ecosystems, including the Great Barrier Reef?

• A. Sponges
• B. Jellyfish
• C. Coral
• D. Anemones

Answer: (C)

Coral reef systems are built by reef-building corals that secrete calcium carbonate skeletons, forming the rigid framework of the reef. These corals grow in colonies and over long periods accumulate carbonate material that creates the solid structure you see in places like the Great Barrier Reef. Many reef-building corals also live in a partnership with photosynthetic algae called zooxanthellae, which provide energy from sunlight and help the corals grow in the warm, shallow, clear waters they prefer.

Other organisms on reefs, like sponges, jellyfish, and anemones, contribute to reef communities but do not create the main calcium carbonate framework. Sponges may add some texture with their own skeletons, but they don’t form the large, stone-like reef backbone. Jellyfish are gelatinous and lack hard skeletons, and anemones are soft-bodied and don’t build the reef itself.