Coral reefs consist of corals which are small invertebrate marine animals that are classified as anthozoans (anthozoans fall under the phylum Cnidaria). Each individual coral, also known as a pulp has an exoskeleton, is shaped cylindrically and has three basic tissue layer: an outer epidermis, an inner layer of cells lining the gastrovascular cavity (internal space for digestion) and a layer called the mesoglea. As corals are individuals they then cluster together creating reefs. These corals sequester carbon dioxide which attracts algae which then attracts other marine organisms creating a hotspot of biodiversity.
Another factor that takes play in reefs remaining healthy is zooxanthellae. Each coral contains a symbiotic algae, zooxanthellae. The coral provides the algae with protection and compounds necessary for its surviva. An example being carbon dioxide. Meanwhile the algae produces oxygen and removes the coral’s waste. This relationship is key in coral reefs flourishing.